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Migration and health research: past, present, and future

• Shira M Goldenberg 1 &
• Florian Fischer 2  

BMC Public Health volume  23 , Article number:  1425 ( 2023 ) Cite this article

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Global migration continues to rise at unprecedented rates. Migrants are an extremely heterogeneous group and face diverse health needs related to infectious diseases, sexual and reproductive health, non-communicable diseases, and healthcare access across the whole lifespan. In this editorial, we set the context and invite contributions for a collection on ‘Migration and health’ at BMC Public Health.

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Human migration is essential to growing economies and represents a critical part of the social and cultural fabric of our societies [ 1 ]. Global migration continues to rise at unprecedented rates, fueled by ’push’ factors such as extreme weather events caused by climate change, conflict and violence, political instability, and poverty, as well as ‘pull’ factors including kinship networks and a desire for social and economic mobility. In 2019, the number of international migrants worldwide – people residing in a country other than their country of birth – reached its highest level, at 272 million [ 2 ].

Migrant populations are extremely diverse. The International Organization on Migration defines migrants as those who move away from their place of usual residence, whether within a country or across an international border, temporarily or permanently, and for a variety of reasons [ 3 ]. The term includes those who migrate voluntarily as well as forced migrants, and encompass refugees, asylum-seekers, those with and without formal immigration status/documentation, and those who migrate for family or economic reasons. Importantly, there is no legally or universally accepted definition for ‘migrants’, as opposed to ‘refugees,’ who are defined in legal terms in the 1951 Refugee Convention [ 4 ]. In large part owing to the nature of the drivers of migration – which include fleeing from persecution, oppression, and violence – marginalized and vulnerable populations are overrepresented among migrants, including sexual and gender minorities, women, pregnant people, and unaccompanied children and adolescents.

Migrants face a broad array of healthcare needs and inequities, including those related to infectious diseases, sexual and reproductive health, chronic diseases, mental health, and violence. While there are many social and economic benefits of migration both among those who migrate and society, migrants face disproportionate exposure to social and structural factors that increase risk of adverse health outcomes in countries of origin, during transit, and in destination communities. These include conflict, trauma, violence, and other human rights violations; exclusionary migration policies; unsafe/inadequate shelter, food, and housing; racism, discrimination, and stigma; labour market exclusion; and exclusion and barriers to healthcare and other services [ 1 , 5 , 6 ]. In destination settings, health inequities may be compounded by language barriers, difficulties navigating health and social services, lack of social support, racism and discrimination, and socio-economic marginalization [ 1 , 5 , 7 ].

The COVID-19 pandemic shone a spotlight on these health inequities and access barriers faced by migrants, and has unfortunately exacerbated many of these concerns [ 7 ]. Migrants are overrepresented in precarious employment sectors, as well as those that were hardest-hit by COVID-19 in many places (e.g., agriculture, factories), and concurrently have consistently lower access to health services in most countries; [ 8 ] this has translated to increased prevalence and transmission of COVID-19 among migrants in many settings globally [ 9 ]. COVID-19 and other health inequities faced by migrants during the pandemic have been further compromised by administrative, financial, legal, and language barriers to accessing the health system, as well as shortages in medicines and healthcare facilities – quite apart from access to disease prevention and health promotion [ 8 ]. Despite all of these challenges and their structural origins, migrants are often vilified and stigmatized on the basis of misguided and xenophobic beliefs and biases that conflate migration with disease transmission – much of which is fostered by political strategies and rhetoric. A growing body of research recognizes the ways in which social and structural factors (e.g., violence, language barriers, criminalization of migrants, health insurance access, stigma, racism, and discrimination) underpin differences in the health of migrants vs. local-born populations. More research of this nature is needed to inform sustainable and pragmatic interventions that address these “upstream” determinants, particularly as it relates to the intersections between immigration policies, racism and discrimination, and health impacts.

In recent years, migration policies in a number of settings have become increasingly xenophobic and restrictive – for example, in the United States, both Republican and Democratic administrations have implemented draconian and restrictive laws aiming to prevent entry and processing of asylum-seekers, all of which are deeply antithetical to public health and human rights principles and commitments. Such policies are in stark contrast to commitments and aspirations laid out in key global policy instruments, including the Global Compact on Migration [ 10 ] and target 10.7 of the 2030 Agenda for Sustainable Development in which Member States committed to cooperate internationally to facilitate safe, orderly and regular migration. Within this policy climate, it is essential to produce research that evaluates the impact of such policies and practices on migrant health outcomes, access to care, and related human rights concerns.

Against this backdrop, the aim of the ‘Migration and health’ collection at BMC Public Health is to bring together research on the health inequities and care access faced by diverse migrant populations around the globe, as well as to feature research highlighting strategies for improving the health of migrants through promising evidence-based interventions. We encourage articles that advance our understanding of the myriad health needs faced by diverse migrant populations globally, including those related to infectious diseases, sexual and reproductive health, non-communicable diseases, mental health, and health care access. Contributions that consider changes in the health status, needs, and determinants of health among migrants across the stages of migration – from countries of origin to the transit stage and receiving communities – are especially encouraged, as are those that examine or identify evidence-based strategies to intervene upon the social and structural drivers of migrant health inequities.

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SG is partially supported by the U.S. NIH (R01 DA028648).

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Shira M Goldenberg

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Goldenberg, S.M., Fischer, F. Migration and health research: past, present, and future. BMC Public Health 23 , 1425 (2023). https://doi.org/10.1186/s12889-023-16363-7

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A theory of migration: the aspirations-capabilities framework

• Hein de Haas   ORCID: orcid.org/0000-0001-7593-6207 1  

Comparative Migration Studies volume  9 , Article number:  8 ( 2021 ) Cite this article

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This paper elaborates an aspirations–capabilities framework to advance our understanding of human mobility as an intrinsic part of broader processes of social change. In order to achieve a more meaningful understanding of agency and structure in migration processes, this framework conceptualises migration as a function of aspirations and capabilities to migrate within given sets of perceived geographical opportunity structures. It distinguishes between the instrumental (means-to-an-end) and intrinsic (directly wellbeing-affecting) dimensions of human mobility. This yields a vision in which moving and staying are seen as complementary manifestations of migratory agency and in which human mobility is defined as people’s capability to choose where to live, including the option to stay, rather than as the act of moving or migrating itself. Drawing on Berlin’s concepts of positive and negative liberty (as manifestations of the widely varying structural conditions under which migration occurs) this paper conceptualises how macro-structural change shapes people’s migratory aspirations and capabilities. The resulting framework helps to understand the complex and often counter-intuitive ways in which processes of social transformation and ‘development’ shape patterns of migration and enable us to integrate the analysis of almost all forms of migratory mobility within one meta-conceptual framework.

Introduction

Migration theory has been at an impasse for several decades (see Arango 2000 ; de Haas 2010a ; Massey et al. 1993 ; Massey  2019 ; Skeldon 2012 ). The field of migration studies has remained a surprisingly under-theorised field of social inquiry. This is unfortunate, as we can only develop a richer understanding of migration processes if we do not conceptually separate them from broader processes of social change of which they are a constituent part. Much thinking on migration remain implicitly or explicitly based on simplistic push–pull models or neo-classical individual income (or ‘utility’) maximising assumptions, despite their manifest inability to explain real-world patterns and processes of migration. Although prior migration theories have been rightfully criticised for their unrealistic assumptions, researchers have generally been better at debunking such theories than at coming up with viable theoretical alternatives.

To overcome this impasse and to advance our understanding of migration processes as an intrinsic part of broader processes of social change and ‘development’, this paper elaborates a theoretical framework that conceptualises migration as a function of people’s capabilities and aspirations to migrate within given sets of perceived geographical opportunity structures . Applying Sen’s ( 1999 ) capabilities framework to migration, this paper defines human mobility as people’s capability (freedom) to choose where to live – including the option to stay – instead of a more or less automated, passive and ‘cause-and-effect’ response to a set of static push and pull factors. The paper draws on Berlin’s ( 1969 ) concepts of positive and negative liberty to theorise the complex and non-linear ways in which macro-structural change can shape migration aspirations and capabilities, as well as to define new, theory-derived categories of human mobility and migration.

Migration theory: what is the problem?

Migration studies is an under-theorised field of social-scientific inquiry, in which the recent trend has been one of theoretical regression rather than progress. Earlier contributions to the field – such as Lee’s ( 1966 ) theory of migration, Mabogunje’s ( 1970 ) migration systems theory, Zelinsky’s ( 1971 ) mobility transition theory, Skeldon's ( 1990 ) work on migration transitions, Harris and Todaro’s ( 1970 ) neo-classical migration theory, Piore’s ( 1979 ) dual labour-market theory, Stark’s ( 1978 , 1991 ) new economics of labour migration and Massey’s ( 1990 ) cumulative causation theory – all tried to come up with generalised understandings of migration phenomena. With the exception of a few authors (Carling 2002 ; Faist 2000 ; Hatton and Williamson 1998 ; Skeldon 1997 ), in more recent decades the systematic theorisation of migration processes has been largely abandoned (see Skeldon 2012 ). In their seminal overview of migration theories, Massey and his colleagues (Massey et al. 1993 , p. 432) concluded that much thinking on migration ‘remains mired in nineteenth-century concepts, models, and assumptions’. Unfortunately, not much has changed since then.

This state of theoretical underdevelopment strongly contrasts with the huge increase in the number of empirical studies on migration. The lack of systematic theorising hampers our ability to meaningfully interpret empirical ‘facts’, to understand how macro-structural factors shape migration processes as well as to explain the huge diversity in migration experiences across different ethnic, gender, skill and class groups. Particularly since the rise of ‘postmodern’ social science in the 1970s and 1980s, big-picture migration theory-making has been largely abandoned. In reaction to the critique on ‘grand theory’ as well as the state-bias and ‘methodological nationalism’ (Wimmer and Glick Schiller 2002 ) inherent in much (policy-driven) migration research, recent work, particularly by anthropologists and sociologists, has focused on studying and conceptualising the (transnational, multicultural, diasporic, creolised) lives, identities and experiences of migrants from an ‘emic’ perspective.

Notwithstanding the considerable merits of such research, this has unfortunately coincided with an increasing gap between sociologists, anthropologists and also geographers conducting qualitative, interpretative micro-studies on migrants’ experiences on the one hand and the branches of economics, sociology and demography that have increasingly focused on quantitative regression analysis to examine the ‘causes’ and ‘impacts’ of migration largely along the (implicit or explicit) lines of the ‘push–pull’ model. While qualitative researchers often seem to have rejected the idea of explanatory migration theories altogether as naïvely positivist, the theoretical veneer of quantitative approaches has also remained extremely thin, as they do generally not go beyond an (often implicit) functionalist ‘push–pull’ perspective, according to which migrants are actors seeking to maximise income or ‘utility’. Both qualitative and quantitative approaches have failed to adequately capture the vital role of difficult-to-quantify structural factors such as inequality, power and states in shaping migration processes, or to develop a meaningful idea of human agency beyond the voluntaristic assumptions of neo-classical models or the portrayal of migrants as more or less passive victims of capitalist forces, as is common in historical-structural theories.

The ‘migration is too complex’ fallacy

The central problem in migration research is the absence of a central body of theories that summarises, generalises and systematises the accumulated insights of a vast amount of empirical research, that can serve as a common frame of reference within which to examine, interpret, understand and explain 'facts' and ‘findings’ from various disciplinary and paradigmatic perspectives, and that can guide future research. Several factors contribute to this lack of progress in our generalised understanding of migration, including:

the ‘receiving country bias’ and the concomitant ignorance of the causes, consequences and experiences of migration from an origin-area perspective, leading to one-sided, biased understandings of migration;

the dominance of government perspectives, ‘methodological nationalism’ (see Wimmer and Glick Schiller 2002 ) and the related tendency to uncritically adopt state categories to classify migrants and migration, which often sustain distorted, ideological views on migration;

disciplinary and methodological divides, particularly between quantitative (positivist) and qualitative (interpretative) approaches;

the divide between the study of ‘forced’ and that of ‘voluntary’ migration; and

the divide between the study of international and that of internal migration.

Researchers have frequently argued that a comprehensive or universal migration theory will never arise because migration is too complex and diverse a phenomenon (see Castles and Miller 2009 ; Salt 1987 ). However, this argument is not convincing for two main reasons. First, it would be misleading to suggest that the goal of social theory is to develop all-explaining, universal theories because social phenomena always need to be understood within the specific historical and social contexts in which they occur and can therefore never be captured by a simple set of formulas, ‘laws’, models or regression equations. Second, complexity can never be a reason to abandon efforts to build better social theories. After all, social phenomena are complex by nature and complexity has not stood in the way of theoretical advancement in other fields of social inquiry. In fact, we need to turn the argument upside down: the complexity which is so characteristic of social processes is the very reason why we need social theories, as they help us to make sense of and to discern patterns amidst the sometimes dazzling diversity – and the apparently random, chaotic and non-systemic nature – of human experiences and social interactions. In other words, social theories help us to see the wood for the (empirical) trees.

Importantly, the notion of complexity does not imply that social phenomena and social processes are chaotic or devoid of regularities, patterns or structure. Rather, complexity implies that they consist of many parts in elaborate, multi-layered arrangements. From a micro-perspective, the diversity of migration experiences may seem bewildering but, once we start to zoom out, regularities and patterns tend to emerge. This reflects the very purpose of social theory: to discern patterns in order to make sense of what is happening around us. For instance, as Ravenstein ( 1885 , particularly for the case of Britain) and Mabogunje ( 1970 , particularly for the case of Africa) have already shown, migration is anything but a random phenomenon. In different geographical and historical settings, they both observed that most migrants move along spatially clustered pathways between very particular communities in origin and destination areas. Similarly, at a macro level, Zelinsky ( 1971 ), Skeldon ( 1990 ) and Hatton and Williamson ( 1998 ) observed clear long-term regularities between demographic, economic and social transitions on the one hand and the sequenced emergence and decline of particular forms of internal and international human mobility on the other.

Paradigmatic classification of migration theories

Since the late-nineteenth century, various theories have emerged in various social-science disciplines which all aim at understanding the processes that drive migration. Such early migration theories can be clustered together into two main paradigms, following a more general division between ‘functionalist’ and ‘historical-structural’ social theory. Despite their various disciplinary origins, theories within each of these two main paradigms share basic assumptions about the nature of society and how society should be studied. For instance, neo-classical equilibrium models (from economics), push–pull models and migration systems theories (mainly from geography and demography) as well as dominant interpretations of migrant network theories (primarily from sociology) can all be situated within the functionalist paradigm of social theory, according to which migration is, by and large, an optimisation strategy of individuals or families making cost–benefit calculations.

Likewise, despite differences in nuance and level of analysis, neo-Marxist conflict theory, dependency theory (Frank 1966 ), world systems theory (Wallerstein 1974 , 1980 ), dual labour-market theory (Piore 1979 ) and critical globalisation theory (see Sassen 1991 ) have broadly similar interpretations of migration as being shaped by structural economic and power inequalities, both within and between societies, as well as the ways in which migration plays a key role in reproducing and reinforcing such inequalities. All these theories can be situated within the historical-structural  paradigm, also known as ‘conflict theory’, which focuses on how powerful elites oppress and exploit poor and vulnerable people, how capital seeks to recruit and exploit labour and how ideology and religion play a key role in justifying exploitation and injustice by making them appear as the normal and natural order of things.

More recent theories that focus on migrants’ everyday experiences, perceptions and identity – such as transnational (Vertovec 2009 ), diaspora (Cohen 1997 ; Safran 1991 ) and creolisation (Cohen 2007 ) theories – can all be situated within the symbolic interactionist perspective in social theory. We can perhaps distinguish a fourth, slightly more hybrid, group of meso-level theories that focus on the continuation or ‘internal dynamics’ (de Haas 2010b ) of migration, such as network theories, migration systems theory (Mabogunje 1970 ) and cumulative causation theory (Massey 1990 ). We can thus reduce what may initially appear as a rather dizzying theoretical complexity by combining existing disciplinary theories (ranging from economics to anthropology) under the conceptual umbrellas of the main social-theory paradigms. For the sake of brevity and because the goal of this paper is to advance an understanding of migration processes as part of broader social change, the following analysis will focus on the classic distinction between functionalist and historical-structural theories.

Limitations of functionalist and historical-structural theories

Functionalist social theory tends to see society as a system, a collection of interdependent parts (individuals, actors), somehow analogous to the functioning of an organism, in which an inherent tendency towards equilibrium exists. Functionalist migration theories generally see migration as a positive phenomenon contributing to productivity, prosperity and, eventually, greater equality in origin and destination societies through bidirectional flows of resources such as money, goods and knowledge. Essentially, they interpret migration as an optimisation strategy, in which individuals (and sometimes families or households) use migration to access higher and more-secure sources of income and other livelihood opportunities.

Neo-classical migration theory, as pioneered by Todaro ( 1969 ) and Harris and Todaro ( 1970 ), is the most prominent representative of functionalist migration theories but there are more theoretical currents that can be grouped under the functionalist migration paradigm. Push–pull models are basically a prototype version of neo-classical migration theories as they interpret migration as a function of income and other opportunity gaps between origin and destination areas. These functionalist models are all based on the explicit or implicit assumption that people make rational decisions in order to maximise income or ‘utility’. The only major exception on this rule seems to be the new economics of labour migration (NELM) pioneered by Stark ( 1978 , 1991 ), which conceptualises migration occurring in contexts of relative poverty and constraints as a household’s or family’s (instead of an individual’s) co-insurance strategy aimed at diversifying (instead of maximising) income through risk-spreading. Although it acknowledges the role of structural constraints in shaping migration decisions, NELM is also ultimately based on the assumption that households are rational actors engaging in a long-term economic optimisation strategy.

At the macro level, from a functionalist perspective, individual optimisation decisions are expected to contribute to a more optimal allocation of factors of production – primarily through the transfer of labour from poor to rich areas and countries and concomitant reverse flows of capital from rich to poor areas – which is expected to decrease economic gaps between origin and destination areas (de Haas 2010a ). However, such accounts typically ignore how poverty, inequality, immigration restrictions, government repression and violence can prevent people from migrating, cause their forced displacement or compel migrants into exploitative work conditions. This explains why the social and economic benefits of migration often accrue disproportionally to the already better-off in origin and destination societies – migrants and non-migrant ‘natives’ alike.

The central problem of functionalist migration theory is its reductionist character. The ‘push–pull’ reasoning on which these explanations are based strongly resonates with intuition but has proved to be inadequate and often plainly misleading in understanding real-world migration processes. Push–pull models are not able to explain migration  as a social process , as they tend to list a number of static factors that obviously play ‘some’ role in migration but without specifying their role and interactions or providing a structural account of the social processes driving population movements. Skeldon ( 1990 , pp. 125–126) therefore argued that push-pull models leave us with ‘a list of factors, all of which can clearly contribute to migration, but which lack a framework to bring them together in an explanatory system’, leading him to conclude that ‘the push–pull theory is but a platitude at best’.

If we reformulate the cornerstone functionalist assumption as ‘most people migrate in the expectation of finding better opportunities at the destination’, probably few would disagree. This assumption, that people basically have good reasons to move, however, is so general that it is of little use in explaining the geographically patterned and socially differentiated nature of migration processes. In other words: knowing what motivates individual people to move does not really help us to explain the processes, patterns and drivers of migration at the structural level.

The real questions are more complex and beg more complex answers. For instance, why do wealthier, more ‘developed’ societies tend to have higher levels of immigration and emigration than poor and ‘underdeveloped’ societies, while push–pull and neo-classical models would predict the contrary? How can we explain that most migration does not occur from the poorest to the richest societies? Why does ‘development’ in origin countries often lead to increased emigration propensities? And why do most people actually not migrate despite the existence of huge income and opportunity gaps within and between countries?

Functionalist migration theories have inherent difficulties explaining the socially and geographically differentiated nature of migration processes, in which structural inequality and discriminatory practices strongly favour the access of particular social groups and classes to attractive, legal migration opportunities, while excluding others by depriving them of rights or compelling them into exploitative situations. People’s ability to make independent migration choices is constrained by states and other structures such as family, community, networks and culture, which ultimately determine the social, economic and human resources which people are able and willing to deploy to migrate. At best, functionalist theories and push–pull models can incorporate such structural constraints as ‘market imperfections’ in theoretical models or cost-increasing factors in regression analyses. However, this exposes their inability to conceptualise how structural forces actively shape migration processes and often stand at the very origins of large-sale migration systems.

Colonialism, warfare, labour recruitment, migration policies, land dispossession, eviction due to infrastructure projects (e.g., road and dam construction) and cultural change are all examples of complex macro-structural change processes that cannot be reduced to ‘factors’ or ‘variables’ affecting migration costs. For instance, much large-scale migration has its origins in active efforts by states and employers to recruit foreign labour (see de Haas et al. 2020a ; Piore 1979 ). This shows the need to conceptualise the role of states, businesses, recruiters and various other migration intermediaries (see Agunias 2009 ) as well organisations (such as the UNHCR, IOM or humanitarian organisations (see Olayo-Méndez 2018 )) in actively shaping migration processes and creating entirely new migration patterns instead of reducing them to cost-increasing constraints in ‘naturally’ occurring migration processes. A full understanding of the role of such structural migration drivers therefore obviously defies their reduction to a few factors  in a mathematical theoretical model or variables in an empirical regression model.

Besides their general inability to conceptualise how structural factors and actors have actively shaped migration processes throughout history, functionalist theories have difficulties in explaining how, in the real world, migration can reinforce pre-existing inequalities. This upsets the underlying equilibrium assumption of functionalist theories, according to which social and market forces, if left to their own devices, would automatically tend towards equilibrium and in which (free) migration is thus expected to lead to income convergence and, eventually, less migration. Myrdal ( 1957 ) already argued that, without redistributive government intervention, socio-economic processes of ‘cumulative causation’ tend to reinforce inequalities between poor and rich areas, rather than the other way around. In the same vein, historical-structural theories argue that structures have, in fact, a tendency to reproduce or even reinforce inequalities, both ‘vertically’ between social groups (such as classes) and ‘horizontally’ across space (i.e., between peripheral rural areas and cities or between rich and poor countries).

For instance, in the poorest countries of Africa and Asia, legal migration opportunities to Europe, North America and other wealthy countries are mainly the prerogative of elite groups, who possess the right diplomas to qualify for a work or study visa and who have the financial means to pay for their migration. If it is mainly rather well-off groups who gain access to the most lucrative forms of legal migration, while others remain stuck in immobility or are relegated to exploitative forms of (often undocumented) migration, this is likely to sustain or even deepen inequalities within origin societies. As Van Hear ( 2014 ) argued, migration and its outcomes are “shaped by the resources that would-be migrants can muster and that in turn the capacity to mobilise such resources is largely determined by socio-economic background or class” (Van Hear 2014 , p. 100).

Such arguments resonate with neo-Marxist political economy and historical-structural theories that emphasise how social, economic, cultural and political structures constrain and direct the behaviour of people in ways that do not generally create greater equilibrium but, rather, reinforce such inequalities. These theories emphasise the role of businesses – and states representing their interests – in shaping migration and see labour migrants, both forced and voluntary, as providing a cheap, exploitable labour force, mainly serving the interests of wealthy groups, areas and countries (de Haas et al. 2020a ; Piore 1979 ). These theories emphasize that economic and political power is unequally distributed and that cultural beliefs (such as religion and tradition) as well as social practices serve to justify and reproduce such inequalities. From this perspective, it is thus the already privileged in destination societies, such as employers and capital-owning elite groups more in general, who mainly benefit from migration. This is reflected in migration rules and political discourses that favour and praise the skilled and wealthy and disdain or vilify the less-skilled, vulnerable, ethnically different and poor migrants and often relegate them to be exploited in the informal sector. From this perspective, the real aim of anti-immigration policies is not to stop immigration but to create an appearance of control (see Massey et al. 1998 , p. 288), with the associated anti-immigrant political discourses serving to justify the economic exploitation of vulnerable migrant groups and to blame migrants for problems not of their own making.

However, the central problem of such historical-structural views is that they leave hardly any room for human agency . They tend to depict migrants as pawns – pushed and pulled around by global macro forces – or as victims of capitalism who have no choice but to migrate in order to survive. Views of migration as a ‘desperate flight from misery’, or that portray migrants as passive victims of smugglers and traffickers, do no justice to the fact that the vast majority of migrants move of their own free will. Indeed, a large body of research evidence shows that most migrants succeed in significantly improving their livelihoods through internal and international migration (de Haas et al. 2020a ; Massey et al. 1998 ; UNDP 2009 ). In addition, juxtaposing mainstream narratives depicting smugglers as ‘unscrupulous and ruthless criminal gangs preying on vulnerable and desperate migrants’, the lived experiences of migrants expose a much more nuanced reality, with smugglers generally functioning as migration facilitators who can be close friends, acquaintances or more distant service deliverers (Zhang et al. 2018 , p. 6).

Historical-structural views are often based on underlying assumptions that much 'South-North' migration is a largely irrational process that would  often not be in the interests of migrants themselves, as they would be blinded by over-optimistic mirages about life abroad and deceived by untrustworthy recruiters, smugglers and traffickers. This assumption is also reproduced in official discourses and policies according to which prospective migrants should be educated about the risks and costs of migration through information campaigns. This clearly denies the fact that, even for less-skilled or undocumented migrants, migration still has huge potential to improve the long-term wellbeing of themselves and their families and that they are therefore willing to endure situations of exploitation and suffering, however unjustified these may be from a moral and ethical point of view.

Towards a more meaningful understanding of migratory agency

At first sight, functionalist and historical-structural accounts of migration seem diametrically opposed in their understanding of migration, in terms both of its social causes and of its consequences for destination and origin areas. However, what both paradigms have in common is a general inability to provide a meaningful understanding of human agency through their portrayal of migrants either as rather soulless individual utility-optimisers or as rather passive victims of global capitalist forces. Numerous studies have highlighted the limited but real ability of migrants to defy government restrictions, discrimination and xenophobia by migrating over closed borders, by buying in the services of recruiters, lawyers, smugglers and various other migration intermediaries, and by forging networks and new identities as well as establishing communities and their own economic structures in destination societies (Agunias 2009 ; De Haan et al. 2000 ; de Haas 2010b ; Stark 1991 ; Zhang et al. 2018 ). It would therefore be just as unrealistic to depict migrants as victims desperately fleeing situations of destitution, opression and human misery as it would be to depict them as entirely rational and free actors who constantly make rational cost–benefit calculations. This shows that neither functionalist nor historical-structural theories provide realistic accounts of migratory agency. The central challenge in advancing migration theory is therefore the elaboration of conceptual tools that improve our ability to simultaneously account for structure and agency in understanding processes and experiences of migration, without discarding the important insights which both functionalist and historial-structural paradigms offer and thus rejecting them altogether.

Theoretical exclusivism versus conceptual eclecticism

While the assumptions of neither functionalist nor historical-structural theories have universal value, both sets of explanations can nevertheless be useful in developing a richer, nuanced and contextualised understanding of migration processes. After all, functionalist theories may have greater explanatory value for some types of relatively unconstrainted forms of human mobility – such as much internal and high-skilled migration – while historical-structural theories may have greater explanatory value for forms of migration where government restrictions, exploitation and involuntariness play a more important role. Migration can be a very empowering experience but can, in other cases, take more exploitative forms.

Thus, instead of rejecting either set of explanations, insights from both paradigms need to be incorporated in a new, overarching theoretical paradigm on migration that can unite them. Instead of seeing theories as exclusive truth claims, we need a vision in which the validity of theoretical assumptions is contingent on the specific conditions under which migration occurs, the specific social and class groups concerned, as well as on levels of analysis. This implies that both the functionalist and historical-structural paradigms can have explanatory power and relevance and can, therefore, to a certain extent, be combined and integrated in a wider meta-theoretical framework which is able to simultaneously incorporate agency and structure in explaining migration and which acknowledges that the vast majority of migrants face some level of constraint yet also have some level of choice.

The way forward is therefore not to develop entirely new theories but to find concepts and analytical tools that help us to build upon and bridge insights provided by existing theories – not only within but also across paradigms. This implies a rejection of the somewhat common idea that we cannot combine social theories that are based on conflicting paradigmatic assumptions, particularly if we combine explanations at different levels of analysis.

A simple example may serve to illustrate the relevance of insights from both functionalist and historical-structural migration theories. From a macro-level perspective, some forms of migration seem rather exploitative – such as undocumented migration from Mexico to the US, Morocco to Spain, Mali to Côte d’Ivoire, Myanmar to Thailand or Indonesia to Malaysia – where uncertain legal status or ‘illegality’ enables employers to hire and fire migrant workers as they please and to pay low wages. At the macro level, such ‘exploitative’ forms of migration can exacerbate economic gaps between origin and destination areas by supplying cheap labour and boosting profits and income growth in destination areas for which reverse resource flows to origin areas such as remittances cannot compensate. Unequal terms of trade, higher productivity and economics of scale can lead to a further concentration of economic activities in wealthy destination countries along with the sustained migration of workers from poor countries to support them (see Martin and Taylor 1996 ). At the micro level, however, it may still make sense for people to migrate if this increases family income significantly and enables them to build a house, afford health care, send their children to school or start a small enterprise. The second insight does not prove the first wrong – vice versa.

This resonates with the argument of Massey and his colleagues (Massey et al. 1993 , p. 432), who stated that there is considerable scope to combine insights from different theories:

A full understanding of contemporary migration processes will not be achieved by relying on the tools of one discipline alone or by focusing on a single level of analysis. Rather, their complex, multifaceted nature requires a sophisticated theory that incorporates a variety of perspectives, levels and assumptions.

Some migration researchers have countered this idea. For instance, Bakewell ( 2010 , p. 1692) argued that Massey et al.’s ( 1993 ) claim that there are no necessary, inherent contradictions between different theories is hard to sustain ‘when one considers very different ontological and epistemological foundations of migration theories’. This is essentially is a Kuhnian argument on the incommensurability of scientific paradigms. As Kuhn ( 1962 ) argued in The Structure of Scientific Revolutions , the proponents of different paradigms live in different worlds and use different vocabularies and criteria determining the legitimacy of both problems and proposed solutions in terms of methodology and analysis. Each paradigm therefore has the tendency to satisfy the criteria it sets for itself and to reject the problem definition as well as evaluation criteria used by other paradigms (Kuhn 1962 , p. 109). Importantly, such a Kuhnian view precludes combination or comparison across scientific paradigms, as adherents of different paradigms seem to be living in largely self-contained ‘truth bubbles’.

Kuhn’s view is extremely valuable for understanding the limited communication and the lack of recognition and appreciation among social scientists conducting research across paradigmatic, methodological and disciplinary divides. Yet there is also good reason to question the full applicability of the Kuhnian incommensurability principle to the social sciences (see also Urry 1973 ). Importantly, Kuhn based his argument on an analysis of the history of the natural sciences. In contrast to natural sciences, social theories typically have no universal bearing but are specific to particular historical and geographical contexts. The positivist universality claims of most natural-science theories can therefore not simply be extended to the social sciences, where theories need to be contextualised and historicised in order to make them meaningful and to be clearer about their specific applicability. Hence, in the social sciences, theories and paradigms do not need to be mutually exclusive a priori. Rather, they offer different explanations of social phenomena which can frequently be combined, particularly if they apply to different historical or social contexts, social groups or levels of analysis, or if they look at the same social phenomenon from different thematic, disciplinary and methodological angles.

It is therefore also dangerous to blindly apply the Popperian falsification principle to the analysis of the social world. For instance, if an empirical analysis conducted within a particular context shows that neo-classical ‘predictor variables’ such as wage differences do not have a significant effect on this particular form of migration, this still does not provide sufficient evidence to reject the theory as a whole. It may simply mean that a particular theory may have little or no explanatory power in that particular context. This does not provide a licence for a sloppy practice of ‘anything goes’ ad-hoc theorising but, rather, makes the case for more analytical precision in carefully assessing the applicability of particular theories in particular settings and at particular levels of analysis. Because theories have often been formulated to explain specific forms of migration occurring in particular geographical and historical contexts, a greater awareness of the history of theories is essential if we are to understand their particular claims and applicability.

Instead of ‘rejecting’ or ‘confirming’ hypotheses and theories, social analyses would gain interpretative depth and theoretical relevance if they indicate which contextual factors may explain certain expected or unexpected empirical observations. One single experiment defying Newton’s law of universal gravitation should indeed suffice to reject his theory. However, in contrast to the natural sciences, in social sciences it is often not about one theory being ‘right’ or ‘wrong’, let alone about testing the validity of an entire theory by plugging in a predictor variable ‘representing’ the theory in a regression model. In this context, Garip ( 2012 , p. 425) rightly argued that migration researchers tend to ‘reduce theories to competing sets of independent variables ... [which] inevitably leads to either/or theoretical stances, rather than an emphasis on the complementarity of different theories’. Instead of stubbornly adhering to theoretical exclusivism, social theory-building should therefore be an inherently eclectic affair, in which an openness should exist to potentially combine different theoretical perspectives as part of an effort to develop more comprehensive, nuanced and, therefore, more realistic conceptual frameworks.

To summarise, migration theories can potentially be combined across five analytical dimensions:

At different levels of analysis : macro-, meso- and micro-level explanations of migration may require different conceptual tools. For instance, forms of exploitative labour migration that seem to fit within the neo-Marxist paradigm can still be rational for migrants and their families.

In different (geographical, regional, national) contexts . For instance, functionalist neo-classical theories may work better to explain relatively unconstrained migration in wealthier countries, while historical-structural approaches may be more useful to explain migration within and from poor or ‘developing’ countries or occurring under conditions of oppression and violence.

Across different social groups : even at the same point in time and in the same geographical and national context, migration is a socially differentiated process; different theories are therefore likely to have varying degrees of applicability to different occupational, skill, income, class or ethnic groups. For instance, neo-classical assumptions may hold relatively well in explaining the migration of higher-skilled migrants, whereas neo-Marxist theories may be more useful in understanding the migration of less-skilled and relatively poor manual workers.

At different points of time . The drivers and internal dynamics of migration processes often change over time and over the various trajectories and successive stages of migration system formation and decline (de Haas 2010b ); so, too, therefore, do the social, cultural and economic mechanisms explaining such migration. For instance, Garip ( 2012 ) identified four distinct types of Mexico–US migrants over the 1970–2000 period and argued that these types gained prevalence during specific time periods depending on the changing conditions in the two countries.

From different thematic or disciplinary perspectives . We can look at the same manifestation of migration from various analytical perspectives. For instance, we can study how social transformation processes shape migration processes simultaneously from cultural, political, economic, technological and demographic perspectives as well as through the use of various methodologies and data. This provides different – and generally complementary – angles from which to study and explain the same social process (see de Haas et al. 2020b )

This highlights the considerable potential to combine different theories to improve our understanding of migration processes across different levels of analysis (and aggregation), contexts, social groups and periods. In this way, disciplines, theories and paradigms become interpretative frameworks that reflect a particular way of viewing the world or dominating certain societies or periods. Such varying perspectives can be complementary (when they stress different dimensions of the same phenomenon) or may seem conflicting (when their fundamental assumptions clash) – although what initially appears to be a clash of assumptions may partly reflect their applicability to different contexts, social groups and levels of analyses. This shows the danger of buying into one particular train of thought, in which theories can easily devolve into intellectual straightjackets rather than conceptual toolboxes. This should compel us to achieve a deeper understanding of the concrete historical, geographical and social contexts in which migration occurs. It also highlights the need to break with bad habits of disciplinary and methodological parochialism.

Migration as an intrinsic part of broader social change

A first essential step in our quest to achieve an more comprehensive theoretical understanding of migration is to connect migration theories to general social-scientific theories. This reflects the need to (re) conceptualise migration as an intrinsic part of broader processes of economic, political, cultural, technological and demographic change embodied in concepts such as social transformation, ‘development’ and globalisation. Footnote 1 This is in opposition to more conventional scientific views which portray migration as either a response to development disequilibria or a function of static ‘push’ and ‘pull’ factors as well as policy views that portray migration either as a ‘problem to be solved’ or, conversely, as a solution to problems (such as population ageing). However, migration is a social process that cannot be seen in isolation from the broader processes of change of which it is a constituent part.

Urbanisation is perhaps the best example to illustrate this fundamental point. Since the onset of the industrial revolution, migration and urbanisation have been intrinsically intertwined processes which can therefore not be conceptualised separately. It is as unconceivable to understand modern urbanisation processes without understanding rural-to-urban migration as it is to understand rural-to-urban migration without understanding urbanisation processes. This exposes the flawed assumptions underlying attempts by governments to curb rural-to-urban migration through rural development programmes (see Rhoda 1983 ), as such policies cannot stop broader processes of social transformation and capitalist expansion which inevitably undermine traditional agrarian livelihoods, encourage the growth of the urban sector and irrevocably change ideas of the ‘good life’ amongst new generations towards more urban lifestyles (see Mabogunje 1970 ; Schewel 2019 ).

While broader processes of social change shape migration, through its social, economic, cultural, demographic and political impacts, to some extent migration also affects these processes in its own right. For instance, remittances can increase income inequality and relative deprivation in origin communities and therefore further increase emigration aspirations, while large-scale immigration can affect the structure and segmentation of labour markets in destination countries (Massey 1990 ; de Haas et al. 2020a ). Although this relationship is reciprocal, it also tends to be a highly asymmetrical one, because migration is generally unlikely to affect the deep structures of origin and destination societies unless it takes on truly massive proportions (see Portes 2010 ) or colonisers subjugate native populations through military force. This reciprocal but asymmetric relation between migration and broader social change is depicted in Fig.  1 .

Prior research has shown that social transformation and economic development often shape migration in complex and often quite counter-intuitive ways that reveal the inadequacy of conventional migration theories. In low-income societies, marginal increases in income, improving education, infrastructure expansion, urbanisation and concomitant transformations from largely agrarian to industrial and service-based economic systems are generally associated with increasing levels of both internal and international migration. In other words, development initially leads to more migration. Only in the longer term, when societies become wealthier and predominantly urban, does emigration tend to decrease and immigration to increase, after which societies transition from being net emigration to net immigration countries. Such mobility or migration transitions were first hypothesised by Zelinsky ( 1971 ) and further elaborated on by Skeldon ( 1990 ,  1997 ). In 2010, I  elaborated a theoretical explanation of the social mechanisms underpinning the occurrence of migration transitions and, drawing on new data, I performed a first global test of the relation between levels of development and levels of immigration and emigration (de Haas 2010c ). These empirical analyses confirmed migration transition theory and were further validated by subsequent studies (Clemens 2014 , 2020 ; de Haas and Fransen 2018 ). Historical studies, such as of the European emigrations to the ‘New World’ between 1850 and 1914 (Hatton and Williamson 1998 ), also seem to confirm the transition model.

This non-linear relationship between development and migration levels clearly challenges functionalist, and historical-structural migration theories as well as push–pull models, which all implicitly or explicitly assume that the reduction of poverty and economic gaps will reduce migration. More in general, these insights highlight the need to conceptualize migration as a normal social process. As long as societies change (which they always do), social stratifications persist (which is equally likely) and people go through life stages (which is inevitable), people will keep on migrating – and settling. Societies are in constant mutation and migration should therefore be seen as a normal process, instead of being normatively cast as an undesirable or desirable process (in public debates), as the ‘antithesis’ of development (as in historical-structural accounts) or as a largely temporary response to development disequilibria (as in neo-classical accounts). The relevant theoretical question is therefore not ‘why people move’ (which tends to yield overly generic and rather meaningless platitudes of the ‘push–pull’ genre) but, rather, how patterns and experiences of migration are shaped by broader processes of social change.

Conceptualising structure and agency in migration processes

The main conceptual problem of conventional theoretical accounts of migration remains their inability to meaningfully conceptualise how individual migrants and groups of migrants exert agency within broader structural constraints. Because of their centrality to our analysis and because of frequent confusion around their meaning, it is important to define the key terms of agency and structure. Agency reflects the limited – but real – ability of human beings (or social groups) to make independent choices and to impose these on the world and, hence, to alter the structures that shape and constrain people’s opportunities or freedoms. Structure can be defined as patterns of social relations, beliefs and behaviour. Factors and institutions such as class, religion, gender, ethnicity, networks and markets as well as cultural belief systems all sustain inequalities and social hierarchies and limit the opportunities that people have – or perceive they have – and the economic, social and cultural resources which they can access – thus significantly constraining their freedom or agency as well as their ideas, knowledge and self-consciousness.

As mentioned above, historical-structural theories tend to portray migrants as relatively passive actors or victims who are pushed around the globe by the macro-forces of global capitalism. Because they focus on the behaviour of actors (individual migrants) and may therefore come across as more ‘agentic’ at first sight, functionalist theories basically argue the same and do not ascribe much, if any, real agency – and therefore power – to migrants. Push–pull and neo-classical gravity models (the latter borrowed from the natural sciences) basically assume that people will migrate if the benefits of migration exceed the costs. This reflects an implicit assumption that people are motivated by individual cost-benefit calculations aimed at income or utility maximisation and will therefore react in automatic, universal and predictable ways to external stimuli or ‘push’ and ‘pull’ factors. As already embedded in the very semantics of the ‘push’ and ‘pull’ terminology, which we therefore need to reject, this reduces people to objects who operate in a social vacuum and who lack a will, perceptions and preferences of their own as well as the ability to actively and subjectively choose between different options. Footnote 2 Functionalist theories and actor-focused empirical applications such as ‘agent-based modelling’, should, however insightful they can be, therefore not be misidentified as ‘agentic’ theories.

Conventional migration theories tend to ignore five vital issues with regard to migratory agency. First, people’s access to economic (material), social (other people), cultural (ideas, knowledge and skills) and bodily (good health, physical condition and habitus) resources shapes their ability to move (or, conversely, their ability to stay), their preferences and aspirations (to stay or to go), their choice in terms of destinations and their ability to obtain work, housing, education and legal status while protecting themselves against abuse and exploitation. Because of social hierarchies and structural inequality, such access to migratory resources tends to be unequally distributed within and across communities and societies.

Second, people’s perceptions of the ‘good life’ and, hence, their life aspirations, vary hugely across different social and cultural contexts. In addition, such aspirations are anything but fixed and tend to change as people move through their life course and as societies change. Depending on people’s subjective life aspirations as well as their (equally) subjective perceptions of opportunities ‘here’ and ‘there’, they may – or may not – develop a desire to migrate. It is therefore unrealistic to simply assume that dissimilar social groups will develop similar aspirations and tendencies to migrate even when exposed to similar set of external factors or stimuli or ‘push’ and ‘pull’ factors.

Third, people do not uniquely migrate out of an instrumental ‘means-to-an-end’ desire to achieve aspired levels of wealth or living standards but may also value migration for more intrinsic reasons – such as wanderlust, curiosity and an innate desire to break free and discover new horizons. This means that not only the ‘functional’ but also the intrinsic, subjective value which people ascribe to mobility should be given a serious place in migration theory. Across societies and throughout history, particularly young people have often harboured a strong desire to leave home – at least temporarily – for a variety of reasons, from the socio-psychological need to separate from their parents, proving their independence and as a rite de passage marking their transition to adulthood. ‘Gap years’ and working holidays are not necessarily a unique prerogative of privileged Western youth but can also be seen as a modern manifestation of a more universal intrinsic desire of many young people to move and discover the world – before settling down. Berriane et al. ( 2013 ), for instance, observed that quite a number of sub-Saharan migrants in Morocco frame their journey in terms of ‘adventure’ and the desire to try out life elsewhere and should not, therefore, be automatically crunched into the stereotypical categories of marginalised or ‘desperate’ economic migrants or refugees.

Fourth, conventional migration theories fail to incorporate mobility and immobility in the same conceptual framework. This is necessary because movement is as much the norm as is sedentary life – and many people experience both over their lifetime. Modern sedentary lifestyles assume residency – but changing residency requires migration. This implies that we need to embed our understanding of migration within broader theoretical frameworks that include non-migratory mobility and people’s desire to move as well as to stay put and have a place ‘to live’.

Last but not least, it is important to bridge the dichotomous divide between the study of voluntary and that of forced migration. While virtually all migrants face some level of constraint, ‘forced migrants’ also have some level of agency as, otherwise, they would not be able to move in the first place. Refugees exercise their agency as far as possible in even the face of appalling circumstances. It is only under extreme conditions such as slavery and deportation that agency may be discounted largely or completely (see de Haas 2009 ). Conversely, most migrants normally cast as ‘voluntary’ face considerable constraints. For instance, many migrants who primarily move for work do so because they face severe constraints on personal development at home and the range of migration options available to them tends to be limited by economic, political and social constraints.

The fact that all migrants face constraints challenges the conventional dichotomy between ‘forced’ and ‘voluntary’ migration. It appears, therefore, more appropriate to conceive of a continuum running from low to high constraints under which migration occurs, rather than applying a dichotomous classification of forced versus voluntary migration to much more complex realities in which all people deal with structural constraints, although to highly varying degrees. In this way, reductionist, dichotomous classifications between forced and voluntary migration (see also Richmond 1988 ) can be overcome or nuanced, in ways that enable us to integrate virtually all forms of migration into one overarching meta-conceptual framework.

Addressing these five challenges requires the elaboration of new concepts of human mobility that can simultaneously account for both agency and structure. However, this is easier said than done, and the crucial question is how to do this in practice . To achieve this, the following sections will argue how a meta-theoretical conceptualisation of migration as a function of aspirations and capabilities to move (1) expands the theoretical concept of human mobility to include movement and non-movement, (2) improves our ability to develop a richer and more realistic understanding of the ways in which macro-level change affects people’s migratory agency and (3) enables us to elaborate new, theory-derived migration and mobility categories.

Migration as a function of capabilities and aspirations

In 2002, Jørgen Carling published a seminal paper  exploring the role of aspirations and ‘abilities’ in migration processes. He introduced the concept of ‘involuntary immobility’ to describe the phenomenon of the growing numbers of people living in Cape Verde (and poorer countries more generally) who wish, but do not have the ability, to migrate (Carling 2002 ). Analysing the case of wartime migration in Mozambique, Lubkemann ( 2008 ) also applied the concept of involuntary immobility to argue that the usual conflation of migration with displacement conceals a large category of people who suffer from ‘displacement in place’ through ‘involuntary immobilisation’ because warfare trapped them in the places they wanted to leave. While this concept has been mainly applied to origin societies, involuntary immobility can also be used to describe situations in places of destination when aspiring return migrants cannot go back because of a lack of resources, border controls or adverse conditions in origin countries. It may also describe situations in which migrants ‘in transit’ are immobilised if they become ‘stuck’ as a consequence of a lack of resources, violence, border controls or a combination thereof (see Collyer et al. 2012 ).

The systematic distinction between the ability (or capability) and the aspiration to migrate allows for richer, nuanced and more realistic migration categorisations. This also resonated with my own fieldwork in south-Moroccan oases between 1993 and 1994 (de Haas 1995 , 1998 ) and between 1998 and 2000 (de Haas 2003 , 2006 ), which inspired me to develop alternative ways of theorising migration, because conventional migration theories struck me as somewhat useless in explaining the migration dynamics I observed. Particularly during my fieldwork in the south-Moroccan Todgha valley (de Haas 2003 , 2006 ), I was confronted with the following puzzle: despite significant increases in income and general living conditions over previous decades, out-migration from the Todgha valley to big cities in Morocco and, particularly, European countries like France, the Netherlands and Spain had continued unabated. This did not fit at all within neo-classical migration theories and push–pull models, which would have predicted decreasing emigration as a consequence of improved local living standards.

This inspired me to adopt the concepts of aspirations and capabilities as theoretical tools enabling me to better understand what I was observing (de Haas 2003 , 2006 , 2014b ). I argued that, although local living conditions had improved significantly in preceding decades, people’s general life aspirations had increased faster, leading to growing migration aspirations. Improved education, increased media exposure alongside the regular return of the migrant ‘role models’ and exposure to their relative wealth had all contributed to rapidly increasing material and changing social aspirations of people living in the valley. Particularly international migration had become so strongly associated with material and social success that many youngsters had become virtually obsessed with leaving. This ‘culture of migration’ also contributed to rapidly changing ideas of the ‘good life’ and an increasing disaffection with traditional, agrarian lifestyles. So, growing aspirations and capabilities to migrate had inspired and enabled increasing numbers of people to leave the valley despite, or paradoxically rather because of , significant improvements in local living standards, income and education.

The core argument of this paper  is that the fragmented insights from different disciplinary theories can be integrated into a single meta-theoretical framework through conceptualising virtually all forms of migration as a function of aspirations and capabilities to migrate within given sets of perceived geographical opportunity structures , in which

Migration aspirations are a function of people’s general life aspirations and perceived geographical opportunity structures.

Migration capabilities are contingent on positive (‘freedom to’) and negative (‘freedom from’) liberties.

The concept of migration aspirations expands the notion of migratory agency into the subjective realm. This addresses the central shortcoming of functionalist and historical-structural theories, which implicitly assume that people respond to external ‘stimuli’ in quite uniform – and therefore predictable – ways. In this view, migration aspirations reflect people’s general life preferences as well as their subjective perceptions about opportunities and life elsewhere. Both general life and more specific migration aspirations are thus affected by culture, education, personal disposition, identification, information and the images to which people are exposed.

Aspirations are conceptually distinct, although empirically not independent from, capabilities. A good example is education in rural areas, which expands not only people’s skills and knowledge but also people’s awareness of alternative, consumerist, urban or foreign lifestyles. This often changes people’s notions of the ‘good life’ and they may subsequently start to aspire to migrate, partly independently from ‘objective’ material conditions at home. However, education may also increase aspirations in a different way, because it may prompt teenagers and young adults to start thinking that these new material and cultural lifestyles are actually within their reach – reflecting the notion of the ‘capacity to aspire’ (Appadurai 2004 ; see also Czaika and Vothknecht 2012 ). In this way, increasing capabilities can increase aspirations. Generally, preferences tend to change and material and consumerist aspirations tend to increase along with broader processes of social transformation usually associated with capitalist development and modernisation (see de Haas et al. 2020b ). However, the extent to which changing preferences translate into migration aspirations depends on the degree to which people perceive that their subjective needs and desires can be fulfilled locally. However, in general, increased access to new ideas through education and the media tends to change people’s ideas about the ‘good life’ in such a way that it increases their desire to explore new horizons and move out of rural places towards towns and cities or foreign lands.

The vital intrinsic dimension of migration aspirations

It is essential to distinguish the (1) instrumental and (2) intrinsic dimensions of migration aspirations. ‘Gap years’ and ‘lifestyle migration’ can be examples of the latter, while labour and student migration are examples of the former although, in practice, intrinsic and instrumental aspirations may occur simultaneously and often reinforce each other. Instrumental aspirations have received the most attention in research and are related to migration as a ‘functional’ or ‘utilitarian’ means to achieve another end, such as a higher income, higher social status, better health care, better education or, in the case of refugees, protection from persecution and violence. Intrinsic aspirations refer to the value which people may attach to the migration experience in and of itself, such as the joy and pleasure derived from exploring new societies, seeing the ‘bright lights’ of the city (Harris and Todaro 1970 , p. 126), or experiencing the social prestige linked to proving oneself and enduring the suffering and taking the risks often associated with migration – to be subsequently seen as a ‘man (or woman) of the world’ or the social status, recognition and respect that usually comes with the ability to provide for the family.

People can also derive wellbeing from having potential  access to mobility freedom, irrespective of whether people use these freedoms or not . The central idea is that the very awareness of having the freedom to move and migrate can add to people’s life satisfaction, in the same way that freedom of speech and religion, the right to organise protest marches or to run for office can contribute to people’s wellbeing, irrespective of whether or not they eventually use those freedoms. Conversely, if people do not enjoy such freedom, they are likely to experience this as a form of wellbeing-decreasing deprivation. For instance, many young Moroccans describe their country as a ‘prison’ because of European migration restrictions. In Carling’s ( 2002 ) terms, they feel stuck in involuntary immobility. This does not mean that they will all migrate if given the opportunity but the feeling of deprivation is real.

So, border walls or other migration restrictions might actually fuel the desire to get to the other side by creating an obsession with ‘getting out’ as soon as the opportunity presents itself, while full mobility rights might paradoxically decrease such aspirations. Before Spain introduced travel visas for Moroccans in 1991, it was common for young Moroccan adults to spend a few (summer) months or years in Spain, often with mixed motives of tourism, pleasure and work. The introduction of visa requirements largely cut off such free circulation, created a market for smuggling and increased the tendency to stay longer for those who still managed to get in – which encouraged the increasingly permanent settlement of Moroccans in Spain (de Haas 2014a ). While numerous empirical studies have indicated that most people would prefer to stay home and many migrants wish to return to their countries of origin, the irony is that the very deprivation of mobility freedom or the expectation of the future tightening of migration regimes may actually encourage non-migrants to get moving (before it is too late) and for migrants to cancel return plans (out of fear of not being able to migrate again).

This is what Vezzoli ( 2015 ) observed in her comparison of migration patterns from Suriname, Guyana and French Guyana, three neighbouring countries located in South America. Guyana and Suriname became independent from Britain and the Netherlands in 1966 and 1975, respectively. French Guyana is a French département and, as a consequence, its inhabitants are full French citizens. The paradox is that more than half of the population of Guyana and Suriname live abroad despite – or paradoxically partly because – migration restrictions imposed by their former colonisers and other destination countries, as this has contributed to an obsession with ‘getting out’. In contrast, the French Guyanese tend to have a more relaxed attitude towards emigration because, as French citizens, they have full mobility rights. Apart from better social security and living conditions, this partly explains why emigration has remained at quite low levels (Vezzoli 2015 ).

Life aspirations can thus often include mobility freedom as an intrinsically valuable and wellbeing-enhancing right, experience and awareness. This intrinsic dimension of migration aspirations is not taken seriously in the predominantly ‘functionalist’ migration literature, or it is set apart as an entirely and essentially different category of migration – e.g., ‘lifestyle migration’ (Benson and O’Reilly 2009 ). We do know from empirical research, however, that intrinsic ‘adventure’ and ‘lifestyle’ motives are not the prerogative of privileged Europeans or North Americans but can also be common among other migrant groups, such as undocumented migrants in England (Bloch et al. 2011 ) or African migrants crossing the Sahara (see Berriane et al. 2013 ; Bredeloup 2008 ; Pian 2009 ). This highlights the need to put the intrinsic dimensions of mobility freedom centre stage when theorising migration.

Migration as freedom

Although not developed to analyse migration, Amartya Sen’s ( 1999 ) capabilities approach, which he proposed to reconceptualise ‘development’, provides useful conceptual tools that can also be fruitfully applied to analysing migration , as it helps us to simultaneously grasp the instrumental and intrinsic dimensions of migration capabilities and aspirations as well as to conceptualise how migration is an intrinsic part of broader development and change. Based on his critique of narrow, income-focused definitions of development, Sen ( 1999 ) conceptualised development as the process of expanding the substantive freedoms that people enjoy. He operationalised this through the concept of human capability , which he defined as the ability of human beings to lead lives they have reason to value and to enhance the substantive choices they have. Sen  argued that income growth itself should not be the litmus test for development theorists but the question whether the capabilities (or freedoms) of people to control their own lives have expanded. Sen posited that freedom is central to the process of development for two reasons. First, there is the intrinsic importance of human freedoms in directly adding to people’s quality of life, which has to be distinguished from the second, instrumental , value of freedoms in also contributing to human and economic progress (Sen 1999 ).

I initially applied Sen’s capabilities approach to the study of migration to evaluate the development impacts of migration and remittances in origin communities, not only in terms of income, but also in terms of wellbeing-enhancing improvements in living standards. Yet I disovered that the capabilities approach was also a valuable concept to understand how, conversely, processes of social transformation and development shape migration (de Haas 2003 , 2006 , 2010a ). Changes in economic, social, cultural and political conditions in origin areas may affect migration propensities in two different ways. First, economic growth and improvements in living standards are likely to increase people’s migration capabilities by increasing their ability to assume the costs and risks of migrating. Second, the extent to which local opportunities allow people to lead the lives they have reason to value (which reflects Sen’s definition of development) at home is also likely to affect their migration aspirations.

Thus, by applying Sen’s capabilities approach to migration, we can learn to see migration not only as an instrumental-functional means-to-an-end to improve people’s living conditions but also as a potentially wellbeing-enhancing factor in its own right. This alludes to the intrinsic, wellbeing-enhancing (non-instrumental and non-utilitarian) dimension of migration and this, at a philosophical level, also expands our understanding of mobility not so much as the act or capability of moving but as the ability to decide where to live, including the option to stay at home . Based on this definition, people may enjoy mobility freedoms without ever using them, while migration can only be seen as genuinely wellbeing-enhancing and empowering if people also have the option to stay. This distinction between the intrinsic and the instrumental dimensions of migration enables us to go beyond common functionalist, instrumentalist views on migration, in which:

the intrinsic dimension of migration is the direct contribution of the freedom of mobility to people’s wellbeing, irrespective of whether they move or not (‘migration as freedom’). It relates to (particularly young) people’s innate desire for adventure, discovery and separation from (the parental) home for shorter or longer periods as well as to the intrinsic wellbeing derived from the awareness of having the optional freedom to move. Such freedoms do not have to result in actual movement in order to be enjoyed: it is the very awareness of having the option – or freedom – of staying or going where one wants that matters most;

the instrumental (functional, means-to-an-end) dimension of migration reflects the role of migration as a way to achieve other personal or family goals such as increased income, education, living standards or, in the case of refugees, personal safety. While people need a certain level of capabilities to be able to migrate, migration can further increase such capabilities. This corroborates the idea that migration is often quite literally an investment of families and individuals in a better future rather than a ‘desperate flight from misery’ as dominant discourses on migration often tend to frame it.

Usually, migratory agency is associated with the act of moving and setting up residency in another place or country. This reflects, however, a one-sided view since, after all, real agency also involve the option to not act (see Emirbayer and Mische 1998 ), as long as a real choice is present. A truly agentic view on migration should therefore capture both non-migratory and migratory behaviour. There is a long-standing controversy in the migration literature about whether migration or sedentary behaviour is the norm. The first argument is that migration is a universal part of the human experience and that we tend to erroneously misrepresent past societies as largely ‘immobile’, with migration being the ‘normal’ pattern. The second argument is that most people, if given the choice, prefer to stay at home (the ‘home preference’) and that migration is in fact rather limited in magnitude if we consider the huge economic inequalities across the globe.

Yet from a theoretical point of view this debate seems somehow futile. First, a truly agentic view on migration does not presume either moving or moving as the norm but, rather, acknowledges that they are two sides of the same freedom-of-mobility coin. Second, at both the practical and the conceptual level, migration is only a meaningful and relevant category in the context of sedentary lifestyles (as the concept of migration implies a change in residence). The lifestyles and livelihoods of hunter-gatherers and nomads are often characterised by permanent mobility and a lack of permanent residence – which renders a category like migration rather obsolete and meaningless. So, migration presumes sedentarism as much as sedentarism presumes migration. The residential lifestyles of both agrarian and capitalist-industrialised societies have thus ‘created’ the need for migration as a continuous adaptive response to social change and social transformation as well as a linguistic and conceptual category.

Third, considering that people make migration decisions as members of social groups, migratory and sedentary behaviour are often interrelated. For instance, one of the strengths of new economics of labour migration (Stark 1978 , 1991 ) is the idea that migration is a strategy by rural households to diversify their income portfolio through the migration of one or a few family members. This means that there is often a strong co-dependency between non-migrant and migrant family members. This implies that migration matters to most of us, whether we move or not. Although only about 3% of the world’s population has migrated across borders and a roughly estimated 12% within borders (de Haas et al. 2020a ), most people in the world are affected by migration in direct or indirect ways, either through family and other social ties or through the impacts of migration on origin and destination societies more in general.

Redefining human mobility

Migration and sedentary behaviour are thus interconnected. We therefore need a truly agentic understanding of human mobility that can simultaneously capture movement and non-movement. Following Sen’s general argument on the intrinsic wellbeing-enhancing value of human freedoms, we should therefore also conceptualise the very capability to move (migrate) as a fundamental human freedom. To capture the idea of migration as a freedom in its own right, we should define human mobility not by the criterion of actual movement but as people’s capability (freedom) to choose where to live – with migration as the associated functioning (see also de Haas 2009 ; de Haas and Rodríguez 2010 ). Essentially, human mobility thus includes the freedom to stay, which we can classify as voluntary immobility (contrasting Carling’s ( 2002 ) concept of involuntary immobility ).

This is related to the concept of capabilities in two different ways: first, people need access to social (other people), cultural (ideas, knowledge and skills Footnote 3 ) and economic (material) resources if they are to exert migratory agency. Under highly constrained conditions of poverty and oppression, people often lack the resources to leave. Second, if people have no realistic option to remain – for instance through war, persecution, deportation or eviction by governments – or if they are pressured by their families to work abroad, they may feel deprived of an essential part of their human mobility freedoms, which is the option to stay. Conversely, if people feel deprived of the capability to move, the concomitant frustration of being ‘trapped’ may fuel migration aspirations and can even create an obsession with ‘getting out’.

Drawing on the capabilities–aspirations framework, Table  1 elaborates a theoretical categorisation of five ideal-typical individual mobility types. Based on our new definition of mobility as ‘people’s capability (freedom) to choose where to live, including the option to stay’, this categorisation of mobility types also includes various forms of immobility . This enables the theoretically desirable inclusion of both movement and non-movement within the same conceptual ambit as manifestations of the two sides of the same freedom-of-mobility coin.

This categorisation builds upon Carling’s ( 2002 ) ‘involuntary immobility’ concept but expands with four other mobility types. It acknowledges a reality in which (cultural) preferences, aspirations and capabilities are deeply affected by macro-structural factors. It is only possible to speak about the ‘voluntariness’ of mobility or immobility if there was a reasonable option to stay. That does not mean that refugees and other groups of ‘distress migrants’ do not have any agency (otherwise they could not have moved in the first place), but that their migration is forced to the extent that they have been deprived of mobility freedoms; they had no real option to remain as that would have put them in serious danger of being persecuted, injured or murdered. For refugees, migration is primarily a response to severe danger at home rather than a positive response to opportunities elsewhere. Obviously, once a decision to leave has been made, such opportunities will play a role in deciding how, when and where to go and people will try to exert their agency as much as possible, although they cannot be conceptualised as the main reason to migrate. Thus, refugees are forced migrants because they had no option to remain.

Similarly, migrants who may be classified as ‘voluntary return migrants’ by governments or international organisations may only be ‘willing’ to return not out of a real, intrinsic desire to do so but because they either have no access to social amenities and shelter or they risk imprisonment, violence and other abuse (such as separation from their children) in destination countries. Under such situations of extreme distress and pressure, migrants may eventually decide to return and be compelled to sign forms confirming consent to their ‘voluntary repatriation’, even if this is against their own intrinsic preferences or desires (see Cleton and Chauvin 2020 ). Boersema et al. ( 2014 ) referred to this category as ‘soft deportation’. From a theoretical perspective, we can classify this as ‘involuntary mobility’. Even if such migrants are not literally forced to move (i.e., by violent means through deportation) they may, under severe threat, feel compelled to move even if is this strongly against their own intrinsic desire. The categories ‘voluntary mobility’ and ‘voluntary immobility’ only apply to people who have the capability to migrate but also have a reasonable option to stay (with the term ‘reasonable’ implying that this would not put them in dangerous, highly exploitative or life-threatening situations) and for whom the decision as to whether or not to go is primarily affected by their (instrumental or intrinsic) migration aspirations.

A final category concerns people with low capabilities and aspirations to move. How can we categorise a person living in poverty, who is neither able to migrate nor has ever imagined doing so? Based on the idea that capabilities affect aspirations (Appadurai 2004 , see also Czaika and Vothknecht 2012 ), we may perhaps say that this person is deprived of the capability to aspire as well as the capability to move. This raises the philosophical question as to what extent we can call this form of immobility “voluntary”? Schewel ( 2015 , 2020 ) therefore proposed the category of acquiescent immobility to describe situations in which people are neither able to migrate nor desire to do so. Schewel argued that because ‘acquiescent’ implies an acceptance of constraints (the Latin origins of the word meaning ‘to remain at rest’) this may be an appropriate term to describe this mobility category. However, we clearly need more research on the formation of aspirations to move or to stay and on the extent to which decisions to stay can indeed be seen as ‘acquiescent’ or, rather, reflect a post-hoc rationalisation of mobility deprivation.

Positive and negative liberty as manifestations of structural conditions

As argued thus far, the capabilities approach enables us to conceptualise human mobility (people’s freedom to choose where to live) as a wellbeing-enhancing capability in its own right (‘migration as freedom’). In order to further enhance our understanding of how these individual migration capabilities and aspirations are shaped by, and interact with, macro-structural processes, it is useful to draw on the distinction between positive and negative liberties made by Isaiah Berlin and to apply it to the study of migration. In his Four Essays on Liberty , Berlin ( 1969 ) made a fundamental distinction between negative and positive liberty (or freedom). In brief, the concept of negative liberty refers to the absence of obstacles, barriers or constraints. This comes close to popular ways of conceiving freedom, which often focus on the role of governments in imposing constraints on people’s freedom or even being an outright threat to people’s lives, for instance through regulation, oppression, violence or war. Positive liberty refers to the ability to take control of one’s life and to realise one’s fundamental purposes. As Berlin argued, positive liberty ‘derives from the wish on the part of the individual to be his own master’ (Berlin 1969 , p. 131). While Berlin’s argument focused on people’s role in choosing who governs society, this concept is also applicable to the ability – or agency – of people to actively change their life circumstances. Berlin’s concept of positive liberty therefore comes very close to Sen’s concept of capabilities as people’s ability to enhance the substantive choices they have.

The twin concepts of negative and positive liberties provide a useful conceptual link between the macro-structural conditions which shape these liberties on the one hand and people’s individual aspirations and capabilities – concepts which embody choice and agency but which are ultimately constrained by these structural conditions – on the other. From this perspective, the absence of external constraint (negative liberty) is not a sufficient condition for people to exert migratory agency, because they need a certain degree of ‘positive liberty’ that will enable them to enjoy genuine mobility freedom – which implies a real choice about where to live. For instance, governments may grant nominal freedom of movement but poor people may still lack positive liberty in the form of capabilities and access to resources that would enable them to actually use such negative liberty.

People may aspire to flee situations of poverty, distress and danger but they still need certain ‘positive liberties’ (capabilities) in the form of resources such as money, social connections, knowledge and physical ability, in order to be able to flee. The most vulnerable populations may not, therefore, have the option to flee and may be trapped into ‘involuntary immobility’. Poor people often only migrate if forced by conflict or disasters – and then mainly move over short distances – while the most vulnerable are often deprived of the possibility to move at all. For instance, when Hurricane Katrina hit New Orleans in 2005, many of the (car-less) poor were trapped in the city (Gemenne 2010 ; see de Haas et al. 2020a , p. 37). In the civil conflict that broke out in Libya in 2011, hundreds of thousands of guestworkers from sub-Saharan Africa were trapped in the country and exposed to abuse, violence, imprisonment and sometimes murder. In contrast to high-skilled migrants from European or other powerful states, they were immobilised because they lacked the resources and connections to move out (de Haas and Sigona 2012 ). This insight has important consequences for the way we analyse migration. For instance, scenarios that predict massive international migration as a result of climate-change are rather unrealistic partly because they ignore evidence that the most vulnerable populations who are most at risk to be negativelly affected by climate change generally lack the resource to move over large distances. In fact, deprivation as a result of climate change-driven environmental change (such as an increased incidence of droughts or flooding) may  deprive them from the capabilties to go elsewhere and therefore immobilize them in situ (see Foresight 2011 ; de Haas 2020 ).

This perspective helps us to understand the complex, often non-linear and frequently counter-intuitive ways in which macro-structural processes of social transformation shape trends and patterns of migration, because negative and positive liberties often impinge in quite different ways – and sometimes opposite directions – upon migration aspirations and capabilities. This renders the analysis of the effects of macro-structural conditions on migration patterns far from straightforward: although the deprivation of negative and positive liberties and awareness of better opportunities elsewhere may increase people’s migration aspirations, the absolute deprivation of the same negative or positive liberties, or both, may prevent people from exerting migratory agency. Conversely, while increases in negative and positive liberties may increase people’s mobility freedom, this does not necessarily lead to more migration as, under such conditions, more people may also be able to realise their intrinsic preference to stay through an increased ability to meet their life aspirations at home. Likewise, as we have seen, increasing mobility freedoms through the liberalisation of migration regimes may paradoxically decrease long-term, permanent emigration as it may take away people’s obsession with ‘getting out’.

The structural formation of migration aspirations and capabilities

Figure  2 depicts the various ways in which life aspirations and capabilities are affected by structurally determined positive and negative liberties and how these may affect mobility freedoms and people’s migration decisions. Negative liberty affects both people’s life aspirations and capabilities; the interaction between these factors explains complex, sometimes counter-intuitive migration outcomes. For instance, while it may seem likely that political oppression and violence will increase migration aspirations, the same factors may also deprive people of the capability of moving – such as through exit restrictions – or actually prompt them to stay so that they can protect family and community members. The concepts of negative and positive freedom therefore enable the incorporation of the role of states and policies in migration theories. From this perspective, mobility deprivation can happen either through negative liberty deprivation – for instance when authoritarian states deprive their citizens of the right to leave – or through positive liberty deprivation – when people lack the access to social, cultural and economic resources needed for realising migration aspirations.

Expanded aspirations–capabilities framework for conceptualising migratory agency

Positive liberty primarily affects people’s capabilities in the form of their access to social, economic and cultural resources or ‘capitals’. Indirectly – and drawing on the notion of the ‘capacity to aspire’ (Appadurai 2004 ; Czaika and Vothknecht 2012 ) – increased capabilities are also likely to increase aspirations, by (1) making people aware of alternative opportunities and lifestyles and by (2) making people believe that migration is ‘within their reach’, that they can actually ‘make it’. For instance, acquiring a school or university degree is likely not only to increase knowledge about opportunities elsewhere but also to instil the belief and self-confidence that it is actually possible to find a job, to live in a strange place or to secure a visa.

Embracing a more ‘modern’ lifestyle, with its concomitant increase in perceived material needs, only prompts aspirations to migrate if people believe that their life goals cannot be fulfilled locally within the foreseeable future and if they believe that better opportunities exist elsewhere. Elaborating on Hirschman’s ( 1978 ) ‘exit or voice’ hypothesis we can argue that, if people are discontent, they can either try to change these circumstances (by raising their voice), consent (acquiesce) or leave. Thus, the imagined opportunities for future local change and people’s belief in their own power and moral obligation to contribute to such change will affect the extent to which increased life aspirations will translate into migration aspirations. A mismatch between personal life aspirations and conditions at home do not therefore necessarily translate into migration: they can also be fulfilled at home – for instance by starting up a business, pursuing an education, joining political movements or taking up arms. It only translates in migration aspirations if people lack, or lose, belief in future local change.

In order to simultaneously incorporate structure and agency migration theory, we need to connect both concepts and conceptualise their dialectics. In this respect, ‘structure’ is often unilaterally seen as a set of constraints. However, this is too limited a view. Essentially, structures are about patterns – or regularities and repetitions in social relations and social behaviour. Structures thus simultaneously constrain the migration of particular social groups while facilitating the migration of other groups along very specific geographical and social pathways. For instance, states and their policies have a strong structuring effect on migration, which means that they can facilitate and actively stimulate (particularly through recruitment) the movement of some (national, age, gender, skill, class and ethnic) groups while simultaneously hindering the movement of others.

Government policies, recruitment and other macro-structural factors thus shape socially differentiated, highly specialised and geographically bundled pathways (also known as ‘migration corridors’) linking very particular social groups and places over space. Once such initial patterns are set, migrant networks, feedback processes known as ‘cumulative causation’ (Massey 1990 ) and various other ‘internal dynamics’ (de Haas 2010b ) tend to give migration processes their own momentum and thereby reproduce such patterns, giving migration an “identifiable geographical structure that persists across space and time” (Mabogunje 1970 , p. 12). However, notwithstanding the importance of such internal dynamics in perpetuating migration and the consolidation of migration systems, the crucial point is that governments, employers and recruiters often play a key role in setting initial migration patterns that are subsequently reproduced over space and time.

The entire set of structural conditions at home and in imagined migration destinations creates complex opportunity structures, endowing different individuals and social groups with various sets of negative and positive liberties, which, depending on how these structural conditions affect people’s capabilities and aspirations and how people perceive these conditions through their social, cultural and personal lenses, may, or may not, make them decide to migrate. In turn, such migratory agency reciprocally affects these structural conditions through various feedback effects, which may stimulate more migration over the paths beaten by initial (pioneer) migrants (de Haas 2010b ).

The aspirations–capabilities framework therefore enables us to explain why social transformation or ‘development’ is initially associated with increasing migration levels (see Skeldon 1990 , 1997 , 2012 ; de Haas 2007 , 2010c ). Particularly if regions or countries transform from a low-income, agrarian and peripheral status to a middle-income, industrialising and urbanising status, migration aspirations and capabilities both tend to increase rapidly, explaining the paradox of development-driven emigration booms. As long as people’s ideas of the ‘good life’ (generally away from traditional rural-agrarian lifestyles Footnote 4 ) and the associated growth in material aspirations change faster than and outmatch local opportunities, this typically leads to growing migration propensities.

Only in the longer term, when local opportunities start to increasingly match aspirations, can we expect migration propensities to go down. While such capabilities and aspirations manifest themselves at an individual level, they are ultimately shaped by macro-structural changes such as the expansion of infrastructure, education and the media. This exposes the potential of the aspirations–capabilities approach to link macro-structural change processes to individual perceptions, experiences and agency with regards to migration decision-making.

Towards new migration categories

As manifestations of structural conditions, the concepts of positive and negative liberty are also useful in operationalising ‘structural conditions’. This enables the development of a four-pronged typology of migration categories as presented in Table  2 , which represents an ideal-typical categorisation of concrete manifestations of migration under different contextual configurations of relatively high and low positive and negative liberty. Table 2 also indicates the explanatory relevance of some of the main migration theories for these different contextual migration categories.

The categorisation presented in Table 2 is tentative and would benefit from further elaboration, verification and refinement. This main purpose of this effort is not to propose a ‘definitive’ categorisation of migration but, rather, to illustrate how the meta-theoretical framework presented in this paper can be helpful in developing a more systematic way of ‘contextualising’ the assumptions of the different theories and, in so doing, achieving greater precision when specifying the varying applicability of different theories. This exemplifies my position that theoretical assumptions should be seen as contextualised statements (or generalisations) rather than mutually exclusive truth claims.

As mentioned earlier, ‘neo-classical’ theories have a relatively higher relevance to explain the more or less free migration of relatively well-off people, under relatively unconstrained conditions characterised by high levels of positive and negative liberty. In their turn, neo-Marxist and other historical-structural theories may be comparatively more powerful in understanding and interpreting the precarious migration which takes place under highly constrained conditions – such as migration restrictions or the lack of state protection against abuse and discrimination. Such constraints reduce the agency of migrants, making them more vulnerable to exploitation by employers, recruiters, state agents or smugglers – and often frustrate their attempts to achieve upward socio-economic mobility through study and work. This can apply to both impoverished labour migrants and the majority of people forcibly displaced by conflict, disasters or persecution who lack the means and contacts to move over large distances. Although they had the resources and the ‘positive liberty’ to leave, they are at high risk of becoming trapped or ‘involuntarily immobilized’ along the journey or at the destination, deprived of the resources and freedom to continue the journey or to return.

In the category of improvement migration , people have relatively low levels of positive liberty (such as manifested by limited financial resources) but face relatively high negative liberty (such as manifested by access to legal migration opportunities and residency in wealthier countries) – which creates the conditions under which migration can be a successful way of achieving upward socio-economic mobility. For instance, recruitment programmes have historically given relatively poor people access to work opportunities abroad, thus fitting within dual labour-market theory (Piore 1979 ). Under other circumstances, family members often pool their resources to invest in the migration of one or more family members. This seems to fit with the assumptions of the new economics of labour migration (NELM), which conceptualises migration as a risk-sharing strategy by households aiming to diversify their income, generate remittances and improve the long-term wellbeing of the family (Stark 1978 , 1991 ). Theories on the internal dynamics of migration processes (Massey 1990 ; de Haas 2010b ) help to explain how migration often facilitates further migration through cost- and risk-lowering network effects. This explains the partial self-perpetuation of such migration even after recruitment has stopped and the original causes of migration such as labour demand have fallen away. This set of theoretical explanations seems to apply, for instance, to Mediterranean ‘guestworkers’ who were initially recruited to work in North-West European countries in the post-WWII decades as well as to Mexican migrant workers who were recruited to work in the US through Bracero programme between 1942 to 1964.

In other situations, people may face high levels of external constraint (negative liberty, such as through oppression, persecution or violent conflict) but still manage to migrate through their access to financial, social and human resources (positive liberty). Examples of this category could include skilled and/or relatively well-off refugees who are actually able to make it to other countries and obtain legal residency. This category, which I have tentatively named distress migration is a form of forced migration that needs further elaboration because existing migration theories seem to apply less easily to this category – reflecting the low level of theorisation of refugee migration. However, based on our definition of human mobility as the capacity to choose where to live, refugees are thus forced migrants because they have no reasonable option to stay even though they have some degree of agency in terms of having the resources and capacity to escape, move to another country and choose where to go.

There seem to be two basic ways of conceptualising ‘forced mobility’ – either as a conscious act to escape external threats and oppression (negative liberty deprivation) or livelihood insecurity and poverty (positive liberty deprivation) or as a literally forced mobility – such as through eviction, deportation or enslavement. In the first case, migrants still have agency and migration can be instrumentally or intrinsically voluntary; in the latter case, agency is entirely ruled out. For distress migrants, the existence of certain positive liberties – for instance speaking the host country language, having family already residing in the country, having the means to pay for a journey that is farther from home – can further increase both their aspiration and capabilities to migrate. Footnote 5 While ‘distress migrants’ have some level of positive liberty and agency (or capability) over where they choose to go, ‘precarious migrants’ lack such resources, generally move over short distances, are more likely to get stuck in situations of ‘involuntary immobility’ and are more vulnerable to exploitation and extortion by state agents, employers or smugglers.

While Table 2 gives an indication of which migration theories seem the most relevant and the ‘best fit’ for these migration categories, it does not mean that each theory exclusively applies to that category but, rather, to where they seem to have the strongest explanatory power. As I argued earlier, there is considerable leeway to combine theories, particularly when applied to different levels of analysis. As argued above, although much migration of less-skilled workers may appear to be exploitative from a macro-structural point of view (fitting historical-structural theory), it may often still be beneficial from the point of view of individual migrants and their families (fitting neo-classical or NELM theories). Migrants may also shift categories over time – for instance if a restrictive turn in policies or increasing racism turns ‘improvement migration’ into ‘precarious migration’, while the reverse may, for instance, apply if ‘precarious migrants’ get access to legal status through regularisation campaigns. The applicability of these categorisations also depends on the type of migration. For instance, the same person may have the positive and negative liberties enabling her to migrate internally but lack the resources and access to documents enabling her to migrate across borders.

This paper has elaborated an aspirations–capabilities framework to advance a new, comprehensive understanding of human mobility as an intrinsic part of broader social transformation processes. Drawing on previous work (Carling 2001 , 2002 ; de Haas 2003 , 2009 ) on aspirations and capabilities, this paper has expanded these concepts and embedded them in the wider theoretical perspectives on capabilities in development theory offered by Sen ( 1999 ) as well as the distinction between negative and positive liberty posited by Berlin ( 1969 ). While neither Sen nor Berlin developed these concepts to explain migration, this paper has argued that they can be fruitfully applied to migration to provide a richer, more agentic and realistic understanding of migration processes. Arguing in favour of conceptual eclecticism to bridge disciplinary and paradigmatic divides, this paper has shown the conceptual exigency and theoretical benefits of conceiving migration as an intrinsic part of broader processes of social transformation. Such a conceptualisation requires the embedding of the analysis of migration into general theories of societal change without reverting to the top-down causal determinism of conventional (historical-structural or functionalist) migration theories.

Except for extreme situations like slavery and deportation, migrants are neither passive subjects nor actors who react in automated and uniform ways to sets of ‘push’ and ‘pull’ factors – whether these be the macro-forces of global capitalism, wage gaps, violence or environmental stress. In order to migrate, people need to take the active decision to move and have the resources to do so. While historical-structural theories tend to portray migrants as passive pawns or victims of the forces of global capitalism, neo-classical and other functionalist migration theories implicitly assume that people’s preferences and, hence, life aspirations are constant across societies and over time, mostly boiling down to individual income (or ‘utility’) maximisation. This reveals that functionalist migration theories, despite their guise as ‘actor-focused’ models, are socially sterile and devoid of any real sense of agency, as individual choices are supposed to be entirely predictable outcomes of individual cost–benefit analyses based on fixed, static sets of assumed preferences.

The crucial flaw in this type of thinking is the assumption that people’s perceptions and preferences are (1) driven by individual utility maximisation; that (2) people’s preferences are uniform across societies and that (3) such preferences are static. So, in various ways, conventional migration theories all tend conceptualise migrants as persons being ‘pulled’ and ‘pushed’ like atoms by somewhat abstract economic, political, demographic or environmental causal forces. This ignores the fact that factors such as culture, education and exposure to media and other sources of images, ideas and knowledge are likely to have a huge impact on (1) people’s preferences and notions of the ‘good life’ and, hence, personal life aspirations, as well as (2) their knowledge, awareness and perception of opportunities ‘here’ and ‘there’.

This paper has argued that we can achieve a more meaningful understanding of agency in migration processes by conceptualising migration as a function of aspirations and capabilities to migrate within given sets of perceived opportunity structures. On this basis, we can define human mobility as people’s capability (freedom) to choose where to live – including the option to stay – rather than the act of moving itself. Moving and staying then become complementary manifestations of the same migratory agency. This conceptualisation enables us to move beyond the futile debate over whether migration or sedentary behaviour is the norm, since a truly agentic view on migration does not presume either moving or staying as the norm, but acknowledges that they are two sides of the same freedom-of-mobility coin. This enables us to overcome dichotomous and simplistic classifications such as between forced and voluntary migration and to integrate the analysis of most forms of migratory mobility within one meta-conceptual framework.

The application to migration studies of Sen’s ( 1999 ) capabilities perspective on development creates the conceptual space to achieve a deeper understanding of the role of capabilities in shaping migration aspirations as well as to make a vital analytical distinction between the instrumental and the intrinsic dimensions of human mobility. In order to acquire a more systematic understanding of the dialectics between structure and agency in migration processes, Berlin’s ( 1969 ) distinction between positive and negative liberties is a useful theoretical tool to elaborate a more structured, systematic and contextualised view of how macro-structural change processes affect people’s aspirations and capabilities to migrate in complex, non-linear and frequently counter-intuitive ways.

The resulting framework creates significant scope for improved theoretical synthesis by integrating different migration theories under one meta-conceptual umbrella. Instead of being mutually exclusive, within this alternative vision different migration theories have various degrees of explanatory power to understand various forms of migration occurring under specific conditions, among particular social groups and migrant categories and at various levels of analysis. This exemplifies the broader argument of this paper, which posits that, in migration theory specifically and social theory more generally, theoretical assumptions should be seen as contextualised statements rather than as mutually exclusive truth claims.

While this paper is hopefully useful in the much-needed effort to elaborate a more comprehensive, contextualised and integrated theorisation of human mobility, significant additional conceptual work remains to be done. For instance, considerable theoretical progress can be achieved by further embedding migration studies within broader theories of social change. This can, for instance, be done through applying insights from fields such as social psychology and behavioural economics. Research in such fields has yielded advanced insights into people’s (often non-rational) behaviour and factors that may affect migration aspirations but have rarely been applied to migration studies. Significant progress can also be achieved by integrating social-scientific and historicising approaches to studying migration. While social-scientific perspectives on migration could benefit from a better historisation and the adoption of longue durée perspectives, historical studies of migration could benefit from the adoption of social-scientific perspectives and theories to elaborate improved explanatory accounts of historical migration trends – for instance through adopting historical comparative methods (see Vezzoli 2015 ). Footnote 6 More generally, as the world and social realities are constantly changing, no social theory will ever be ‘final’, as ongoing processes of social transformation will perpetually create the need for theoretical innovation in order to make sense of these changing realities and the diverse ways in which people give meaning to them.

Future theoretical work can help us to address several remaining conceptual puzzles. For instance, we may wonder to what extent we can really separate intrinsic from instrumental migration aspirations, as they often seem conflated in practice. After all, what appears to be an intrinsic and subjective desire for adventure and the discovery of new horizons could, at least subconsciously, also fulfil a ‘functional’ role in the psychological separation-individuation process of adolescents and young adults, as a way to acquire new knowledge, meet future partners, find a job and establish independence. Conversely, what appears to be a move abroad to earn more money can be difficult to separate from the social prestige which successful migration – particularly when it initially involves significant risks and courage – can bring, particularly in communities where migration has become a rite de passage and is cast in positive cultural-normative ways.

Another, related conceptual puzzle is that of voluntariness. To what extent can we classify migration as voluntary if a migrant does not want to move but does so for the sake of the long-term economic future of the family? Perhaps we can argue here that such a migrant has no intrinsic desire to move but that the decision to move still emanates from an autonomous decision and real willingness to sacrifice short- to medium-term individual wellbeing (for instance, being separated from loved ones or being deprived of sexual relations or the alienating experiences of living in a strange and sometimes hostile society) from the (instrumental) wish to improve the long-term wellbeing of the family (presumably after return or family reunion). However, what if family members are put under immense social pressure to migrate against their own intrinsic desire? This could for instance apply to labour migrants who move abroad to work because of their family’s social expectations, although they may personally resent this; but it can also apply to adolescents sent to boarding school abroad by their affluent parents. What, therefore, can we say about children and adolescents who often have little input into the mobility decisions of their parents and who may feel that they are ‘moved around’ as if they were a piece of luggage. We cannot ignore the considerable emotional stress and social costs implicated in the loss of friends, a familiar environment, alienation and the constant need to adapt to new situations, even if they live a life of material privilege. This begs the question as to whether we can conceptualise such children as forced migrants.

This highlights the inherently blurred lines between the concepts of ‘voluntary’ and ‘forced’ migration. There are parallels with similar debates, such as those on the difficulties in conceptualising forced versus voluntary marriage (see Enright 2009 ). In such situations there is often a conflict between the desire to be a member of social groups for psychological and social-security reasons on the one hand and the personal drive towards autonomy on the other. This shows the importance of developing conceptual tools that can help us to gain more nuanced understandings of the interaction between structure and agency in social action. This also suggests that the aspirations–capabilities framework developed in this paper can be useful for other domains of social theory.

Availability of data and materials

This paper does not use any primary research data, and is fully based on other publications and the author’s analysis.

This paper is not the appropriate place to extensively discuss the pros and cons of these different concepts, which all refer to social change but have different foci and disciplinary origins. For the sake of brevity, this paper will use the terms social transformation and development as shorthands to indicate broader, more fundamental processes of social change (for a more elaborate discussion, see de Haas et al. 2020b ).

This also exemplifies the fundamentally problematic nature of transposing natural-science notions of causality to the social sciences, in both theorising and empirical analysis.

Economists would rather call this ‘human capital’.

In urbanized societies, and later in the lifecycle when people establish families, urbanites may aspire to live in the countryside again in search for calm, nature and community, or find a compromise by moving to suburbs.

I thank Sarah Salehi for bringing my attention to this point.

From this perspective, there is, indeed, good reason to consider history (also) as a social science.

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Acknowledgments

The research leading to this article is part of the MADE (Migration as Development) project and has received funding from the European Research Council under the European Community’s Horizon 2020 Programme (H2020/2015-2020)/ERC Consolidator Grant Agreement 648496. The author is grateful to Oliver Bakewell, Naluwembe Binaisa, Mathias Czaika, Katharina Natter, Edo Mahendra, Sarah Salehi, Kerilyn Schewel, Sorana Toma, María Villares-Varela and Simona Vezzoli for their generous comments on earlier versions of this paper. He thanks Sally Kingsborough for proofreading an earlier version of this text and Jenny Money for final editing.

Funding to conduct the research and write the article was provided by the European Research Council (see acknowledgements).

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de Haas, H. A theory of migration: the aspirations-capabilities framework. CMS 9 , 8 (2021). https://doi.org/10.1186/s40878-020-00210-4

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Anticipating sea‐level rise and human migration: A review of empirical evidence and avenues for future research

Sem j. duijndam.

1 Institute for Environmental Studies (IVM), Vrije Universiteit Amsterdam, Amsterdam The Netherlands

Wouter J. W. Botzen

2 Utrecht University School of Economics (U.S.E.), Utrecht University, Utrecht The Netherlands

3 Risk Management and Decision Processes Center, The Wharton School, University of Pennsylvania, Philadelphia Pennsylvania, USA

Liselotte C. Hagedoorn

Jeroen c. j. h. aerts.

4 Deltares, Delft The Netherlands

Associated Data

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

Sea‐level rise (SLR) threatens millions of people living in coastal areas through permanent inundation and other SLR‐related hazards. Migration is one way for people to adapt to these coastal changes, but presents an enormous policy challenge given the number of people affected. Knowledge about the relationship between SLR‐related hazards and migration is therefore important to allow for anticipatory policymaking. In recent years, an increasing number of empirical studies have investigated, using survey or census data, how SLR‐related hazards including flooding, salinization, and erosion together with non‐environmental factors influence migration behavior. In this article, we provide a systematic literature review of this empirical work. Our review findings indicate that flooding is not necessarily associated with increased migration. Severe flood events even tend to decrease long‐term migration in developing countries, although more research is needed to better understand the underpinnings of this finding. Salinization and erosion do generally lead to migration, but the number of studies is sparse. Several non‐environmental factors including wealth and place attachment influence migration alongside SLR‐related hazards. Based on the review, we propose a research agenda by outlining knowledge gaps and promising avenues for future research on this topic. Promising research avenues include using behavioral experiments to investigate migration behavior under future SLR scenarios, studying migration among other adaptation strategies, and complementing empirical research with dynamic migration modeling. We conclude that more empirical research on the SLR‐migration nexus is needed to properly understand and anticipate the complex dynamics of migration under SLR, and to design adequate policy responses.

This article is categorized under:

• Climate Economics < Aggregation Techniques for Impacts and Mitigation Costs
• Vulnerability and Adaptation to Climate Change < Learning from Cases and Analogies
• Assessing Impacts of Climate Change < Evaluating Future Impacts of Climate Change

Sea‐level rise (SLR) threatens millions of people living in coastal areas. Migration is one way for people to adapt to SLR‐related hazards, with major implications for people and policy. A more comprehensive understanding of migration dynamics under SLR is therefore vital.

Photo: Flood event in Jakarta, Indonesia, 2017. Photo source: World Meteorological Organization on Flickr https://tinyurl.com/8azc3vnr

1. INTRODUCTION

According to the Intergovernmental Panel on Climate Change (IPCC) sixth assessment report, global mean sea levels have risen by around 0.2 m since 1900 (IPCC,  2021 ). Because of accelerating climatic change, the yearly rate of sea‐level rise has increased from 1.3 mm over the period 1901–1971 to 3.7 mm over the period 2006–2018, and is expected to increase further in the decades to come. Global mean sea‐level rise by 2100 relative to 1995–2014 is expected to be between 0.3 and 1 m depending on greenhouse gas emission scenarios. Higher levels approaching 2 m by 2100 and 5 m by 2150 cannot be ruled out because of deep uncertainty about ice sheet processes (IPCC,  2021 ). Sea‐level rise (SLR) posits an enormous threat to coastal populations, as it will lead to more frequent and intense flooding from storm surges, tidal extremes, waves, and backwater effects, and to coastal and riverbank erosion, water and soil salinization, and in some cases permanent inundation (Alam et al.,  2016 ; Ketabchi et al.,  2016 ; Nicholls & Cazenave,  2010 ; Vitousek et al.,  2017 ). Next to climate‐induced SLR, other (non‐climatic) processes can further exacerbate these hazards, such as subsidence, dam construction, and industrial (dredging) activities (Dunn et al.,  2018 , 2019 ; Nicholls, Lincke, et al.,  2021 ; Vasilopoulos et al.,  2021 ). Economic development and population growth are expected to exacerbate vulnerability to SLR, although projections of these processes are uncertain as well (Jongman et al.,  2012 ; Nicholls, Hanson, et al.,  2021 ). These impacts affect coastal societies in numerous ways, such as by reducing agricultural production, threatening freshwater supplies, and damaging and destroying properties and critical infrastructure (Wrathall et al.,  2019 ).

SLR‐related hazards could potentially displace tens of millions of people living in coastal areas by the end of this century (Hino et al.,  2017 ; Lincke & Hinkel,  2021 ; Nicholls et al.,  2011 ). Depending on population growth, more than one billion people could be populating the low‐elevation coastal zone and be vulnerable to SLR‐related hazards (Hauer,  2017 ; Neumann et al.,  2015 ). However, other common and more restricted measures to assess “at‐risk” populations, including the 100‐year floodplain and areas inundated under SLR, suggest a much lower number (Hauer et al.,  2020 ). Although of the SLR‐related hazards flooding threatens the greatest number of people, livelihoods of millions of people are also threatened by salinization and erosion impacts (Chen & Mueller,  2018 ; Hinkel et al.,  2013 ; Nicholls et al.,  2018 ). Policy measures focused on protection (i.e., armoring coasts to prevent SLR hazards) or accommodation (i.e., adaptation measures that facilitate living with SLR hazards) can mitigate the impacts of SLR. However, the high costs of these measures make it unlikely that they will be implemented along all threatened coasts (Hauer et al.,  2020 ). Especially in developing countries and along low‐populated coastlines, protection is in many areas projected to be economically unfavorable under 21st century SLR (Lincke & Hinkel,  2018 ). When protection and accommodation are not feasible, retreat might be the only option left for adapting to SLR. Retreat can be centrally planned with the goal to relocate affected populations to safer areas, but as such government‐led resettlements are often expensive, complicated, and contested, its applicability seems limited to smaller populations highly vulnerable to SLR (Hauer et al.,  2020 ; Wilmsen & Webber,  2015 ). The largest share of future SLR‐related migration will therefore likely be the result of people that autonomously decide to migrate in anticipation of, or in reaction to, SLR‐related hazards (McLeman,  2014 ). Such migration can be facilitated by government policy, for instance by prohibiting development in flood‐prone areas, not investing in flood protection, and property buyouts (Siders,  2013 ).

The locations from which, and how many, people will eventually migrate as a result of SLR and where they would migrate to is difficult to predict. Nevertheless, gaining a better understanding of future migration flows is of critical academic and policy relevance. It enables anticipatory policy planning to reduce the societal and economic costs of migration, can provide better insights into the welfare impacts of climate change, and allows for improved understanding and modeling of future coastal risk. Besides uncertainty about future SLR and policy responses, an important reason why forecasting migration under SLR is so difficult is the complexity of the migration decision‐making process, where next to SLR‐related hazards also demographic, cultural, economic, social, political, and individual/household‐level factors play a role (Black et al.,  2011 ; Hauer et al.,  2020 ; Wrathall et al.,  2019 ). At present, economic and social motives instead of environmental concerns are often the main reason for people to migrate, also in vulnerable locations (Adger et al.,  2021 ; Nicholls et al.,  2020 ; Safra de Campos et al.,  2020 ). Despite this complexity, we can already learn how SLR might influence migration from the growing empirical literature that investigates, using survey or census data, how SLR‐related hazards including flooding (Bohra‐Mishra et al.,  2014 ; Call et al.,  2017 ; Codjoe et al.,  2017 ), erosion (Bernzen et al.,  2019 ; Goldbach,  2017 ), and salinization (Bernzen et al.,  2019 ; Chen & Mueller,  2018 , 2019 ) impact migration or migration intentions. Although these hazards may not always be directly caused by SLR in the specific study settings, they reflect hazards that will be exacerbated by SLR, and are therefore relevant analogies.

This article presents the first systematic literature review that brings together these empirical studies. Our review focuses on those studies employing quantitative multivariate regression approaches because, despite the great value of qualitative research, this restriction facilitates the comparison of research findings and the isolation of SLR‐related hazards from other factors influencing the migration decision‐making process (Kaczan & Orgill‐Meyer,  2020 ). While previous review studies have been done on the impact of climate‐related events on human migration (see, e.g., Berlemann & Steinhardt,  2017 ; Black et al.,  2013 ; Kaczan & Orgill‐Meyer,  2020 ; Klaiber,  2014 ), none of them centered around the impacts of SLR‐related hazards and as a result many relevant studies were excluded from these reviews. Moreover, the majority of the 15 studies included in this review were published since 2017, which indicates that reviews published before are outdated and miss many new studies. Kaczan and Orgill‐Meyer ( 2020 ) explicitly recognize the need for a systematic review of the impacts of SLR on migration, but yet do not consider these impacts themselves in their review of quantitative empirical studies on climate‐driven migration. Our study aims to fill this gap in the literature. Notwithstanding SLR is primarily a future affair, a “wait and see” approach is not an attractive option; instead the information available now can be utilized for making informed choices about how to anticipate and manage SLR‐induced migration.

To capitalize on the available information and to guide future research, this article provides two main outputs. First, embedding the findings from our literature review in a conceptual framework of human migration, adapted from Black et al. ( 2011 ) and Hauer et al. ( 2020 ), emerging patterns on the impact of SLR‐related hazards on human migration are discussed, as well as the role of non‐environmental factors. Second, this article proposes a research agenda for future empirical research on SLR and migration. Although this article takes stock of current knowledge, the evidence available is still sparse and confined to limited geographical contexts, which highlights the need for more empirical work on this topic (McLeman,  2014 ). We also argue that future empirical research should be more closely aligned to promising migration modeling approaches, such as agent‐based models (ABMs). ABMs can simulate the expected number of migrants under different SLR scenarios, where they come from and go to, and when they leave, while considering complex individual‐level and/or household‐level migration decision making processes (Kniveton et al.,  2011 ; Speelman,  2015 ; Thober et al.,  2018 ; Wrathall et al.,  2019 ). This is relevant for policy making and goes beyond what can be achieved by empirical survey research alone (Williams et al.,  2017 ; Wrathall et al.,  2019 ). Key decisions that have to be made by developers of ABMs are the variables to include in the model and the decision‐making rules that agents (simulated individuals or households) follow. In the proposed research agenda, we discuss how empirical research can aid in making these decisions more well‐informed and improving modeling outcomes.

The remainder of this article is structured as follows. Section  2 discusses the conceptual framework. Section  3 explains the methodology used for the systematic literature review. Section  4 presents and discusses the review results. Section  5 proposes a research agenda for future empirical research on SLR and human migration. Section  6 concludes.

2. CONCEPTUAL FRAMEWORK

Migration can be defined as the permanent or semi‐permanent change of residence (Lee,  1966 ). 1 Studying the impacts of environmental change on migration has become of increasing relevance due to climate change and widespread environmental degradation (McLeman & Gemenne,  2018 ). While initially studied predominantly by scholars from the broader natural sciences, the environmental migration literature is increasingly integrated with concepts, theories, and methodologies from the social sciences, and it is acknowledged that environmental factors are just one of many factors that influence the migration decision (Adger et al.,  2018 , 2021 ; Hunter et al.,  2015 ; Nicholls et al.,  2020 ; Safra de Campos et al.,  2020 ).

Figure  1 presents a conceptual framework of migration under SLR‐related hazards. This framework is modified from the original work of Black et al. ( 2011 ), and the later adaptation of Hauer et al. ( 2020 ) who applied the framework to SLR‐induced migration. The main benefit of this framework is that it recognizes the complexity of the migration decision‐making process and the associated drivers of migration (Hunter et al.,  2015 ). In our adapted version, we take advantage of the richness of non‐environmental factors proposed by Black et al. ( 2011 ) and the application to SLR by Hauer et al. ( 2020 ). The framework depicts five macro drivers of migration in the context of SLR: economic, political, social, demographic, and SLR‐related hazards as a subset of environmental drivers. SLR‐related hazards in the conceptual framework and in the remainder of this article refer to hazards that are expected to be exacerbated by climate‐induced SLR, being flooding, salinization, and erosion. However, these hazards may also have other (non‐climatic) causes such as subsidence, dam construction, and industrial (dredging) activities (Dunn et al.,  2018 , 2019 ; Nicholls, Lincke, et al.,  2021 ; Vasilopoulos et al.,  2021 ). Environmental drivers of migration refer to the hazard people face (Black et al.,  2011 ). Hence, studying people's migration response to (perceptions of) SLR‐related hazards, whether or not caused by climate‐induced SLR, provides us with important lessons on how people may respond to similar hazards under increased future SLR. SLR‐related hazards in the framework can influence migration directly or indirectly through the other four drivers. Within the pentagon, spatiotemporal variabilities in the migration response are shown. SLR‐related hazards can drive migration gradually (e.g., salinization) or suddenly (e.g., flooding from storm surges), and migration can be in anticipation or in reaction to SLR‐related hazards. Furthermore, both actual and perceived levels of risk can play a role. Whether drivers indeed translate to migration behavior is mediated by personal/household characteristics such as education, wealth, and preferences, and by intervening obstacles and facilitators such as the costs of migration, protection or retreat policies, and social networks. The framework reflects that both structural forces and human agency shape migration decisions, and it can be applied to different spatiotemporal types of migration. This conceptualization helps to classify and interpret the findings of our systematic literature review, and to identify knowledge gaps and fruitful avenues for future research.

Migration in the context of SLR‐related hazards, modified from Black et al. ( 2011 ) and Hauer et al. ( 2020 )

To synthesize current knowledge on the impact of SLR‐related hazards on migration, this study conducts a systematic review of the empirical literature. Appendix  S1 provides a detailed description of the article inclusion criteria and the search and screening procedure of the review. To be included in the review, the following article inclusion criteria had to be met. First, the research should analyze migration intentions or actual migration as the dependent variable. Second, the research should study the effects of SLR‐related hazards, as an independent variable, on actual migration or migration intentions. Both studies looking at coastal and riverine flooding are included, as SLR through backwater effects can also increase the incidence and extent of upstream river flooding (Ikeuchi et al.,  2015 ). Third, the study should utilize individual‐level or household‐level data. Fourth, data should be analyzed using multivariate regression analysis.

We conducted the systematic literature search over the period November and December 2020 to identify peer‐reviewed scientific articles meeting all the inclusion criteria. A search term including keywords on SLR, migration, and empirical research methods was used and yielded 5353 articles. These articles were subsequently screened and filtered for eligibility following a three‐step procedure: title analysis, abstract and keyword analysis, and finally a thorough full‐text analysis. This resulted in 13 studies meeting all the inclusion criteria. We then screened the references of these 13 studies and the references of a selection of review articles dealing with migration and the environment, which yielded an additional two studies. The article selection process is summarized in Figure  2 .

Selection process of articles included in the systematic literature review

4. LESSONS FROM EMPIRICAL RESEARCH ON SLR ‐RELATED HAZARDS AND HUMAN MIGRATION

4.1. characteristics of articles included in the review.

Table  1 provides an overview of the 15 articles included in the review, incorporating the following characteristics: (a) author(s) and year, (b) study area, (c) SLR‐related hazards studied, (d) how migration is measured, (e) type of migration studied in terms of migration destination and whether migration is temporary or permanent, and (f) survey or census data characteristics. The publication years of the articles included in this review lie between 2008 and 2020, with the majority of studies being published in the period 2017–2020. Geographically, the articles cover seven countries, being Bangladesh (five studies), the United States (four studies), Indonesia (two studies), Ghana (two studies), Australia (one study), Canada (one study), and Pakistan (one study). Most case studies comprise (at least partly) the coastal zone, except two. Regarding socio‐economic context, six studies are situated in developed countries, and nine studies in developing countries. In terms of SLR‐related hazards, the effect of flooding on migration is investigated in all included studies, and to a lesser extent salinization (three studies) and erosion (two studies) are also researched. For flood type, six studies focus on coastal flooding, three studies on riverine flooding, while six studies do not distinguish between flood types.

Overview of studies included in the review

Abbreviations: HH, households; HY, household‐years; IN, individuals.

The studies either use actual migration data (nine studies) or gauge migration intentions in survey questionnaires (six studies). Six out of the fifteen studies specify the migration destination (i.e., whether migration is characterized as short‐distance or long‐distance, internal, or international). Nine studies do not make this specification in their regression analyses, among which are all studies that investigate migration intentions. Six studies also specify the temporal dimension of migration (i.e., whether migration is temporary or permanent). 2 Lastly, studies differ in terms of whether they use cross‐sectional data (nine) or panel data (six) for their regression analyses.

4.2. SLR ‐related hazards and human migration

Table  2 presents the results of the 15 studies included in the review. The results are separately presented for the impacts of flooding, salinization, and erosion. Because of the heterogeneity in research approaches, the specific measurements of the independent variables are also described in the table. For each finding, the sign of the effect is shown, where a “+” indicates a positive impact on migration, and a “–” a negative impact. In addition, the significance level of the result is shown ( p  < 0.10, p  < 0.05, or p  < 0.01). 3 Sometimes a range of significance levels is provided in case a study presents results of multiple model estimations. Non‐significant results are denoted by NS. Results are separated by migration destination, if applicable.

Empirical evidence of relationship between SLR‐related hazards and human migration

Note : −/+ is the sign of the effect, significance level in parentheses, NS means effect is not significant. Results displayed in merged cells indicate that the measure encompasses multiple migration destinations.

Abbreviations: CM, migration destination combined or unspecified; INTM, international migration; LIM, long‐distance internal migration; SIM, short‐distance internal migration.

4.2.1. Flooding

All studies included in this review studied the effect of flooding on migration. Different types of flood indicators are studied: flood experience, flood severity or frequency, economic damages from flooding, geographical indicators of flood risk, and flood risk perceptions. Of the eight flood experience indicators five are reported as insignificant, two as having a positive impact on migration, and one as having a negative impact on migration. Positive impacts of flood experience on migration are found in US studies by Paxson and Rouse ( 2008 ) and Schwaller et al. ( 2020 ), for experience with home flooding specifically. Home flooding can cause serious economic damage and experiencing it first‐hand can strongly increase risk perceptions of flooding, as explained by the availability heuristic (Botzen et al.,  2021 ), which can promote migration behavior. However, for the other studies we find insignificant or even negative impacts of flood experience on migration. This discrepancy might be due to the fact that these studies were conducted in developing countries (except for Haney,  2019 , who finds a positive, albeit insignificant, effect on migration intentions in a Canadian study). People in developing countries are often more dependent on coastal and riverine resources for their livelihoods, and also face higher financial constraints, which may limit their ability to migrate. These financial constraints can be further exacerbated by flood events. This is exemplified by Call et al. ( 2017 ) who find that a river flood event in the month of occurrence significantly reduces temporary migration in Bangladesh, with the odds of migration being 17% lower in a month of flooding compared with a month without flooding. Besides financial constraints, the flood type studied could also explain the divergence in results. River flooding, as opposed to coastal flooding, provides freshwater that can boost agricultural productivity and may reduce migration in agricultural households (Banerjee,  2010 ; Chen & Mueller,  2019 ). As will be explained in Section  5 in greater detail, more research evidence is needed to validate and clarify this possible multidirectional relationship between flooding and migration.

A similar discrepancy between developed and developing country case studies can be observed when we look at indicators of flood severity or frequency. Studies focusing on Bangladesh, Indonesia, and Pakistan find that more severe floods in terms of deaths, destroyed houses, or exposed households significantly decrease long‐term migration (Bohra‐Mishra et al.,  2014 ; Chen & Mueller,  2019 ; Mueller et al.,  2014 ). Contrastingly, studies in Australia and New York find that more severe or frequent flooding increases intentions to migrate (Boon,  2014 ; Buchanan et al.,  2019 ). However, the measurement of migration could also play a role here. Namely, the studies that focused on developing countries looked at actual migration flows, while those focusing on developed countries studied migration intentions. Flood type (coastal or riverine), which in some of these studies is unspecified, could also moderate the effects found. Nonetheless, this does not undermine the finding that severe flooding tends to restrict long‐term migration in developing country contexts. Although severe flooding can generate large temporary population displacements in developing countries (IDMC,  2017 ), displacement may often not result in permanent migration because people prefer to return to their place of residence for social or economic reasons, or because severe floods deplete household resources to finance migration (McLeman,  2014 ). Looking at less severe “moderate” flood events, Gray and Mueller ( 2012 ) find that this increases short‐distance migration and decreases long‐distance migration in Bangladesh. In contrast to severe flood events, moderate flood events might leave enough household resources to allow for short‐distance migration (Kaczan & Orgill‐Meyer,  2020 ). In light of the discussion on the possibly important role of financial constraints in developing countries, it is surprising that Bohra‐Mishra et al. ( 2014 ) and Gray and Mueller ( 2012 ) find that indicators of economic losses from flooding are insignificantly or weakly positively associated with migration. However, both studies use measures of absolute economic losses. Absolute economic losses from flooding may be higher for richer households, for whom migration may still be affordable.

Risk measured in terms of the distance people live from rivers and coasts is significantly associated with migration in three out of four studies looking at this geographical indicator. Goldbach ( 2017 ) and Bernzen et al. ( 2019 ) find in, respectively, Bangladesh and Indonesian case studies that people living closer to a coast or river are more likely to migrate. In contrast, Call et al. ( 2017 ) in a Bangladesh case study find that people living closer to a river are less likely to migrate. The choice of living close to coastal or riverine waterbodies reflects a trade‐off between greater economic and livelihood opportunities and increased flooding and other SLR‐related hazards. However, riverine flooding also comes with benefits by irrigating the land with freshwater (Banerjee,  2010 ). These benefits of riverine flooding may explain the positive effect found by Call et al. ( 2017 ), who focus exclusively on the distance to rivers.

Two studies in our review investigate the relationship between migration and self‐reported risk perception. Haney ( 2019 ) finds that people in the Canadian inland city of Calgary who believe that flooding will become more frequent in the future do not report higher migration intentions, while people who worry about future flooding affecting their neighborhood are significantly more likely to intend to migrate. In fact, worried individuals report a 67% lower likelihood of staying in their neighborhood in the coming year, and a 50% lower likelihood of staying for the coming 5 years, compared with non‐worried individuals. Song and Peng ( 2017 ) in a US study find that both a belief that SLR is occurring and a belief that SLR will intensify extreme weather events is significantly positively associated with migration intentions. Both findings illustrate the important role that risk perceptions can play alongside actual risk, as is also pointed out in Figure  1 . The influence of risk perceptions is particularly relevant for SLR, which main impacts lie in the future and can be gauged by self‐reported risk perceptions.

Chen and Mueller ( 2019 ), Gray and Mueller ( 2012 ), and Mueller et al. ( 2014 ) compare the migration impacts of flooding by gender. Chen and Mueller ( 2019 ) find that in Bangladesh more severe flooding decreases migration among men, but not among women. Gray and Mueller ( 2012 ) in another Bangladesh study find that indicators of flood severity are negatively associated with migration among men, albeit insignificantly, while they tend to be positively associated with migration among women, and significantly so for moderate flood severity. Mueller et al. ( 2014 ) find that flooding in Pakistan significantly deters short‐distance migration for both males and females but find no significant effects for long‐distance migration. The finding of the two Bangladesh studies that flooding has more negative effects on migration for men than for women is somewhat unexpected, given that men generally have more migration decision‐making power than women in developing countries, and men are also more likely than women to move for economic purposes (Kaczan & Orgill‐Meyer,  2020 ). It could be that migration decreases more among men than among women when communities have to be rebuild after a severe flood event. In addition, migration for marriage, one of the most important reasons for migration among Bangladeshi women, might be little affected or even increase after flooding as it can be seen as a risk diversification strategy for resource‐constrained households (Rosenzweig & Stark,  1989 ).

Whether studies focus on migration intention or on actual migration also seems to matter for the study outcomes. In studies that investigate migration intention, the effects of the flooding indicators are either insignificant or positive. Studies that investigate actual migration find that flooding can both decrease and increase migration, depending on the type of flood indicator and study context. Besides variations in study context, the intention–behavior gap could be a reason for the observed differences, as intentions do not always lead to action (Echegaray & Hansstein,  2017 ). Although people might intend to leave because of flooding, they may change their minds over time or find themselves physically, financially, or in any other way constrained in their migration decision. These diverging findings could also disclose that flooding impacts migration over longer time horizons than shortly after flooding events; flooding can influence migration intentions or migration decisions, but this might not yet be revealed through actual migration behavior. Reduced (financial) capability to migrate shortly after flood events can be one reason for this (Kaczan & Orgill‐Meyer,  2020 ).

4.2.2. Salinization and erosion

In contrast to flooding, much less research has been done on the impact of salinization on migration, and the studies included in this review are confined to Bangladesh. However, the results that are available provide a more consistent picture of the relationship between migration and salinization than is the case for flooding. In most cases, experience with or being geographically at risk of salinization increases migration (Bernzen et al.,  2019 ; Chen & Mueller,  2018 , 2019 ). Although Chen and Mueller ( 2018 ) find that salinization deters international migration overall, salinization still has a significant positive influence on international migration to nearby countries in South Asia. Perhaps negative economic effects as a result of salinization can obstruct long‐distance international moves. Chen and Mueller ( 2019 ) find that a one standard deviation increase in soil salinity increases migration from coastal Bangladesh to India and other countries in the region by a factor of 18, leading to an expected yearly increase in outmigration of 17,874 people to India alone (based on 2011 data). Chen and Mueller ( 2019 ) also examine whether the impact of salinization differs by gender, but find that salinization increases migration among both men and women. Salinization in Bangladesh greatly reduces yields of rice and other important crops, and as a result threatens agricultural production and people's livelihoods (Clarke et al., 2015; Hussain et al., 2018). There is now a widespread conversion from paddy cultivation to aquaculture in saline‐affected areas in the country, increasing rural unemployment and poverty, and stimulating out‐migration (Lázár et al.,  2020 ; Nicholls et al.,  2018 ). This can all explain the strong positive effect of salinization on migration found in our review. In addition, while sudden financial losses from extreme flooding events can deter people's capability to migrate, salinization is more of a gradual process where people have the time to plan migration and collect the necessary resources (Chen & Mueller,  2019 ; Kaczan & Orgill‐Meyer,  2020 ). These findings on the impact of salinization in coastal Bangladesh correspond to Li et al. (2014) who in a case study in inland China find that individuals who experienced problematic salinization as a result of excessive groundwater extraction were much more inclined to migrate than individuals who did not experience these difficulties.

Only two studies in this review looked at how erosion in coastal areas influences migration. Goldbach ( 2017 ) finds that coastal erosion does not affect migration in both Ghana and Indonesia. A reason for this insignificant effect could be that coastal erosion only impacts migration for people living directly adjacent to the coast, while in the study of Goldbach ( 2017 ) most respondents lived a few kilometers away from the shoreline. In another study, Bernzen et al. ( 2019 ) focused on loss of arable land in Bangladesh and find that this does affect migration. Over 93% of respondents reported that riverbank erosion was the reason for the arable land loss. Future SLR, by intensifying the hydrological cycle and river flooding, is expected to enhance the loss of arable land as a result of riverbank erosion (Alam et al.,  2016 ). Loss of arable land impedes agricultural and housing opportunities and, consequently, may force people to migrate (Islam & Rashid, 2011). In fact, in Bernzen et al. ( 2019 ) those who experienced arable land loss were 2.5 times more likely to migrate than those who did not. In the scientific and in the public discourse flooding as a result of SLR captures most attention (Nicholls,  2011 ), but the previous findings illustrate that salinization and erosion and their relationship with migration should be seriously considered as well.

4.3. Other factors that influence migration

Table  A1 in the Appendix presents an overview of non‐environmental factors and their effects on migration, as obtained from the results of the 15 studies included in our review. These factors cover many, but certainly not all, elements shown in the conceptual framework in Figure  1 . Especially information on the impact of macrolevel factors, such as government policies, is still lacking. We only discuss factors that have been investigated in at least two study settings, so that the patterns found are more reliable.

We identify several factors that play a role in explaining migration behavior next to SLR‐related hazards. First, individuals with higher income/wealth are in most cases more likely to migrate than poorer individuals. Although the poor and marginalized are often hit hardest by natural disasters, a lack of economic resources can inhibit their capability to migrate, which can be further exacerbated by natural hazard events (Black et al.,  2011 ; Kaczan & Orgill‐Meyer,  2020 ). However, Gray and Mueller ( 2012 ) find that coming from a richer household significantly increases migration among men but not among women in Bangladesh, reflecting potential gender inequalities in the capability to capitalize on higher household resources. Second, homeowners are less likely to migrate. Homeownership indicates a substantial economic investment that ties people to their place of residence, and may also indicate place attachment (Haney,  2019 ). Third, and related to the previous point, place attachment is associated with a lower intention to migrate, which is a commonly held view in the literature (Adams & Kay,  2019 ; Swapan & Sadeque,  2021 ). Fourth, two US studies find that black people are more likely to migrate than people of other ethnicities (Paxson & Rouse,  2008 ; Song & Peng,  2017 ). This could reflect that black communities in the United States are disproportionally located in areas most vulnerable to climate change (Shepard & Corbin‐Mark,  2009 ), as well as that black people are more likely to be affected by the phenomenon of climate gentrification (Shokry et al.,  2020 ). This raises an important issue, and policy should prevent that already existing inequalities will be aggravated by SLR. Fifth, people with social networks outside of their place of residence are more inclined to migrate. External social networks reduce information costs and make it easier for migrants to integrate into their new destination, and are central in network theories of migration (Haug,  2008 ; McLeman,  2014 ; Ryan,  2011 ). Finally, individual preferences including risk aversion and impatience have been found to lower the likelihood of migration. This can be explained by the fact that migration involves a high level of uncertainty, and it might take time before one can reap the benefits of a migration investment (Goldbach,  2017 ). We observe no clear influence of age, education, or gender. Although younger, male, and higher educated individuals are often depicted as being more likely to take part in environmental migration, especially internationally (Kaczan & Orgill‐Meyer,  2020 ; Obokata et al.,  2014 ), this general pattern is not observed for the studies included in this review.

5. A RESEARCH AGENDA FOR FUTURE EMPIRICAL RESEARCH ON SLR AND MIGRATION

The previous section presented the results of a systematic literature review concerning empirical research on the impact of SLR‐related hazards on human migration. Based on these results, the following section proposes a research agenda by discussing the main gaps that still exist in the literature and fruitful avenues for future research. With this research agenda, we aim to help bring the science forward and promote research that provides policymakers and other stakeholders with the information they need to anticipate and manage SLR‐related migration. The research agenda discusses seven main themes, identifying research avenues for: (a) empirical research for varying geographical contexts and SLR risks, (b) studying a broader set of explanatory variables, (c) utilizing behavioral experiments and risk perception data, (d) differentiating impacts of SLR for subpopulations, (r) defining migration specifics, (f) researching migration together with other adaptation options, and (g) making empirical research complementary to dynamic migration modeling. Consistent with our systematic literature review, this research agenda focuses on quantitative research. Although not discussed in detail here, we acknowledge the importance of qualitative research. Qualitative research allows for a more in‐depth and nuanced understanding of migration motives, and is of high value in identifying the role of non‐environmental factors like gender and culture, and how these factors interact with environmental change such as SLR (Gioli & Milan,  2018 ; McLeman & Gemenne,  2018 ; Zickgraf,  2019 ). In addition, qualitative work offers important insights and experiences from marginalized and indigenous people (Albert et al.,  2018 ; Farbotko,  2018 ), whose specific views are not reflected in our review study.

• Empirical research for varying geographical contexts and SLR risks . Despite the wealth of knowledge gained from the 15 articles included in the systematic literature review, our understanding of the issue is still limited. One major limitation is the geographical scope of the current literature. Only seven country contexts are included in our review. Quantitative empirical research is lacking in some of the countries expected to be among the hardest hit by SLR, including China, India, Vietnam, and the African countries of Egypt and Nigeria (Hauer et al.,  2020 ). Studies are also lacking in several developed countries with considerable population numbers in the low‐elevation coastal zone, such as the Netherlands and the United Kingdom (Neumann et al.,  2015 ). Migration is generally a less pressing issue in these contexts because coastal protection is put in place and planned for, and is often economically feasible under future SLR scenarios (Lincke & Hinkel,  2018 ; McEvoy et al.,  2021 ). For instance, in the Netherlands coastal protection currently insulates the population from SLR changes and flood risk perceptions are generally very low (Kellens et al.,  2013 ; Mol et al.,  2020 ). Such case study locations would only become of substantial relevance for SLR migration research would protection fail or risk perceptions increase for other reasons like public awareness programs. Another limitation of present research is the uneven distribution of SLR‐related hazards that are studied, with erosion and salinization being understudied compared with flooding. This balance should be improved as worldwide many countries already experience the impacts of salinization as well as coastal and riverbank erosion, which will increase under future SLR (Das et al.,  2014 ; Mentaschi et al.,  2018 ; Smajgl et al.,  2015 ). Future studies focusing on flooding should differentiate between the type of flood event, as the impacts of coastal compared with riverine flooding may be different, sometimes interacting in coastal zones, but are often not accounted for in current studies. These studies could also help to validate and clarify the multidirectional relationship between flooding and migration suggested from the findings of our review, disentangling the role of possibly decisive factors such as flood type and financial constraints. Finally, when studying the impacts of SLR‐related hazards on migration, studies should consider not only the role of climate‐induced SLR but also that of other (non‐climatic) factors driving these hazards, including subsidence (Nicholls, Lincke, et al.,  2021 ).
• Broadening of the set of explanatory variables . The conceptual framework portrayed in Figure  1 theorizes the factors that can be important in explaining migration in the context of SLR. However, many of these factors are not or barely analyzed in the empirical literature. Macrolevel drivers, including economic, social, demographic, and political drivers, are especially understudied. Of particular importance is the impact of government policies on migration decisions, such as adaptation or disaster insurance policies (Wrathall et al.,  2019 ). As these are important tools that governments could use to steer migration, research is needed to provide insights into their effectiveness. Besides using data from available datasets, the effects of macrolevel variables can also be tested in behavioral field experiments or by collecting survey data on people's perceptions, which could be necessary if the geographical scope of the study does not include variation in macrolevel context. Future research could also focus more on the influence of intervening obstacles or facilitators of migration. This includes the costs of migration, which is considered to be of high importance in the migration decision. However, in the literature its impact is often only gauged indirectly by looking at household wealth measures (Kaczan & Orgill‐Meyer,  2020 ). The effect of pull, compared with push, factors also deserves more scholarly attention.
• Utilization of behavioral experiments and risk perception data . Many of the studies included in the literature review presented in this article have analyzed how SLR‐related hazards (i.e., flooding, salinization, and erosion) in the past have influenced migration. Although learning from the past gives us important insights, the dynamics of these hazards under future SLR will be markedly different and so will be their impact on migration. When flooding becomes more frequent and severe, and erosion and salinization increases, migration thresholds may be reached that have not been reached in the past. Currently, knowledge about migration thresholds for people under climate change and SLR is increasing but still limited, and more research is needed on what levels and manifestations of SLR trigger (inevitable) migration (Hauer et al.,  2020 ; McLeman,  2018 ). To circumvent the drawbacks of using data on past migration, behavioral experiments and risk perception studies can be utilized. However, these studies are currently underrepresented in the literature. Research on risk perceptions can help us better understand how SLR already influences migration also beyond hazard exposure or impacts, for instance by studying if people who understand or are worried about SLR are more likely to migrate from vulnerable areas. This also allows for the incorporation of local knowledge in addition to scientific measures of SLR risk (McMichael et al.,  2021 ). This can also improve our understanding of anticipatory migration patterns, instead of reactive migration where most studies so far have focused on. Behavioral experiments, on the other hand, can be utilized to identify when thresholds are reached and people start to migrate. This can be done by presenting respondents with migration and adaptation choices under realistic future SLR scenarios for permanent inundation, flooding, salinization, and/or erosion and analyzing the choices they make. Choice experiments are an example of behavioral experiments that could be used for such purposes (Dachary‐Bernard et al.,  2019 ; Train,  2009 ). Results of behavioral experiments could provide pivotal information on when thresholds are reached for people to migrate, and consequently on when to anticipate migration under different SLR scenarios. Longitudinal, in addition to cross‐sectional, studies could help identify if results differ before and after experience with SLR‐related events such as flooding, or whether people actually migrate when they experience the SLR‐related hazards they were presented with in hypothetical experiments (Bubeck et al.,  2012 ).
• Differentiation of the impacts of SLR for subpopulations . The impacts of SLR on migration are not expected to be uniform across populations. For instance, vulnerable population groups like women, the elderly, the poor, and minorities, tend to be less capable of acting on climate risks, especially in developing countries (Gioli & Milan,  2018 ; Otto et al.,  2017 ; Rahman,  2013 ). However, this heterogeneity is not properly accounted for in the analytical strategies of most of the quantitative empirical literature, which is characterized by the use of linear regressions on the full sample. Better accounting for population heterogeneity is imperative, and prevents oversimplification of the environment‐migration relationship. To do this, future studies could estimate separate regressions for different population groups, such as is done by Mueller et al. ( 2014 ) for gender, or include interaction effects between SLR and socio‐demographic indicators (e.g., interacting soil salinity with income). It should be taken into account that for reliable results these analyses might require large sample sizes.
• Defining migration specifics . Migration specifics, including when, where and for how long people migrate, are often not well defined, whereas this is pivotal in properly understanding migration dynamics and their causal factors. For instance, only 6 out of the 15 studies in our review specify the migration destination or whether migration is temporary or permanent. Especially migration intention research tends to ignore migration specifics, where often broad dichotomous questions are analyzed on whether respondents plan to migrate in the future, but without considering its particularities. Migration intention research can help identify migration thresholds under future SLR, but loses much of its value when we do not know what this migration will look like. Another important aspect in this domain is that of migrant agency, which is about the agency or freedom an individual/household has in the migration decision‐making process (Bakewell,  2010 ; McLeman,  2014 ). Migrant agency represents a continuum with fully voluntary and involuntary (forced) migration as the two extremes. The degree of migrant agency in the context of SLR risk may vary (McLeman,  2014 ), and is important to examine in future research.
• Researching migration as part of a broader array of adaptation options . Most studies research migration as the only outcome, whereas people can adapt to SLR in many other ways than migration, such as by flood proofing their houses or by cultivating saline‐tolerant crops (Jamero et al.,  2017 ; McLeman,  2014 ). Therefore, one of the greatest challenges for future research is to break the silos of the migration and mainstream adaptation literature, and to bundle strengths to explain how people respond to SLR risk. Compared with the migration literature, the literature on in‐situ adaptation to SLR‐related hazards is much larger and further developed, with most attention being focused on flood adaptation (Bubeck et al.,  2012 ; Koerth et al.,  2017 ; Paik et al.,  2020 ). Combining both strands of literature could provide important insights into the circumstances under which people will implement private adaptation measures to protect themselves against SLR, when people will decide to migrate, how these relate to each other, and which response will be more likely under future SLR scenarios. A choice experiment analysis in the United States by Buchanan et al. ( 2019 ) where respondents under different flood scenarios choose whether to take up insurance, elevate their homes, or migrate, provides a good example of such an integrative approach.
• Making empirical research complementary to migration modeling research . It is of high policy relevance to obtain reliable forecasts on the key dimensions of human migration under future SLR: how many people will migrate, where they will migrate to, the timing of their migration, and their socio‐economic situation before and after migration (McLeman & Gemenne,  2018 ; Wrathall et al.,  2019 ). Dynamic migration modeling approaches can be employed to obtain such forecasts, and examples include gravity models (Afifi & Warner,  2008 ), radiation models (Davis et al.,  2018 ), machine learning models (Robinson et al.,  2020 ), and agent‐based models (Bell et al.,  2021 ). Although all these model types have their merits, in the remainder our focus will be on agent‐based models (ABMs) because of their unique capability to model complex individual decision‐making processes while incorporating socio‐ecological interactions at the individual/household level (Klabunde & Willekens,  2016 ). ABMs can take into account heterogeneity in the characteristics of individuals/households, and aggregate micro‐level results to population‐level migration outcomes (Klabunde & Willekens,  2016 ; Kniveton et al.,  2011 ; Thober et al.,  2018 ; Wrathall et al.,  2019 ). The use of ABMs has become increasingly popular among migration scholars, and a few ABM studies have simulated migration under future SLR (e.g., Bell et al.,  2021 ; Hassani‐Mahmooei & Parris,  2012 ). Empirical studies, like the studies included in our review, have an important role to play in developing realistic ABMs, and as a result in making their outcomes useful for policymakers (Gray et al.,  2017 ; Schlüter et al.,  2017 ; Williams et al.,  2017 ). We highlight two ways in which empirical research can contribute, both reflecting key decisions that have to be made by ABM modelers. First, ABM modelers select and parametrize a set of variables that influence migration behavior in the model. Quantitative survey‐based research can facilitate this by providing insights into the main factors that influence migration and their effect sizes. Second, ABM modelers decide about the behavioral decision‐rules that agents follow (i.e., how individuals/households make decisions in the model), which are preferably guided by behavioral decision theory (Groeneveld et al.,  2017 ). The choice and formalization of a theory can determine to a large extent the outcomes of the model, and it is therefore important that a theory is chosen that is most appropriate for the specific research context (Klabunde & Willekens,  2016 ; Schlüter et al.,  2017 ; Wens et al.,  2020 ). Some of the most commonly used behavioral theories in social‐ecological ABMs include economic theories such as Subjective Expected Utility Theory and Prospect Theory and psycho‐sociological theories such as the Theory of Planned Behavior and Protection Motivation Theory (Aerts,  2020 ; Schlüter et al.,  2017 ). Empirical survey‐based research can aid in the choice and the subsequent calibration of a behavioral theory. For instance, our review of the empirical literature illustrates that economic wealth can play an important role in whether someone migrates after SLR disasters. This seems to indicate that people's (economic) capability to migrate is an important factor in the migration decision, which is one of the key elements in the Theory of Planned Behavior and Protection Motivation Theory, and can be incorporated in economic decision models as budget constraints. Applying behavioral theories that approach the true complexity of human behavior would be an important step in designing realistic ABMs, as at present still much of the ABM literature is not firmly grounded in behavioral theory or assumes rational agent behavior (Gray et al.,  2017 ; Groeneveld et al.,  2017 ; Schlüter et al.,  2017 ). There is also potential in integrating the migration and adaptation literature in ABM modeling, so that future pathways of both SLR‐induced adaptation and migration can be simulated. Indeed, much ABM development is already going on in the flood adaptation literature (Haer et al.,  2019 ; Zhuo & Han,  2020 ), and we encourage exploring cross‐fertilizations between both strands of research.

6. CONCLUSION

SLR presents a substantial threat to hundreds of millions of people living in coastal areas. Migration is one way for people to adapt, and may increasingly become the preferred option when SLR impacts intensify in the future. Knowledge about migration dynamics under SLR is important to allow for anticipatory policymaking and to better understand the welfare impacts of climate change. In this article, we present the results of a systematic review of the empirical literature and bring together existing knowledge on the relationship between migration behavior and SLR‐related hazards including flooding, salinization, and erosion. This review adds to existing reviews on climate change and human migration by focusing specifically on SLR‐related hazards, which is valuable given the growth in empirical studies on this topic in recent years. Findings from our review indicate no clear association between flooding and higher levels of migration. Severe flooding is often even associated with decreased levels of long‐term migration in developing countries, although the underpinnings of this finding remain unclear from present research. Salinization and erosion have been investigated in fewer studies, in which they tend to be associated with increased levels of migration. Important factors that influence migration alongside SLR‐related hazards include income, homeownership, place attachment, race, social networks, and risk attitudes.

Despite an increase in scholarly attention in recent years, the number of studies on SLR‐related hazards and migration is still limited and there are promising ways to improve upon existing findings. Therefore, we also presented a research agenda outlining knowledge gaps in the literature and fruitful avenues for future research. We highlight that dynamics of SLR‐related hazards such as flooding will be markedly different under future SLR than they were in the past. As a result, we encourage research to investigate how people will likely respond to future SLR scenarios, for instance by utilizing behavioral experiments. We also encourage scholars to clearly define migration specifics, expand the geographic contexts and explanatory variables that are investigated, differentiate impacts of SLR for subpopulation groups, and study migration as part of a broader array of adaptation strategies that people can choose. Finally, we discuss how empirical research can aid agent‐based modeling, which is a promising approach to forecast migration flows under future SLR scenarios. What is clear is that more empirical research is needed if we want to properly understand the complex dynamics of migration under SLR and anticipate it in a timely manner. We hope that our work can give directions for such endeavors.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

AUTHOR CONTRIBUTIONS

Sem Duijndam: Conceptualization (lead); data curation (lead); formal analysis (lead); investigation (lead); methodology (lead); validation (equal); visualization (lead); writing – original draft (lead); writing – review and editing (lead). Wouter Botzen: Conceptualization (supporting); data curation (supporting); formal analysis (supporting); investigation (supporting); methodology (supporting); validation (equal); visualization (supporting); writing – original draft (supporting); writing – review and editing (supporting). Liselotte Hagedoorn: Conceptualization (supporting); data curation (supporting); formal analysis (supporting); investigation (supporting); methodology (supporting); validation (equal); visualization (supporting); writing – original draft (supporting); writing – review and editing (supporting). Jeroen Aerts: Conceptualization (supporting); data curation (supporting); formal analysis (supporting); funding acquisition (lead); investigation (supporting); methodology (supporting); project administration (lead); validation (equal); visualization (supporting); writing – original draft (supporting); writing – review and editing (supporting).

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Supporting information

Appendix S1 Supporting Information

ACKNOWLEDGMENT

This research received funding from the European Research Council through the ERC Advanced Grant project COASTMOVE (grant number 884442).

Other variables and their influence on human migration in the context of SLR

Influence of other variables on human migration in the context of SLR

Duijndam, S. J. , Botzen, W. J. W. , Hagedoorn, L. C. , & Aerts, J. C. J. H. (2022). Anticipating sea‐level rise and human migration: A review of empirical evidence and avenues for future research . Wiley Interdisciplinary Reviews: Climate Change , 13 ( 1 ), e747. 10.1002/wcc.747 [ PMC free article ] [ PubMed ] [ CrossRef ] [ Google Scholar ]

Edited by: Julie Rozenberg, Domain Editor and Mike Hulme, Editor‐in‐Chief

Funding information European Research Council, ERC Advanced Grant COASTMOVE, Grant/Award Number: 884442

1 Migration as a result of SLR‐related hazards is characterized as environmental migration. We define environmental migrants in this article following the definition of Kaczan and Orgill‐Meyer ( 2020 ) and the International Organization for Migration ( 2009 ): “Environmentally induced migrants are persons or groups of persons who, for compelling reasons of sudden or progressive changes in the environment that adversely affect their lives or living conditions, are obliged to leave their habitual homes, or choose to do so, either temporarily or permanently, and who move either within their country or abroad” (Kaczan & Orgill‐Meyer,  2020 , p. 282).

2 Most migration intention studies do not specify whether migration is temporary or permanent, and only investigate whether respondents intend to “migrate” from their place of residence. Some studies focusing on actual migration flows do also not make this specification.

3 We do not report the size of the regression coefficients. Because studies did not report standardized regression coefficients, the size of the regression coefficients depends on the scales and units of the independent variables used in the specific studies, which impedes a meaningful comparison across studies. We therefore recommend future research to report standardized regression coefficients.

DATA AVAILABILITY STATEMENT

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A systematic review of climate migration research: gaps in existing literature

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• Published: 16 April 2022
• Volume 2 , article number  47 , ( 2022 )

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• Rajan Chandra Ghosh   ORCID: orcid.org/0000-0001-9027-6649 1 , 2 &
• Caroline Orchiston   ORCID: orcid.org/0000-0002-3171-2006 1  

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Climatic disasters are displacing millions of people every year across the world. Growing academic attention in recent decades has addressed different dimensions of the nexus between climatic events and human migration. Based on a systematic review approach, this study investigates how climate-induced migration studies are framed in the published literature and identifies key gaps in existing studies. 161 journal articles were systematically selected and reviewed (published between 1990 and 2019). Result shows diverse academic discourses on policies, climate vulnerabilities, adaptation, resilience, conflict, security, and environmental issues across a range of disciplines. It identifies Asia as the most studied area followed by Oceania, illustrating that the greatest focus of research to date has been tropical and subtropical climatic regions. Moreover, this study identifies the impact of climate-induced migration on livelihoods, socio-economic conditions, culture, security, and health of climate-induced migrants. Specifically, this review demonstrates that very little is known about the livelihood outcomes of climate migrants in their international destination and their impacts on host communities. The study offers a research agenda to guide academic endeavors toward addressing current gaps in knowledge, including a pressing need for global and national policies to address climate migration as a significant global challenge.

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Introduction

Population displacement can be driven by climatic hazards such as floods, droughts (hydrologic), and storms (atmospheric), and geophysical hazards such as earthquakes, volcanic eruptions, and tsunami (Smith and Smith 2013 ). The interactions between natural hazard events, and social, political, and human factors, frequently act to intensify the negative effects of climatic and geophysical hazards, leading to political and social unrest, increased social vulnerability, and human suffering. As a consequence of these adverse effects, people migrate from their native land, causing stress, uncertainty, and loss of lives and properties. However, such migration can also have positive impacts on migrants’ lives. For example, migrants may be able to diversify their livelihood and have greater access to education or healthcare.

In 2020, 30.7 million people from 149 countries and territories were displaced due to different natural disasters. Among them, climatic disasters were solely responsible for displacing 30 million people within their own country, with the highest recorded displacement occurring in 2010 when 38.3 million people were displaced (IDMC 2021a ; IOM 2021 ). It is difficult to estimate the actual number of people that moved due to the impacts of climate change (Mcleman 2019 ), because peoples’ migration decisions are triggered by a range of contextual factors (de Haas 2021 ). Nevertheless, the Internal Displacement Monitoring Centre (IDMC) states that approximately 283.4 million people were displaced internally between the years 2008 and 2020 because of climatic disasters across the globe (Table 1 ). This number represents almost 89% of the total disaster-induced displacement that occurred during this timeframe (IDMC 2021a ).

People who move from their homes due to climate-driven hazards are described in a range of ways, including climate migrants, environmental migrants, climate refugees, environmental refugees, and so on (Perkiss and Moerman 2018 ). The process of migration related to climate-driven hazards is variously described as environmental migration, environmental displacement, climate-induced migration or climigration (Bronen 2008 ).

In this research, we focus on climate-induced migration more specifically induced by slow-onset climatic disasters (sea-level rise, drought, salinity etc.), rapid onset extreme climatic events (storms, floods etc.), or both (precipitation, erosion etc.). This study investigates how climate change-induced migration studies are framed in the existing literature and identifies key gaps in the published literature.

There is a significant ongoing debate about the links between climate change and human migration in the academic literature. Some researchers strongly believe that climate change directly causes people to move, whereas the others argue that climate change is just one of the contextual factors in peoples’ migration decisions (Laczko and Aghazarm 2009 ). Although there are scholarly opinions that call into question climate change as a primary cause of migration (Black 2001 ; Black et al. 2011 ; McLeman 2014 ), there is also evidence that climate change causes severe environmental effects and exacerbates the vulnerabilities of people that force them to leave their place of living (Bronen and Chapin 2013 ; Laczko and Aghazarm 2009 ; McLeman 2014 ).

Moreover, the relationship between the adverse effects of climate change and different types of human mobility (migration, displacement, or planned relocation) has become increasingly recognized in recent years (Kälin and Cantor 2017 ). It is assumed in general that the number of climate displaced people is likely to increase in future (Mcleman 2019 ; Wilkinson et al. 2016 ), and climate change could permanently displace an estimated 150 million to nearly 1 billion people as a critical driver by 2050 (Held 2016 ; Perkiss and Moerman 2018 ). As the number of climate migrants increases rapidly in some areas of the world (IDMC 2017 ), it is now confirmed as a significant global challenge (Apap 2019 ) and recognized as a considerable threat to human populations (Ionesco et al. 2017 ).

Climate migration has multifaceted impacts on peoples’ livelihoods. Being displaced from their home, people migrate within their own country, described as internal migration, or across borders to other countries known as international migration. Internal movements of climate migrants occur mostly to nearby major cities or large urban centers (Poncelet et al. 2010 ). Climate migrants who try to move internationally are significantly challenged by two different security problems. Firstly, they cannot live in their own homeland because of worsening climatic impacts and are forced to leave their ancestral land. Secondly, they cannot move to other countries quickly to find a safer place because, according to international law, climate migrants are not refugees and they are not supported by the UN Refugee Convention or any international formal protection policies (Apap 2019 ; Mcleman 2019 ). In this situation, they live with significant livelihood uncertainty. The United Nation’s Sustainable Development Goals (SDGs) and the Sendai Framework for Disaster Risk Reduction (SFDRR) recognize them as a key group that is highly exposed and vulnerable because of their circumstances (Ionesco et al. 2017 ). Hence, policy development to address complex climate migration issues has become an emerging priority around the globe (Apap 2019 ).

In order to address this global challenge, there has been growing academic and policy attention focused on regional (Kampala Convention-2009 by African Union), national (Nansen Initiative—2012 by Norway and Switzerland), and international (Global Compact for Safe, Orderly and Regular Migration- 2018 by United Nations) levels of climate-induced migration in recent years. Myers’s ( 2002 ) seminal article signposted environmentally driven migration as one of the most significant challenges of the twenty-first century, and later, similar assumptions were made by Christian Aid (Baird et al. 2007 ), IOM (Brown 2008 ), and Care International (Warner et al. 2009 ). Such predictions led to a proliferation of the academic discourse on migration, focused on national and international security, policy frameworks, and human rights (Boncour and Burson 2009 ). Other studies have focused on vulnerability assessment, risk reduction, adaptation, resettlement, relocation, sustainability, and resilience, considering pre-, during and post-disaster circumstances of climate migration (Bronen 2011 ; Bronen and Chapin 2013 ; IDMC 2019 ; IOM 2021 ; King et al. 2014 ).

This research contributes to the discourse by identifying the gaps in the published literature regarding climate migration. A systematic literature review was undertaken to shed light on the current extent of academic literature, including gaps in knowledge to develop a climate migration research agenda. Two notable review papers provided a solid foundation for this endeavor. First, Piguet et al. ( 2018 ) developed a comprehensive review of publications on environment-induced migration from a global perspective based on a bibliographic database—CliMig. Their detailed mapping of environmentally induced migration research focused on five categories of climatic hazards (droughts, floods, hurricanes, sea-level rise, and rainfall); however, it did not include salinity and erosion which are also climate-driven and has direct effects on internal and international migration (Chen and Mueller 2018 ; Mallick and Sultana 2017 ; Rahman and Gain 2020 ).

The second key review paper was by Obokata et al. ( 2014 ), which provided an evidence-based explanation of the environmental factors leading to migration, and the non-environmental factors that influence the migration behaviors of people. Their scope of analysis was limited to international migration and excluded other types of migration, such as internal climate-induced migration.

Although migration, or more specifically environmental migration, was occurring over many decades of the twentieth century, the IPCC First Assessment report was released in 1990, which presented the first indications of the risks of climate change-induced human movement (IPCC 1990 ). This milestone report then stimulated the academic discourse, and consequently, a rapid increase in climate migration publication resulted. For this reason, the current study undertook a systematic review of literature across three decades beginning in 1990 and ending in 2019. This study aims to understand how the published literature has framed the climate-induced migration discourse. This paper identifies the key gaps in existing scholarship in this field and proposes a research agenda for future consideration on current and emerging climate migration issues.

In the following section, we outline the systematic review method and identify how journal articles were searched, selected, reviewed, and analyzed. In the next section, we present the results of this study. Results are organized into four subsections that illustrate the reviewed literature in the following ways—spatial and temporal trends, disciplinary foci, triggering forces of migration, and other key issues. Finally, we conclude by identifying research gaps, addressing the limitations of this study, and presenting a research agenda.

Methodology

We have adopted a systematic review methodology for this study because it provides an …overall picture of the evidence in a topic area which is needed to direct future research efforts (Petticrew and Robert 2006 ). Systematic reviews reduce the bias of a traditional narrative review, although it is challenging to eliminate researcher bias while interpreting and synthesizing results (Doyle et al. 2019 ). It also limits systematic bias by identifying, evaluating, and synthesizing all relevant studies to answer specific questions or sets of questions, and produces a scientific summary of the evidence in any research area (Petticrew and Robert 2006 ). Moreover, systematic reviews effectively address the research question and identify knowledge gaps and future research priorities (Mallett et al. 2012 ). We have adopted this approach following the methodology developed by Berrang-Ford et al. ( 2011 ) which was tested in the field of environmental and climate change studies, with measurable outcomes. We have conducted the review following these four steps—article search, selection, review, and analysis (Fig.  1 ).

Systematic review flowchart

Article search

We conducted a comprehensive literature search to identify the published academic literature on climate-induced migration to develop a clear understanding of this field of study. We identified sixteen commonly used keywords to search for articles that are predominantly used in the literature. ProQuest central database was selected and used in consultation with a skilled subject librarian to search for the relevant articles for this study. We conducted this literature search in July 2019 using the key thesaurus terms, presented in Table 2 . All keywords were then searched individually in the publication’s title and abstract. We only considered English language peer-reviewed articles for this study, published between the years 1990 and 2019 (up to June).

Article selection

The main purpose of this process was to ensure the selection of appropriate literatures for further analysis. We approached the Preferred Reporting Items for Systematic Reviews and Meta Analyses (PRISMA), a systematic evaluation tool, which was also used by Huq et al. ( 2021 ). In stage one of the selection process, 277 articles were counted based on our search criteria. In stage two, we excluded 25 duplicates, and 252 articles remained for further assessment. In the third and final stage of the detailed assessment of each paper, we identified a further 91 publications that were not relevant to our study but appeared in our searched list because search terms were briefly mentioned in their title and/or abstract without being described in further detail. As these articles did not fit with the aim and content of this research, we excluded those 91 and selected a final 161 articles for this study.

Article review

All the selected articles were then considered for detailed review in order to achieve the purpose of the study. A questionnaire (Online Attachment—A) was developed partially following Berrang-Ford et al. ( 2011 ); Obokata et al. ( 2014 ) and Piguet et al. ( 2018 ) to investigate how climate migration studies are framed in the published literature. Then each article was reviewed in detail in response to the individual parameters of the questionnaire such as general information ( article title, authors name, publication year, journal, discipline, content ), methodological approach ( qualitative, quantitative, mixed ), focused study areas ( country, climatic zones ), source of migrants ( rural, urban ), migration types ( internal, international ), impacts of climate migration ( social, economic, political, health, cultural, environmental, security ), causes of migration ( climatic: flood, sea-level rise, drought etc ., other: socio-economic, political, cultural ), target communities ( displaced community, receiving community ), and livelihoods ( housing, income, employment, etc . ) of climate migrants described in the publications.

Article analysis

All the data were recorded in Microsoft Office Excel spreadsheets. Relevant data for each parameter were filtered, analyzed, and summarized using the necessary Excel tools. Referencing was compiled through Mendeley Desktop.

Spatial and temporal trend

General information.

In this section, the publication date of the reviewed articles was used in order to identify the development of the academic discourse in climate migration studies over the last three decades (1990–2019). Results show the increasing focus of academic attention on this area of research over that timeframe. The study found only four publications between the years 1990 and 1999. During 2000–2009, an additional 16 articles were published, which was followed by an almost 90 percent (141 publications) increase in reviewed articles over the period of 2010–2019 (Table 3 ).

Reviewed study areas

In 84 reviewed articles, the study reported research focused on a particular location, and in some cases, they considered two or more areas for their research. Therefore, multiple counting for each study has been considered, which represents all the continents except Antarctica. The analysis shows that Asia (38%) is the continent with the greatest number of climate migration studies, followed by Oceania (20%), North America (17%), and Africa (14%). In contrast, Europe and South America have received less attention, with 7% and 5%, respectively. Table 4 presents the distribution of study areas by continent focused on the reviewed papers.

Climatic zones of the reviewed studies

This study identified the climatic zones of the study areas in order to find out which zones are most commonly studied among the reviewed studies. We adopted the climatic zones of the world from Peel et al. ( 2007 ), which is the updated version of Koppen’s climate classification, and categorizes the world climate into five major zones, i.e., (i) tropical, (ii) arid, (iii) temperate, (iv) cold, and (v) polar. This review shows that 86 publications mentioned their study areas, equating to 54% of the total reviewed papers. Among them, 81% referred to a specific region as their study area. The study areas were then classified into the above-mentioned climatic zones with one reference offered randomly for each country as an example of the range of research that has been conducted.

This study reveals that 49% of this group (among 81%) focused on tropical climatic areas such as Bangladesh (Islam et al. 2014 ), Cambodia (Jacobson et al. 2019 ), Kiribati (Bedford et al. 2016 ), Papua New Guinea (Connell and Lutkehaus 2017 ), Philippines (Tanyag 2018 ), Tuvalu (Locke 2009 ), and Vanuatu (Perumal 2018 ) among others, and 16% focused on arid climatic zones such as African Sahel (McLeman and Hunter 2010 ), Israel (Weinthal et al. 2015 ), Peru (Scheffran 2008 ), and Senegal (Nawrotzki et al. 2016a , b ). In addition to these, 13% of authors focused on temperate regions, i.e., Mexico (Nawrotzki et al. 2016a , b ), Nepal (Chapagain and Gentle 2015 ), Taiwan (Kang 2013 ), UK (Abel et al. 2013 ), and the USA (Rice et al. 2015 ) for their study and 3% focused on cold climatic areas, i.e., Alaska: USA (Marino and Lazrus 2015 ), Canada (Omeziri and Gore 2014 ), and northern parts of China (Ye et al. 2012 ). No studies were found based on polar regions (Fig.  2 ). Some studies did not specify a region or country of study but instead focused on broader regions such as Africa (White 2012 ), Asia–Pacific (Mayer 2013 ), Europe (Werz and Hoffman 2016 ), Latin America (Wiegel 2017 ), and Pacific (Hingley 2017 ).

Climatic zones of the reviewed study areas-adopted from Peel (2007)

Migration types and sources of climate migrants

Migration types here refer to whether migration was internal (within a country or region) or international (across borders), and sources of climate migrants refer to people from rural or urban source regions. Most authors (73%) mentioned nothing regarding migration types, but a quarter (27%) explicitly discussed internal or international migration. Among them, 11% described climate migration within countries and 10% investigated cross-border migration. Some authors (6%) were concerned with both internal and international climate migration. Source regions for climate migrants were not often considered, with only 19 publications mentioning the origin of migrants. Among these, 11 articles stated that migration occurred from rural areas, and two publications discussed migration from urban areas. Also, six articles described climate migration from both rural and urban areas.

Disciplinary foci

Research discipline.

This study reveals that climate migration studies are becoming more focal issues in different research disciplines that include more than 40 subject areas. Hence, we developed a typology for the reviewed articles based on the relevant research themes. The typology consists of six research disciplines, each of which includes different subjects, as follows.

Social sciences: Social sciences, Sociology, Political Science, International Relations, Comprehensive Works, Population Studies, Anthropology, Social Services and Welfare, History, Philosophy, Ethnic Interests, Civil Rights, Women's Studies

Geography and environment: Meteorology, Environmental Studies, Energy, Conservation, Earth Sciences, Geography, Agriculture, Geology, Biology, Archaeology, Pollution

Business studies and development: Management, Business and Economics, International Commerce, International Development and Assistance, Economics, Insurance, Investments, Accounting

Law, policy, and planning: Law, Military, Civil Defense, Criminology and Security, Environmental policy

Health and medical science: Public Health, Psychology, Medical Sciences, Physical Fitness, and Hygiene

Other: Literature, Library and Information Sciences, Physics, Technology

Among the reviewed publications, some articles were discussed from the perspective of one particular discipline, while others came from two or more disciplines. Therefore, multiple counting for each discipline was considered during the analysis. The study reveals that Social Science covers the highest percentage of publications (41%), followed by Geography and Environment (30%), Business Studies and development (10%), Law, policy and planning (9%), and Health and medical science (7%). Only 2% of publications are not covered by any of these disciplines.

Primary research themes

The authors discussed a diverse range of themes in the reviewed articles. Key themes have been classified into eight categories based on their topics and focusing subjects. Some of the publications focused on multiple themes, which were counted separately under each theme. Most of the authors (27%) focused on Politics and policy issues, and almost a fifth (18%) of total articles focused on the themes of population, health, and development issues. Human rights, conflicts, and security issues were discussed in 16% of papers, and climate, vulnerability, adaptation, and resilience topics were the focus of 12% of publications. In 11% of publications, the authors focused on identity and cultural issues, and socio-economic topics comprised a further 9% of the total. Environmental issues were discussed by 4% of reviewed articles and 3% of publications did not fit into any of the above categories and are described as Other.

Methodological approaches

This review identified that researchers applied both qualitative and quantitative methods in climate migration research. A total of 82% of the reviewed articles used qualitative methodologies, and 9% quantitative. In addition to these, 9% of articles used mixed methods in climate migration research. Of those who used qualitative studies, most were review-based (86%), comprising systematic review, empirical evidence-based review, critical synthesis review, critical discourse review, and policy review. Only 14% of qualitative studies used interview methods (7%), case studies (6%), and focus group discussion (1%). Data sources reported in the reviewed literature for the quantitative research included secondary data (73%), historical data (13%), remote sensing data (7%), and survey data (7%).

Triggering forces of migration

Climatic causes of migration.

The reviewed publications outlined a range of different causes of climate migration. This study reveals nineteen climate-related causes of migration. We merged these causes into eight categories, defined as (i) climate change (climate change, global warming, temperature, environmental change, climate-induced natural disaster, meteorological events, extreme weather, heatwave), (ii) flood, (iii) sea-level rise (sea-level rise, melting glacier), (iv) drought (drought, desertification), (v) storm (storm, cyclone, hurricane, typhoon), (vi) salinity (salinity, tidal surge), (vii) precipitation-induced landslide, and (viii) erosion (coastal erosion, river erosion). “Climate change” is defined as a separate category because some publications named climate change as an overarching driver of migration, rather than specifying any particular hazard. In 70 publications, authors mentioned particular climatic events that were solely responsible for human migration, and 53 of these articles predominantly identified climate change as the main driver of migration, followed by sea-level rise (6), drought (4), flood (3), storm (2), and precipitation-induced landslide (2). In the remaining articles, scholars identified two or more climatic events that were collectively responsible for human displacement. Based on these articles, multiple counting for each climatic event was considered and the results show that climate change was the most commonly cited cause in 126 articles, along with other climatic causes. The authors also identified sea-level rise, drought, flood, and storms as the significant drivers of peoples’ migration along with other climatic drivers, which were mentioned in 51, 46, 44, and 43 articles, respectively. Precipitation-induced landslide and erosion were recognized in 17 and 12 articles, respectively, as the causes of human displacement, whereas eight articles identified salinity as the main reason.

Influencing causes of migration

Although this review was focused on identifying the climatic causes of human displacement, some other causes emerged during the analysis that also influence migration. In 68 publications, economic, social, environmental, political, cultural, and psychological causes were stated as drivers of migration, in addition to the climatic causes. Among these, economic causes (32%) have been identified as the most common driver, followed by social (25%) and environmental (22%) causes. Some articles described political causes (16%), and the remainder mentioned cultural (3%) and psychological (1%) drivers of migration.

Other key issues

• Impacts of climate migration

One of the key findings of this review concerns the impacts of climate migration. In 48 publications, authors described a range of different impacts caused by climate migration, such as social, economic, political, health, cultural, environmental, and security. All the impacts were identified based on the location of climate migrants which are classified into the following three categories: (i) impacts on the place of origin, (ii) impacts on the place of destination, and (iii) impacts on both origin and destination. The review demonstrates that the impacts of climate migration were more frequently identified for the place of origin rather than for the destination. In the place of origin, authors discussed the economic, social, and cultural impacts, compared to political, security, health, and environmental impacts. In contrast, in the destination, scholars were more focused on security and cultural impacts. Overall, security, cultural and economic impacts were the most frequently discussed themes by the authors of reviewed literature in comparison with other impacts (Table 5 ).

Discussed communities

More than half of the reviewed articles ( N  = 81) described climate migrants and/or their receiving communities. In most of the discussions, authors talked about both displaced and host communities together (57%). In more than two-fifths of articles, they considered only displaced communities (42%). In contrast, none of the authors of the reviewed literature discussed host communities in detail in their publications, except Dorent ( 2011 ). Only a few authors briefly mentioned host communities during the discussion of climate migration impacts.

Livelihoods of climate migrants

This review demonstrates that the overall livelihood of climate migrants has not been a key focus in any of the reviewed literature. However, a few separate parameters of livelihoods, including housing, income and employment, health, access to resources, and education were mentioned in 23 articles. The analysis shows that the livelihoods of migrants in their place of origin (71%) were more likely to be considered compared to their destination (11%). In some articles (18%), authors addressed the livelihoods of climate migrants considering both their place of origin and destination. In total, all the articles which considered livelihoods had a specific focus on internal migration, and none mentioned the livelihoods of climate migrants in terms of international migration.

Discussion and research gaps

Climate change-induced migration is neither new (Nagra 2017 ), nor a future hypothetical phenomenon—it is a current reality (Coughlin 2018 ). This review provides a comprehensive analysis of how this field of study is framed in the existing literature. The academic discourse on human migration due to climate change is suggestive of a long-standing causal connection, which is hard to dissociate (Milán-García et al. 2021 ; Parrish et al. 2020 ; Piguet et al. 2011 ).

The review of spatial and temporal trends of climate-induced migration studies illustrates the growth in the field since the release of 1st IPCC report in 1990. In addition, this review has explored some basic questions that are useful to guide future research in this field of study, for instance, which study areas have received greater or lesser focus? Where are these study areas located in relation to global climatic zones? How are people migrating, i.e., internally, or internationally? What are the spatial sources of climate-induced migrants, i.e., rural, or urban environments?

This review also demonstrates that the expansion of climate migration research increased rapidly after 2000, although the studies in this field began before 2000 (Table 3 ). It denotes that the global academia and policymakers have emphasized their focus on this topic in recent decades (Milán-García et al. 2021 ; Piguet et al. 2011 ). Moreover, this review identifies the Asia–Pacific region as the global ‘hotspot’ of climate migration research (Table 4 ). This reflects the IDMC ( 2019 ) report that states more than 80% of the total displacement between 2008 and 2018 occurred within this region. Moreover, a significant proportion of global environmental displacement will continue to occur in the Asia–Pacific region (Mayer, 2013 ). Therefore, this region could be considered as a critical ‘living laboratory’ for future climate migration research.

Climate migration is mostly occurring internally (IDMC 2021a ; Laczko and Aghazarm 2009 ), and in recent years, it has been widely acknowledged in the policy areas (Fussell et al. 2014 ; The World Bank 2018 ). Nevertheless, this study reveals that only a quarter of the reviewed studies for example, Chapagain and Gentle ( 2015 ), Islam et al. ( 2014 ), and Prasain ( 2018 ) have considered the migration types (internal or international) and sources (rural or urban) of climate migrants in their research. Thus, this review identifies the gap and need for contributions to the academic discourse that investigate migration types, the origin of migrants, and their patterns of migration.

The review of the disciplinary foci of climate-induced migration literature reveals that a broader range of disciplines are now focusing on this research topic, which suggests that greater interdisciplinarity is developing in the discourse. IDMC ( 2021b ) data presented in Table 1 show that climate-induced disasters are displacing millions of people every year, but surprisingly none of the reviewed publications appeared under the subject category of disaster management in the database. This reflects the emergent nature of the academic discourse on climate migration and disaster management, which includes recent studies by Ye et al. ( 2012 ), Tanyag ( 2018 ), and Hamza et al. ( 2017 ). In addition, politics and policy issues regarding climate migration were discussed by scholars; however, no country-specific policies were found during the review that considered both the origin and host communities of climate migrants.

Campbell ( 2014 ) argues that there is insufficient empirical evidence within climate migration research. However, this review reveals that research in this area has been undertaken using a range of methodologies, from qualitative (review, case study, interview, focus group discussion etc.) to quantitative (based on survey data, secondary data, historical data, and remote sensing data), which has produced a strong foundation of work to guide future pathways for interdisciplinary climate migration research. A significant proportion of the research to date has been review-based. Also, there is a lack of empirical studies in this research field that consider the application of geographic information system and remote sensing.

It is clear from reviewing the triggering forces of climate-induced migration literature that climatic events are dominantly responsible for climate migration, which is supported by Rahman and Gain ( 2020 ), Connell and Lutkehaus ( 2017 ), Gemenne ( 2015 ), and Kniveton et al. ( 2012 ). Despite this, there are some other influencing push and/or pull factors such as socio-economic, political, cultural, etc., which are likely to compound (or be compounded by) climate impacts, to trigger the migration process (Black et al. 2011 ; de Haas 2011 , 2021 ; Fussell et al. 2014 ). While there remains ample anecdotal evidence of the relationship between climate change impacts and migration, the specific reasons for people to decide to migrate are interwoven with indirect pressures, such as livelihood disruption, poverty, war, or disaster (Werz and Hoffman 2016 ). Moreover, why people choose to stay at their places is also essential in the context of creeping environmental and climate-induced migration (Mallick and Schanze 2020 ).

One of the other key issues reviewed in this study is that the literature to date fails to build an understanding of the impacts of climate migration on both the origin (source regions) and destination of the climate migrants. There are very few studies such as Comstock and Cook ( 2018 ), Maurel and Tuccio ( 2016 ), Pryce and Chen ( 2011 ), Rahaman et al. ( 2018 ), Rice et al. ( 2015 ), and Schwan and Yu ( 2017 ) that investigate different aspects of socio-economic impacts (housing, health, social, economic, etc.) of climate migration in the destination region, and this presents a clear gap in knowledge that requires further study. Also, no current research has been identified during the review that focused on the environmental impacts of climate migration.

In addition, this review identifies that there was less attention paid to the impacts of climate migration on host communities compared to displaced populations in their new locations. Given that migration will continue to increase globally, there is likely to be a growing need to understand the range of potential impacts on host communities. Although some countries and regions are developing policies to manage internal migration, there are no formal protection policies for cross-border climate migration (Nishimura 2015 ; OHCHR 2018 ; Olsson 2015 ; Zaman 2021 ). Therefore, policy arrangements for managing the needs of climate displaced people in their new communities need to be developed to account for issues related to impacts, livelihoods, community cohesion, and cultural diversity and values. Future research should address the significant gap in understanding the livelihoods of climate migrants in their cross border or international destination. More specifically, in developed countries where the employment sector is more formalized, there is less room for informal economic practices that are common in developing contexts. More formal employment arrangements make it challenging for migrants to establish new livelihoods, alongside other challenges such as language barriers, and other financial, social, cultural and well-being issues.

Limitations and future research scope

Limitations of this study.

There are some limitations to this systematic review; firstly, this review used ProQuest as the sole database for the analysis, and future work could extend the scope to include other major databases. Secondly, this study only considered English language literature, and there are likely to be significant publications in other languages relating to climate migration that were not included in this analysis. Thirdly, looking at pre-1990 or post-2019 literature could add more exciting findings to the search list, which would provide more informative literature. Finally, the outputs of this review are limited to the nature of the search terms, and thus, if other words or texts such as climate-induced relocation or mobility were used, it might extend the range of the review.

Toward a research agenda for climate migration

This review has highlighted several exciting future research opportunities that will build on the strong foundation of work over the past decades in the field of climate migration studies. These include the following research themes; (i) a richer understanding of the full range of impacts (such as social, economic, environmental, and cultural) of climate migration on host communities; (ii) in-depth analysis of the livelihoods of climate-induced migrants in their new destination; (iii) evidence-based research on internal and international climate migration with their sources; (iv) long-term migration policy development at national, regional, or international levels considering both climate migrants and host communities; (v) scope and application of geographic information systems and remote sensing in this area of research, and (vi) developing sustainable livelihood frameworks for climate migrants. The authors believe that academic contributions to these research themes will drive climate migration challenges toward long-term solutions, particularly in those countries that are going to be hosting increasing numbers of climate migrants in future.

This study aimed to understand the past three decades of academic endeavor on climate migration and to identify the gaps in the existing literature in order to inform a research agenda for future research. Climate change, climate-induced migration, and climate migrants are now considered significant global challenges. Climate migrants are identified as a vulnerable group, and a consideration of issues for this group is essential in addressing the goals of the SDGs and SFDRRR. There is a growing body of knowledge that reflects the global relevance of climate migration as a major current and future challenge (Boncour and Burson 2009 ). Addressing the issues and challenges of this form of migration will improve the survival and certain resettlement rights of climate migrants (Miller 2017 ). Therefore, this review contributes a research agenda for future climate migration studies. This study has revealed a critical need to establish a universally agreed definition of ‘climate-induced migrants’ and ‘climate-induced migration,’ which remains unclear to date. Lack of clarity only acts to reduce the visibility of issues related to climate-induced migration. In addition, there is a crucial need to improve the evidence base for climate-induced migration by improving current global datasets, to inform local, regional, and global policy development. Policies need to be future-looking in preparation for a rapid and significant increase in climate-related migration across the globe, within and across national borders. For instance, it is important for receiving countries to anticipate an upsurge in migration by developing appropriate policies to support new migrants, particularly regarding visa and immigration arrangements. Addressing current gaps in knowledge will lead to improved pathways to manage this global migration challenge, which is now a critical need if we are to achieve a sustainable future in a climate-challenged world.

Data availability

Data are available from the corresponding author upon reasonable request.

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Acknowledgements

The authors would like to thank Dr Douglas Hill, Dr Ashraful Alam and Dr Bishawjit Mallick for their feedback on the initial draft of this article.

This research has been supported by a University of Otago Doctoral Scholarship. Open Access funding is enabled and organized by CAUL and its Member Institutions.

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• Published: 15 May 2024

Macro-scale relationship between body mass and timing of bird migration

• Xiaodan Wang   ORCID: orcid.org/0000-0001-5714-0395 1 , 2 ,
• Marius Somveille 2 ,
• Adriaan M. Dokter   ORCID: orcid.org/0000-0001-6573-066X 3 ,
• Wenhua Cao 1 ,
• Chuyu Cheng 1 ,
• Jiajia Liu   ORCID: orcid.org/0000-0002-1923-5964 1 &
• Zhijun Ma   ORCID: orcid.org/0000-0001-7459-9448 1  

Nature Communications volume  15 , Article number:  4111 ( 2024 ) Cite this article

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• Animal migration
• Conservation biology
• Phylogenetics

Clarifying migration timing and its link with underlying drivers is fundamental to understanding the evolution of bird migration. However, previous studies have focused mainly on environmental drivers such as the latitudes of seasonal distributions and migration distance, while the effect of intrinsic biological traits remains unclear. Here, we compile a global dataset on the annual cycle of migratory birds obtained by tracking 1531 individuals and 177 populations from 186 species, and investigate how body mass, a key intrinsic biological trait, influenced timings of the annual cycle using Bayesian structural equation models. We find that body mass has a strong direct effect on departure date from non-breeding and breeding sites, and indirect effects on arrival date at breeding and non-breeding sites, mainly through its effects on migration distance and a carry-over effect. Our results suggest that environmental factors strongly affect the timing of spring migration, while body mass affects the timing of both spring and autumn migration. Our study provides a new foundation for future research on the causes of species distribution and movement.

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Introduction.

Every year, billions of migratory birds fly thousands or even tens of thousands of kilometres between breeding and non-breeding sites with predictable timing. Bird migration is not only a fascinating natural phenomenon, but also an important global ecological process that involves interaction with local biological communities and promotes the flow of materials, energy, and information along migration routes 1 . The annual cycle of migratory birds is composed of a series of events and biological activities that are strictly timed, generally by internal biological clock 2 , to match the periodical changes in the environment in order to maximize fitness 3 . Clarifying migration timing and its link with underlying drivers is fundamental to understanding the development, maintenance, and evolution of bird migration 4 , 5 as well as to predict the responses of migratory birds to global environmental changes 6 , 7 .

Seasonal changes in environmental conditions are the major external driving force for migration 1 , 8 . Within this context, latitude, which is strongly associated with local temperature and phenology, is a key factor that affects the migration schedule of birds 9 . Because suitable breeding times occur later at higher latitudes, birds that breed at higher latitudes generally depart from non-breeding sites and arrive at breeding sites later than those that breed at lower latitudes 10 , 11 , 12 . The latitude of the non-breeding sites can also affect migration timing 13 , 14 . Birds wintering closer to the equator, where environmental conditions are relatively stable and thus birds sense late external signals to migrate, tend to start their spring migration and arrive at their breeding sites later than those wintering at higher latitudes 14 , 15 . In addition, the geographic locations of breeding and non-breeding sites determine migration distance, which, in turn, affects migration duration 16 and, ultimately, migration timing 17 . With increasing distance, birds spend longer on migratory flight and require more refuelling, thus causing an earlier departure and/or later arrival.

Migration timing can also be influenced by intrinsic biological traits. In particular, body mass affects the physiology, behaviour, and ecology of organisms 18 , 19 , 20 and, thus, is closely associated with migration timing as it can affect the amount of time required for various activities across the annual cycle. An analysis of the scaling of the annual cycle in birds suggests that larger species spend longer periods breeding one brood, which is largely driven by longer incubation periods due to larger eggs 21 , and longer time to grow into adult size 22 . It has also been shown that migration speed (i.e., migration distance divided by total travel time) decreases with increasing body mass during powered flapping flights 23 , 24 , 25 . Thus, the migration of larger species is expected to be subject to stronger time constraints than that of smaller species 26 . Because breeding and migration are all time-consuming activities, with increasing time investment with body mass 21 , we expect that to maximize fitness larger species exhibit earlier migration timing in spring, which has been observed in some shorebird species 27 . Although the duration of the breeding period per brood for larger species is generally longer, smaller species may produce multiple broods during the breeding season when the timing is suitable. Therefore, there might be no consistent effect of body mass on the departure date of migratory birds from their breeding sites 27 .

Moreover, body mass affects the latitudinal distribution of species 28 , and thus can in turn indirectly affect migration timing. Larger species have a better tolerance to cold than smaller species 26 and therefore they can live in colder areas at higher latitudes, following Bergmann’s rule 26 . However, breeding seasons are relatively short at high latitudes, which may limit the ability of larger species to breed there as they tend to have a higher time requirement for the breeding period 21 . We therefore predict that larger species tend to breed at lower latitudes than smaller species, but that they spend the non-breeding season at relatively high latitudes compared to smaller species due to their higher tolerance to colder climate, which would result in a shorter migration distance and migration period.

Although it is widely recognized that bird migration is regulated by both external and internal factors 3 , 16 , 29 , most previous studies on migration timing focused on the influence of environmental factors and on a limited number of species and flyways 12 , 30 . Only a few papers discussed the relationship between body mass and migration timing and they only addressed one-way migration period in a few bird groups 27 , 30 . For example, Zhao et al. found that departure date from the non-breeding sites and arrival date at the breeding sites were negatively related to body mass based on seven species in family Scolopacidae that wintered in Oceania 27 . According to radar observation in the northeastern United States, La Sorte et al. indicated that small-sized nocturnal migrants tended to depart from breeding sites earlier 30 . A comprehensive understanding of the global spatiotemporal dynamics of bird migration and its determinants, including both intrinsic biological traits and external drivers, is still lacking.

Due to the rapid development of tracking technology, the annual schedules of many migratory birds have been accurately described. These data now make it possible for a comprehensive analysis of migration timing and related factors to be conducted in multiple species on a global scale. To fully understand the impacts of intrinsic biological traits and external drivers on the migration timing of birds, we compiled available data on the full annual cycles of 1531 individuals and 177 populations of 186 bird species of 44 families of 19 orders that breed in the Northern Hemisphere from 306 published tracking studies (Fig.  1 , Supplementary Fig.  1 , Supplementary Table  1 and Supplementary Data  1 ). Using Bayesian phylogenetic structural equation models (SEMs), we analyzed the effects of environmental factors (latitude of breeding and non-breeding sites and migration distance) and body mass on the timing of the four key events of the annual cycle of migratory birds: departure from non-breeding sites and arrival at breeding sites during spring migration, and departure from breeding sites and arrival at non-breeding sites during autumn migration.

The figure shows data from 1531 individuals and 177 populations, obtained from 306 studies of 186 species. Each dot represents the median longitude and latitude for each species in each study.

We tested the following four hypotheses: (1) body mass of migratory birds affects the latitudinal distribution of their breeding and non-breeding sites, as well as their migration distance; with larger species tending to breed at low latitudes but wintering at relatively high latitudes, and thus exhibiting shorter migration distances than smaller species; (2) body mass affects migration timing, with larger species exhibiting earlier migration timing in spring; (3) latitudes of breeding and non-breeding sites affect migration timing, with low-latitude breeding sites, high-latitude non-breeding sites, related to earlier migration timing; and (4) migration distance affects migration timing, with shorter migrations related to later migration departure and earlier arrival.

In this work, we investigate the key factors that affect the migration timing of birds. We demonstrate that environmental factors (breeding and non-breeding latitude and migration distance) strongly affect the timing of spring migration, while intrinsic biological trait (body mass) affects the timing of both spring and autumn migration. Our findings highlight the impacts of size-related traits on the spatiotemporal patterns of migratory birds.

Time allocation of migratory birds throughout the annual cycle

Among the 186 species included in this study, the mean dates of departure from non-breeding site, arrival at breeding site, departure from breeding site, and arrival at non-breeding site were 27 March (SD, ±24 days), 2 May (± 27 days), 26 August (± 35 days), and 16 October (± 35 days), respectively (Supplementary Fig.  2 ). The non-breeding period was the longest in duration (mean ± SD, 163 ± 46 days, 44.5% of the total cycle), followed by the breeding period (116 ± 47 days, 31.8%). The total migration period, including both spring and autumn migration, accounted for 23.6% of the annual cycle. The autumn migration period (51 ± 36 days) was generally longer than the spring migration period (35 ± 23 days, paired t test, t  = 29.02, df  = 1707, P  < 0.001, Supplementary Fig.  2 ).

Bayesian linear mixed models indicated that body mass was significantly associated with the length of the breeding and non-breeding periods, with larger birds staying longer on breeding sites (95% credible interval (CI) [0.20, 0.58]) (Supplementary Fig.  3b and 4b ) and a shorter amount of time on non-breeding sites (95% CI [−0.63, −0.18]) (Supplementary Fig.  3d and 4d ). However, body mass did not significantly impact the duration of the spring (95% CI [−0.17, 0.26]) (Supplementary Fig.  3a and 4a ) or autumn (95% CI [−0.15, 0.16]) migration periods (Supplementary Fig.  3c and 4c ).

Effect of body mass on latitudinal distribution

SEM analyses demonstrated a significant correlation between body mass and non-breeding latitudes of the tracked birds, with larger species tending to winter at higher latitudes (95% CI [0.33, 0.78] with full dataset, Figs.  2 and 3 ; 95% CI [0.32, 0.79] with data of birds captured at breeding sites, Supplementary Fig.  5b ). However, body mass did not exhibit a significant latitudinal gradient in the breeding season (95% CI [−0.12, 0.35] with full dataset, Fig.  2 and Supplementary Fig.  6a ; 95% CI [−0.54, 0.23] with data of birds captured at non-breeding and stopover sites, Supplementary Figs.  5a and 6b ). SEM analyses also showed that migration distance was negatively related to body mass (95% CI [−0.57, −0.20], Fig.  2 and Supplementary Fig.  7c ). We found that 34 of the total 186 species in this study had an average latitude of non-breeding sites in south of the equator. However, the average latitudes of non-breeding sites for each species weighing more than 1.1 kg (36 species, including some geese, cranes, storks, and eagles, etc.) were all located north of the equator (Supplementary Fig.  6c ).

Arrows represent the direct effects of variables on the migration timing of birds. Red arrows and values (correlation coefficients) represent positive significant effects (credible intervals excluding zero); blue arrows and values (correlation coefficients) represent negative significant effects (credible intervals excluding zero). The dashed-line arrows represent non‐significant relationships (credible intervals including zero). R 2 m , marginal R square, representing only the variance of the fixed effects, R 2 c , conditional R square, representing both the fixed and random effects. The absolute value of the non-breeding latitude was used as the non-breeding latitude.

Direct effects of body mass on the non-breeding latitude ( a ), where absolute values were used for birds wintering in the Southern Hemisphere, departure date from the non-breeding site ( b ), departure date from the breeding site ( c ), and arrival date at non-breeding site ( d ) of migratory birds. Light bands represent 95% credible intervals. Dots represent each tracked individual or population. The density distribution of each variable is plotted alongside the scatter plots.

Effect of body mass on migration timing

Body mass showed strong direct effects on the departure date from non-breeding and breeding sites, with larger species tending to start migration earlier in spring and later in autumn (Figs.  2 and 3 ; Table  1 ). Moreover, body mass had indirect and negative effects on the arrival date at the breeding site, mediated by migration distance, non-breeding latitude and the date of departure from the non-breeding site (Fig.  2 ; Table  1 ). Additionally, body mass directly affected the arrival date at the non-breeding site and exhibited indirect effects mediated by migration timing in earlier periods (positive effect) and migration distance (negative effect), with larger species tending to end their autumn migration later (Fig.  2 , Supplementary Fig.  8 ; Table  1 ).

Further analysis on Passeriformes (53 tracked species) indicated that breeding latitude but not body mass significantly affected annual migration timing (Supplementary Fig.  9 ).

Effect of geographical distribution on migration timing

SEM analyses showed that higher-latitude breeders generally have later migration timings than lower-latitude breeders throughout the entire annual cycle (Figs.  2 and 4a, b ; Table  1 ). We also found that breeding latitude directly influenced migration timing in spring but indirectly influenced migration timing in autumn (Fig.  2 ; Table  1 ), the latter being attributed to migration distance and carry-over effects of timing in earlier periods (Fig.  2 ). Non-breeding latitude also exerted indirect effects on the timing of arrival at breeding sites and non-breeding sites during migration by affecting migration distance (Fig.  2 ; Table  1 ). Migration distance exerted a direct effect on arrival dates in both spring and autumn migration: the longer the migration distance, the longer the migration periods and the later the date of arrival at the migration destination (Figs.  2 , 4c, d and Supplementary Fig.  3a, c ). However, migration distance had no significant impact on the departure dates from non-breeding or breeding sites (Fig.  2 ; Table  1 ).

The relationships between migration timings in spring and breeding latitudes ( a , b ) and the relationships between arrival dates and migration distances ( c , d ). Light bands represent 95% credible intervals. Dots represent each tracked individual or population. The density distribution of each variable is plotted alongside the scatter plots.

Effects of other variables on migration timing

Carry-over effects affect migration timing within an annual cycle. In particular, departure date strongly impacts arrival date in both spring and autumn migration (Fig.  2 , Supplementary Fig.  8 ). In addition, the SEM model variance explained by phylogenetic relationship and year of tracking was small compared with intraspecific variance or paper ID and species ID (Supplementary Fig.  10 ). Flight mode (soaring or flapping) had no significant impact on migration distance and any timing of the four key events of the annual cycle (Supplementary Fig.  11 ).

When sex was included in the models, it was observed to directly impact the arrival date at the breeding site, with males arriving at breeding sites earlier than females (Supplementary Figs.  12 and 13 ). The impacts of other variables on migration timing were consistent with earlier descriptions when sex was not included in the models (Supplementary Fig.  12 and Supplementary Table  2 ), suggesting the results are robust.

Using the most extensive dataset on bird migration timing examined to date, our results indicate that body mass is an important determinant of bird migration, affecting migration timing directly but also indirectly through influencing latitudinal distribution and migration distance. Our findings highlight that size-related capabilities and constraints strongly influence the spatiotemporal patterns of bird migration across the full annual cycle.

In support to our first hypothesis, we found that body mass does not exhibit a significant latitudinal gradient in migratory birds during their breeding season, according to data of all the birds or data of birds captured outside the breeding sites. This result contrasts with the pattern observed in sedentary birds 31 and other animal groups such as mammals 32 , fish 33 , and marine copepods 34 . This may be because temperatures remain generally mild to warm across the Northern Hemisphere during the breeding season (i.e., northern summer), and migratory birds can avoid cold winter temperatures. Therefore, larger body mass might not be required for adaptation to colder climates at breeding sites in migratory birds. Additionally, the suitable breeding period is short at high latitudes, which limits the breeding of large species that require long breeding periods 10 . To mitigate the time constraint of a short breeding period at high latitudes, some large species, such as geese and swans, adopt a capital breeding strategy and accumulate fuel and nutrients from stopover sites to support breeding activities 35 , thus the time investment at breeding sites is reduced and their chicks can match the seasonal food peak at breeding sites. Such adaptation might explain why we do not see an inverse relationship between body mass and breeding latitude in migratory birds.

During the non-breeding season, we found that the distribution of migratory birds is consistent with Bergmann’s rule as larger species tend to winter at higher latitudes. This result supports our first hypothesis, and it could be due to the higher tolerance of large birds to cold 26 , 28 . Our results also indicate that larger birds tend to migrate shorter distances than smaller birds, which is consistent with their higher time requirements for breeding 36 . Consequently, long-distance migratory birds, especially those wintering in the Southern Hemisphere, tend to be smaller species (Fig.  2 , Supplementary Fig.  6c ), which have relatively lower time and energy requirements for breeding per brood compared with larger species 21 , 36 , potentially allowing them to invest more time and energy in migration activities. This result contrasts with a previous study on the migration of seven species in family Scolopacidae, which found that body mass did not affect migration distance 27 . However these previous results were likely due to the fact that all the species in the study were relatively small-sized species (body mass from 50 to 750 g) with concentrated non-breeding sites in Oceania 27 , thus making the relationship undetectable.

In support to our second hypothesis, we found that larger birds spent longer time at the breeding sites than smaller birds, largely driven by a direct effect of body mass on the departure date from breeding sites, i.e., larger birds start their autumn migration later than smaller birds. We found that body mass also affects the arrival date at breeding sites but this is an indirect effect mediated by migration distance, i.e., larger birds have shorter migration distance which leads to earlier arrival on breeding sites. The smaller, indirect effect of body mass on arrival date at breeding sites is likely affected by constraints due to the prevalence of harsh environmental conditions if they arrive too early 1 . The longer duration in the breeding sites of larger birds was compensated by a shorter duration in the non-breeding site, while migration duration is not related to body mass during either spring or autumn despite larger species migrating shorter distances, which is consistent with that the migration speed of large species is generally slower than that of small species 23 , 24 , 25 . These results suggest that large species increase their investment in time and energy for breeding by reducing the time spent at non-breeding sites. We found that the migration timing of Passeriformes was strongly affected by breeding latitude but not body mass (Supplementary Fig.  9 ). This might be due to the smaller body mass of Passeriformes (<90 g for the 53 species in this study) makes them sensitive to periodical environmental changes, and thus annual migration timing is endogenously controlled 12 . This also suggests the impacts of body mass on migration timing mainly occur among taxonomic groups.

Our results show that birds breeding at higher latitude generally have later migration timing throughout the annual cycle, which supports our third hypothesis and is consistent with previous intraspecific studies that found that breeding latitude is the main driver of migration timing 10 , 12 , 37 , 38 . More specifically, our results indicate that latitude of breeding sites directly and positively influences migration timing in spring, i.e., birds breeding at higher latitude migrate later in the spring, but its influence on migration timing in autumn is indirect and mediated by migration distance and carry-over effects.

We found that migration distance also has a direct effect on timing, with longer-distance migrants terminating their migration later than shorter-distance migrants, which is consistent with our fourth hypothesis. However, in contrast with our hypotheses, non-breeding latitude did not affect migration timing directly and migration distance did not affect departure dates. This could be partially due to effects going in opposite directions cancelling each other: birds wintering at low latitude are expected to depart late but they also tend to migrate longer distances, which is expected to lead to earlier departure, thus potentially explaining why we did not find any effect.

This study illustrates how environmental factors (latitude and distance), intrinsic biological traits (body mass), and their interaction can influence key timings in the annual cycle of migratory birds. In particular, our results suggest that external factors strongly constrain the timing of spring migration as migratory birds must time their arrival on their breeding sites to match favourable environmental conditions and peak in food supply, while body mass affects the timing of both spring and autumn migration as larger birds require more time to breed. In response to global warming, many migratory birds advance migration timing in spring. In addition, some species are showing shrinking body sizes 39 , 40 , shifts in their breeding and non-breeding ranges 41 , 42 and corresponding changes in migration distances. Our results suggest that all these changes can also affect migration timing both directly and indirectly, and this work therefore provides a reference for further studies investigating how the migration timing of migratory animals is adjusted to adapt to climate change.

Literature search

We conducted a search for all papers that were published between 1 January 1900 and 1 January 2022 and that described the migration timing of individual birds. In the Web of Science (All Databases) we used the search terms “TS = (GPS OR PTT OR satellite OR geolocat*) AND (bird* OR aves OR avian) AND migrat*) AND PY= (1900-2021)”, and in Scopus we used the search terms “TITLE-ABS KEY (gps OR ptt OR satellite OR geolocat*) AND TITLE-ABS KEY (bird* OR aves OR avian) AND TITLE-ABS-KEY (migrat*)) AND PUBYEAR < 2022”. A total of 2706 papers were retrieved after duplicates were removed. We examined the title and abstract of each paper to exclude irrelevant studies (2194), such as studies on habitat use, local movements, home ranges or the development of transmitters. We then read the full text of each remaining paper and extracted the studies that tracked bird migration for at least one year (including both northward and southward migration) with complete data of all four key events of the annual cycle (departure from non-breeding sites, arrival at breeding sites, departure from breeding sites, and arrival at non-breeding sites). If multiple papers used the same data sources, only the earliest published paper was included. We collected an additional 30 papers by checking the references of the searched papers. Finally, a total of 306 papers were included in our database. A flowchart of the selection process is shown in Supplementary Fig.  1 .

Data collection

We extracted the following complete data from each selected paper: species and/or subspecies name, individual identification, geographical coordinates of breeding and non-breeding sites, departure date from non-breeding site, arrival date at breeding site, departure date from breeding site, arrival date at non-breeding site, capture site and year when tracking was conducted. We also recorded the sex of individuals if the information was provided. Because birds of different ages show differences in migration timing 43 , 44 , 45 , we only collected data for adult birds, excluding data for juveniles and subadults. We excluded birds breeding in the Southern Hemisphere because they have different migration patterns from those breeding in the Northern Hemisphere. Birds breeding in the tropics as well as seabirds (e.g., penguins, petrels, pelagic gulls and terns, and auks) were also excluded from the database because their migration behaviour often lacks regular seasonal variation 46 . However, we included four species of gulls and three species of terns that often move in inland and coastal regions (Supplementary Data  1 ). If the data were depicted only in figures in the original studies, we used GetData Graph Digitizer (V2.26, www.getdata-graph-digitizer.com ) to obtain original values, including the migration schedules and geographical coordinates of breeding and non-breeding sites. Some studies reported migration timing only at the population level and not at the individual level; for those, we extracted population-level data to represent a proxy for individual-level data. We generalized both individual-level data (1531) and population-level data (177) as “tracking data” in the description of this paper. Overall, in our analysis, we used 1708 full annual tracking data of 186 species in 44 families and 19 orders (Supplementary Table  1 and Supplementary Data  1 ). The 186 species were 12.0% of the total 1550 migratory species breeding in the Northern Hemisphere 47 .

Some individuals stayed at more than one non-breeding site within one season. For those individuals, we used the first non-breeding site they visited and the date of arrival at that site as the non-breeding site and the arrival date for the autumn migration, respectively, and we used the last non-breeding site they visited and the date of departure from that site as the non-breeding site and the departure date from the non-breeding site in the spring migration. The period between arrival at the first non-breeding site and departure from the last non-breeding site was used as the non-breeding period. We defined the distance between the breeding site and the first non-breeding site as the migration distance in autumn and the distance between the last non-breeding site and the breeding site as the migration distance in spring. We calculated the great circle distance between the breeding and non-breeding sites as the migration distance for each individual using the distm function in the Geosphere package in R 48 . For the studies that reported migration and distribution data only at the population level, we used the mean data at the population level.

The body mass of migratory birds varies throughout the year, especially in long-distance migratory species, due to the large amount of fuel deposition and consumption in the migration season 49 . We used the minimum body masses, which are equivalent to the lean body mass, of females and males at the species or subspecies level, obtained from 50 , as the body mass of each individual in this study. If sex was not reported in the study, we used the average of the minimum body masses of both sexes. The body mass of the bird species included in this paper ranged from 8.7 g to 7600 g, covering most of the body mass range of all migratory bird species in the world (Supplementary Fig.  14 ).

Data analysis

To detect the time allocation of migratory birds throughout the annual cycle, we combined both tracking data reported at the individual level (1531) and population average (177) of a total of 186 species as “tracking data” (1708) and calculated the mean migration timing for each species and then the mean across species of the four key events of the annual cycle (departure from non-breeding sites, arrival at breeding sites, departure from breeding sites, and arrival at non-breeding sites). We defined the period between departure from the non-breeding sites and arrival at the breeding sites as the spring migration period, the period between arrival at the breeding sites and departure from the breeding sites as the breeding period, the period between departure from the breeding sites and arrival at the non-breeding sites as the autumn migration period, and the period between arrival at the non-breeding site and departure from the non-breeding sites as the non-breeding period.

To test the effects of geographical factors (latitude of breeding and non-breeding sites and migration distance) and body mass on the timing of the four key events of the annual cycle of migratory birds, we performed Bayesian phylogenetic piecewise structural equation modelling (SEM) 51 on individual tracking data using the brms package 52 , 53 . The SEM was constructed according to the hypothesized relationships among the variables below:

The latitudes of breeding and non-breeding sites determine the phenology and the migration distance, which is closely related to migration duration, and thus directly 10 and indirectly (via migration distance) 54 influences the timing of the start and end of both northward and southward migration.

Body mass is related to the latitude of breeding and non-breeding sites following Bergmann’s rule 26 ; body mass also impacts the cost of migration 24 . Consequently, body mass can directly and indirectly affect migration distance 26 , 55 and the timing of the start and end of migration 27 .

The timing at earlier phases of the annual cycle influences the timing at later phases 14 , so the migration timing at the earlier phases of the same migration cycle was included as an explanatory variable in the models. Birds can reset migration timing during the non-breeding period 56 , so the migration timing at the earlier annual cycle has no carry-over effects on the migration timing at the following annual cycle.

Because the latitude of the non-breeding site had a strong impact on migration distance (95% CI [−0.89, −0.84]) (the farther south the non-breeding site was, the longer the migration distance was) (Supplementary Fig.  15 ), the absolute value of the latitudes of non-breeding sites in Southern Hemisphere was used in the SEM to avoid the issue of multicollinearity. To address the issue of nonrandom sampling in analyzing the relationship between body mass and breeding/non-breeding latitudes, we further assessed the relationship between body mass and breeding latitude by using the combined data of birds captured at stopover and non-breeding sites. We also assessed the relationship between body mass and non-breeding latitude by using the data of birds captured at breeding sites. Considering that flight mode is related with energetic consumption during flight and thus can affect migration ranges and migration timing 57 , we classified flight mode into soaring and flapping 24 and included flight mode in the SEMs.

To control for the potential effects of phylogenetic relatedness on migration timing, phylogenetic relationship among species was included in the models as a random effect. We built a maximum clade credibility tree derived from the phylogenetic tree distributions 58 . We pruned the global phylogenetic tree of birds from BirdTree.org using the option ‘Hackett All Species: a set of 10000 trees with 9993 OTUs each’ to download bird species datasets 59 . We randomly sampled 1000 pseudo-posterior distributions and then constructed the maximum clade credibility tree using common ancestor heights with TreeAnnotator software from the BEAST package 60 , 61 . We calculated phylogenetic similarity at the species level with a variance-covariance matrix and included “paper” nested in “species” as a nested random effect in these models. We also used year as a random effect to control its potential effect on migration timing.

We used Bayesian linear mixed models to determine the effect of body mass on the durations of the four periods (breeding, non-breeding, spring and autumn migration) of migratory birds using the brms package 52 . Breeding latitude, non-breeding latitude and migration distance were also included in the models as explanatory variables; phylogenetic relationship among species (as mentioned above), year of tracking, paper ID and species ID were included in the models as random effects.

All the explanatory variables were scaled (mean zero, unit variance) prior to analysis. We did not detect strong multicollinearity among the variables in the analysis (all variance inflation factors (VIFs) < 2). We ran four Markov chains with 10,000 iterations and a burn-in of the first 1000 iterations per chain to contribute to summarizing the posterior distribution of estimation. Using the standard priors, the model converged with Rubin–Gelman statistics (<1.1) according to Gelman and Rubin’s diagnostic 62 model assessment was performed using approximate leave-one-out cross-validation (LOOIC) in the loo package of R 63 . We also used the pp_check function to assess the validity of Bayesian SEM model with posterior predictive checks in brms package 52 (Supplementary Fig.  16 ). To confirm causal pathways, we performed piecewise structural equation modelling using piecewiseSEM package before performing Bayesian SEM model 53 . This ensured results robustness in Bayesian SEM. The coefficient estimates of population-level effects with a 95% credible interval did not overlap zero.

Sex can influence migration timing 64 , the selection of non-breeding sites 65 and, thus, migration distance due to differences in the roles in reproductive activities and resource competition between the males and the females. Hence, we further used SEM to investigate whether migration timing varied between sexes across the annual cycle using a dataset including data from 1077 individuals, accounting for 63% of the full dataset, after removing records with “unknown sex”.

Reporting summary

Further information on research design is available in the  Nature Portfolio Reporting Summary linked to this article.

Data availability

The data in this paper come from published papers, which are listed in Supplementary Data  1 . All migration timing data used for the analyses are available in an online repository at Figshre 66 . The body mass data is available from the Handbook of Avian Body Masses 50 .  Source data are provided with this paper.

Code availability

The data analysis codes used in this study are all open-source software, which is available in an online repository at Figshare 66 .

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Acknowledgements

This study was financially supported by the National Key Research and Development Program of China (grant number 2023YFF1304504) and the National Natural Science Foundation of China (grant numbers 31830089 and 31772467). We thank Chenchen Feng, Shaoping Zang, and Qianyan Zhou for their assistance in checking and correcting data. We thank all researchers contributing to bird tracking and sharing tracking data.

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Xiaodan Wang, Wenhua Cao, Chuyu Cheng, Jiajia Liu & Zhijun Ma

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Z.M. and X.W. conceived the project. X.W., W.C. and C.C. collected the migration dataset. X.W. performed the analyses. X.W., M.S., A.D., W.C., C.C., J.L. and Z.M. contributed to the first draft and the revisions.

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Wang, X., Somveille, M., Dokter, A.M. et al. Macro-scale relationship between body mass and timing of bird migration. Nat Commun 15 , 4111 (2024). https://doi.org/10.1038/s41467-024-48248-7

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Work from Home and Interstate Migration

Working Paper 2024-012A by Alexander Bick, Adam Blandin, Karel Mertens, and Hannah Rubinton

Interstate migration by working-age adults in the US declined substantially during the Great Recession and remained subdued through 2019. We document that interstate migration rose sharply following the 2020 Covid-19 outbreak, nearly recovering to pre-Great recession levels, and provide evidence that this reversal was primarily driven by the rise in work from home (WFH). Before the pandemic, interstate migration by WFH workers was consistently 50% higher than for commuters. Since the Covid-19 outbreak, this migration gap persisted while the WFH share tripled. Using quasi-panel data and plausibly exogenous changes in employer WFH policies, we address concerns about omitted variables or reverse causality and conclude that access to WFH induces greater interstate migration. An aggregate accounting exercise suggests that over half of the rise in interstate migration since 2019 can be accounted for by the rise in the WFH share. Moreover, both actual WFH and pre-pandemic WFH potential, based on occupation shares, can account for a sizable share of cross-state variation in migration.

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https://doi.org/10.20955/wp.2024.012

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Study explains why the brain can robustly recognize images, even without color

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Even though the human visual system has sophisticated machinery for processing color, the brain has no problem recognizing objects in black-and-white images. A new study from MIT offers a possible explanation for how the brain comes to be so adept at identifying both color and color-degraded images.

Using experimental data and computational modeling, the researchers found evidence suggesting the roots of this ability may lie in development. Early in life, when newborns receive strongly limited color information, the brain is forced to learn to distinguish objects based on their luminance, or intensity of light they emit, rather than their color. Later in life, when the retina and cortex are better equipped to process colors, the brain incorporates color information as well but also maintains its previously acquired ability to recognize images without critical reliance on color cues.

The findings are consistent with previous work showing that initially degraded visual and auditory input can actually be beneficial to the early development of perceptual systems.

“This general idea, that there is something important about the initial limitations that we have in our perceptual system, transcends color vision and visual acuity. Some of the work that our lab has done in the context of audition also suggests that there’s something important about placing limits on the richness of information that the neonatal system is initially exposed to,” says Pawan Sinha, a professor of brain and cognitive sciences at MIT and the senior author of the study.

The findings also help to explain why children who are born blind but have their vision restored later in life, through the removal of congenital cataracts, have much more difficulty identifying objects presented in black and white. Those children, who receive rich color input as soon as their sight is restored, may develop an overreliance on color that makes them much less resilient to changes or removal of color information.

MIT postdocs Marin Vogelsang and Lukas Vogelsang, and Project Prakash research scientist Priti Gupta, are the lead authors of the study, which appears today in Science . Sidney Diamond, a retired neurologist who is now an MIT research affiliate, and additional members of the Project Prakash team are also authors of the paper.

Seeing in black and white

The researchers’ exploration of how early experience with color affects later object recognition grew out of a simple observation from a study of children who had their sight restored after being born with congenital cataracts. In 2005, Sinha launched Project Prakash (the Sanskrit word for “light”), an effort in India to identify and treat children with reversible forms of vision loss.

Many of those children suffer from blindness due to dense bilateral cataracts. This condition often goes untreated in India, which has the world’s largest population of blind children, estimated between 200,000 and 700,000.

Children who receive treatment through Project Prakash may also participate in studies of their visual development, many of which have helped scientists learn more about how the brain's organization changes following restoration of sight, how the brain estimates brightness, and other phenomena related to vision.

In this study, Sinha and his colleagues gave children a simple test of object recognition, presenting both color and black-and-white images. For children born with normal sight, converting color images to grayscale had no effect at all on their ability to recognize the depicted object. However, when children who underwent cataract removal were presented with black-and-white images, their performance dropped significantly.

This led the researchers to hypothesize that the nature of visual inputs children are exposed to early in life may play a crucial role in shaping resilience to color changes and the ability to identify objects presented in black-and-white images. In normally sighted newborns, retinal cone cells are not well-developed at birth, resulting in babies having poor visual acuity and poor color vision. Over the first years of life, their vision improves markedly as the cone system develops.

Because the immature visual system receives significantly reduced color information, the researchers hypothesized that during this time, the baby brain is forced to gain proficiency at recognizing images with reduced color cues. Additionally, they proposed, children who are born with cataracts and have them removed later may learn to rely too much on color cues when identifying objects, because, as they experimentally demonstrated in the paper, with mature retinas, they commence their post-operative journeys with good color vision.

To rigorously test that hypothesis, the researchers used a standard convolutional neural network, AlexNet, as a computational model of vision. They trained the network to recognize objects, giving it different types of input during training. As part of one training regimen, they initially showed the model grayscale images only, then introduced color images later on. This roughly mimics the developmental progression of chromatic enrichment as babies’ eyesight matures over the first years of life.

Another training regimen comprised only color images. This approximates the experience of the Project Prakash children, because they can process full color information as soon as their cataracts are removed.

The researchers found that the developmentally inspired model could accurately recognize objects in either type of image and was also resilient to other color manipulations. However, the Prakash-proxy model trained only on color images did not show good generalization to grayscale or hue-manipulated images.

“What happens is that this Prakash-like model is very good with colored images, but it’s very poor with anything else. When not starting out with initially color-degraded training, these models just don’t generalize, perhaps because of their over-reliance on specific color cues,” Lukas Vogelsang says.

The robust generalization of the developmentally inspired model is not merely a consequence of it having been trained on both color and grayscale images; the temporal ordering of these images makes a big difference. Another object-recognition model that was trained on color images first, followed by grayscale images, did not do as well at identifying black-and-white objects.

“It’s not just the steps of the developmental choreography that are important, but also the order in which they are played out,” Sinha says.

The advantages of limited sensory input

By analyzing the internal organization of the models, the researchers found that those that begin with grayscale inputs learn to rely on luminance to identify objects. Once they begin receiving color input, they don’t change their approach very much, since they’ve already learned a strategy that works well. Models that began with color images did shift their approach once grayscale images were introduced, but could not shift enough to make them as accurate as the models that were given grayscale images first.

A similar phenomenon may occur in the human brain, which has more plasticity early in life, and can easily learn to identify objects based on their luminance alone. Early in life, the paucity of color information may in fact be beneficial to the developing brain, as it learns to identify objects based on sparse information.

“As a newborn, the normally sighted child is deprived, in a certain sense, of color vision. And that turns out to be an advantage,” Diamond says.

Researchers in Sinha’s lab have observed that limitations in early sensory input can also benefit other aspects of vision, as well as the auditory system. In 2022, they used computational models to show that early exposure to only low-frequency sounds, similar to those that babies hear in the womb, improves performance on auditory tasks that require analyzing sounds over a longer period of time, such as recognizing emotions. They now plan to explore whether this phenomenon extends to other aspects of development, such as language acquisition.

The research was funded by the National Eye Institute of NIH and the Intelligence Advanced Research Projects Activity.

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The Effects of Immigration in a Developing Country: Brazil in the Age of Mass Migration

We study the effects of immigration in Brazil during the Age of Mass Migration, focusing on the country's agricultural sector in 1920. This context combines the widely recognized value of historical perspective in studies of the effects of immigration with Brazil's unique position among major immigrant destinations of the period as a low-income country with a large agricultural sector and weak institutions to shed light on the effect of immigration in countries at an early stage of development. Instrumenting for a municipality's immigrant share using the interaction of aggregate immigrant inflows and the expansion of Brazil's railway network, we find that a greater immigrant share in a municipality led to an increase in farm values and that the bulk of the effect was the product of more intense cultivation of land. Finally, we find that it is unlikely that immigration's effect on agriculture slowed Brazil's structural transformation.

For helpful comments, we thank Leah Boustan, William Collins, Martín Fernández-Sánchez, Gordon Hanson, Richard Hornbeck, David Jaeger, Brian Marein, Aldo Musacchio, Blanca Sanchez-Alonso, Sandra Sequeira, Koleman Strumpf, Marco Tabellini, and João Tampellini; seminar participants at the University of Bonn, University of York, University of Newcastle, Wageningen University, Wake Forest University, and Vanderbilt University; and conference participants at the 2021 Markets and Policy in History Workshop, the 2022 Economic History Society Conference, the 2022 European Historical Economics Society Conference, the 2022 World Economic History Congress, and the 2024 LACEA-EHN Workshop on Historical Development. Jared Katz and Víctor Luna provided excellent research assistance. Renato Colistete kindly shared data with us. Financial support was provided by Vanderbilt University and the University of St. Andrews. For the purpose of open access, the authors have applied a creative commons attribution (CC BY) licence to any author accepted manuscript version arising. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research.

MARC RIS BibTeΧ

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