Human Migration

Human migration denotes any movement by human beings from one locality to another, often over long distances or in large groups. Its participants are called migrants, and depending on setting and perspective they are also described as emigrants, immigrants, or settlers. Migration is distinct from a single trip or daily commute in that it usually involves a change of residence, whether permanent or long-term, and it can be internal, moving within a country such as from the countryside to the city, or international, crossing state borders and therefore falling under sovereignty and international-law.

Movement is among the oldest features of human-evolution. Early members of the genus Homo spread out of Africa and across Eurasia, and later expansions by farmers, seafarers, pastoralists, traders, conquerors and refugees reshaped continents and the linguistic, ethnic and genetic composition of their populations. Migration remains central to contemporary demography and economics: international migrants numbered roughly 304 million in 2024, and far larger numbers are displaced inside their own countries. These flows affect labor markets, urbanization, public finance, cultural life and the politics of admission and citizenship in nearly every country.

This entry first sets out the terminology and the kinds of evidence used to study past movements, then traces the history of migration from the out-of-Africa dispersal through agricultural, imperial, colonial and industrial periods. It then examines the typology of migration, its main causes including forced displacement and environmental drivers, the leading theoretical models, current statistics, and the economic, cultural and governance consequences. The entry closes with unresolved debates about prehistory, causation and data.

Definition and basic terminology

Migration is, in its most basic sense, a movement of people that changes where they live. Demographers generally use the term for relocations that establish a new usual residence, whether across a state border or within one. A change of residence is the defining feature: Migration is the movement of people away from their usual place of residence to a new place of residence, either across an international border or within a State. Because there is no single accepted definition across disciplines, scholars and agencies tend to specify their own criteria, and the precise cut-off between migration and other mobility depends on those choices.

The distinction that matters most is between a move that alters residence and movements that leave it intact. Seasonal labour, daily commuting, and repeated travel between two homes are temporary movements that do not establish a new base. Circular migration is the category closest to the boundary. It is usually described as the fluid movement of people between areas, usually for the purpose of employment. Its defining features include temporariness, repetition, and legal and rights-based conditions, which means it is classified by how it recurs rather than by a single relocation.

Several terms describe the same people from different positions. "Migrant" is the broadest label and is used in everyday and policy language for anyone who moves away from a usual residence. "Emigrant" and "immigrant" describe one and the same movement seen from the origin and the destination. The word "settler" carries a historical and political charge. In colonial contexts it referred to people who moved to establish permanent residence, often on land taken from or already occupied by others, and in postcolonial and indigenous-rights debates it remains contested. The older term "alien" is still found in legal texts, but the IOM glossary notes that it tends to carry a negative connotation, and other terms should be preferred, such as "non-nationals" or "migrants". The terms also depend on who is doing the classifying. A person may be a "migrant" to a state's statistical office, a "refugee" under international law, and an "economic migrant" in political rhetoric, and each label implies different legal entitlements.

The state boundary is the central divide between internal and international migration. Internal migration is the movement of people within a single country, including rural–urban shifts, and it is far more common than cross-border movement. Internal migration is the dominant form of human migration globally, although it receives less attention in policy and statistics. International migration matters because crossing a border brings the person under another state's authority over admission, residence, work, and eventual citizenship. The boundary therefore determines which legal regime applies, such as visas, asylum procedures, and protection against expulsion. It also determines which statistics are collected. Border-crossing records, foreign-born censuses, and passport data capture international flows that internal population registers do not. The boundary does not always align with the cause of a move. Internally displaced persons have been forced from their homes but have not crossed an internationally recognized border, so they are counted as internal and fall outside the refugee framework.

Measurement conventions shape the numbers that appear in public debate. The United Nations' 1998 recommendations define an international migrant as any person who changes his or her country of usual residence. The UN distinguishes long-term from short-term migrants. The former change residence for at least a year, while the latter do so for at least three months but less than a year. A person's usual residence is the country in which the person has a place to live where he or she normally spends the daily period of rest. The UN's global migrant stock estimates, which count people present in a country at a given time, rely on birth rather than citizenship wherever possible. When data on country of birth are available, they are given precedence because such data reflect a change in the country of residence more accurately than do data on country of citizenship. Citizenship is used only where birth data are missing. This convention has consequences. Under citizenship-based counting, children of migrants born in countries that grant citizenship by descent are counted as migrants, whereas those born in countries that use birthplace-based citizenship are not. People born abroad who later naturalize drop out of the count altogether. Purpose and legal status, such as whether a move is for work, study, family, or asylum, and whether it is regular or irregular, are not part of the basic UN definition, which is why these categories must be measured separately.

Sources of evidence about past migrations

Archaeology

Archaeology reconstructs past movements from material remains: settlement sites, hearths, burials, tools and pottery. When a new site appears in a region with a distinctive tool kit or ceramic style, researchers may infer that a population arrived there. Stone tools and pottery are useful because their forms, decoration and manufacturing methods vary across space and time. Still, a shared style does not automatically prove a shared population, since objects can also spread through exchange, imitation or the adoption of a technology. Around the Pacific, for example, Austronesian studies compare red-slipped pottery and Taiwanese-sourced nephrite with other artifacts such as cord-marked pottery, and some scholars argue that the spread of such items reflects diffusion of culture more than large-scale movement of people.

Dating turns these finds into a sequence. Radiocarbon dating works on organic material, luminescence dating on buried sediments, and isotopic methods on volcanic and cave deposits, and each has its own time range and margin of error. Because dates for different sites often rest on different methods, comparing the timing of a dispersal across regions requires careful calibration.

Genetics

Genetic evidence works on different timescales and from a different kind of record. Mitochondrial DNA (mtDNA) passes from mother to child, while the Y chromosome passes from father to son, so each traces only one line of descent. Because their mutations accumulate in branching patterns, the resulting family trees (phylogenies) preserve a rough chronological record of how lineages separated and spread. The Y chromosome is especially informative because it carries far more sites than the roughly 16,000 base pairs of mtDNA, which helps distinguish populations. In 1987 a study of mtDNA led to the proposal that all living humans trace their maternal ancestry to a population in Africa around 200,000 years ago, a finding later refined by subsequent work.

Ancient DNA has added direct samples from past individuals, so researchers no longer rely only on the genomes of living people. The Xiaohe cemetery in the Tarim Basin, for example, yielded remains whose mtDNA included both East Eurasian and West Eurasian lineages, while the sampled males carried only a West Eurasian Y lineage. Such mixed profiles point to past contact between groups. Admixture analysis takes this further by estimating when and from where gene flow entered a population. A study of Yemen found that its present-day populations received their initial gene flow from Arabia and the Levant, with a later and smaller contribution from East Africa. The same study noted that mtDNA showed less differentiation between Arabian and North African groups than the Y-chromosome data suggested, a reminder that different markers can point in different directions.

Genetics can also fill gaps left by the archaeological record. Fossils are rare and often fragmentary, and the earliest movement from Africa to Australia is visible in mitochondrial and Y-chromosome lineages even though physical artifacts along much of that route are largely missing.

Historical linguistics

Historical linguistics infers dispersals from the geographical distribution of language families and from the reconstruction of ancestral languages. Once related languages are grouped into a family tree, scholars use the tree's branching order and the vocabulary shared by each branch to locate a probable homeland. For the Austronesian family, which stretches from Easter Island to Madagascar, linguistic subgrouping points to Taiwan as the origin, and archaeological and genetic work has been read as broadly consistent with that view. The fine-grained detail remains contested. A recent linguistic argument, based on a grammatical affix, located the starting point of the expansion among the Amis people of eastern Taiwan, and it also highlighted a roughly thousand-year gap between the archaeological and linguistic timelines that needed explanation.

Reconstructed vocabulary can also reveal what ancestral speakers knew. For the Bantu expansion across sub-Saharan Africa, the reconstructed lexicon indicates that ancestral speakers had pottery-related terms before they left the Nigeria-Cameroon border region, roughly 5,000 to 4,000 years ago. Some scholars have also argued that reconstructed words for crops show that Bantu speakers farmed by the time they settled south of the Congo rainforest. Linguistic methods have limits, however. Early attempts that relied on glottochronology, a method based on the rate of vocabulary replacement, have been widely rejected, and one analysis concluded that neither linguistic nor archaeological data conclusively support the idea that farming and language spread together in the initial dispersal in West-Central Africa.

Documentary and quantitative sources

Written sources supply dates and named actors, but each has its own limits. Chronicles are usually composed by outsiders or elites, cover only some regions and events, and may be shaped by political purposes. Port, passenger and migration records capture formal movements but miss undocumented crossings and many short-distance moves. Censuses and population registers offer systematic counts, yet their coverage, definitions and categories change over time, and their undercounts are often greatest for mobile or marginal groups. In the Austronesian case, Arabic chronicles record the presence of Austronesian-speaking peoples in the Persian Gulf with secure dates, while the movement of Javanese to Suriname is documented well enough that descendants can still be interviewed. Researchers therefore assign different levels of chronological certainty to different expansions.

Combining sources is the usual method. A study of pottery in northern Botswana, for instance, used petrographic analysis of ceramic fabrics together with historical linguistics to track the arrival of pottery-using migrants between the first and fourth centuries CE, and proposed that pottery evidence might reflect the convergence of two branches of Bantu languages. Pottery in this sense serves as an interface between archaeology and linguistics, because the same object can be read through both disciplines. Agreement across independent sources strengthens an inference; disagreement, such as a mismatch between a linguistic tree and an archaeological sequence, usually prompts a revision of one of the models.

Early Homo and the out-of-Africa dispersal

The oldest well-dated hominin remains outside Africa come from Dmanisi in Georgia, where stone tools and fossils fall between about 1.85 and 1.77 million years ago, making the Dmanisi hominins the earliest well-dated hominin fossils in Eurasia. For decades the standard account credited a single species, Homo erectus, with this first exit from Africa around 1.8 million years ago. A 2025 study in PLOS ONE challenged that picture. Its authors measured tooth crown areas and argued that the Dmanisi material represents two taxa, often classified as Homo georgicus, representing the more primitive form, and Homo caucasi, representing the more human-like species. The taxonomy remains contested: some researchers attribute the large size differences among the skulls to sexual dimorphism within one species. The site's classification is controversial and disputed, but it has shifted attention toward the possibility of several hominin groups moving out of Africa together.

Homo erectus fossils later appear farther east. We also have evidence of H. erectus from sites in eastern Asia, including Indonesia and China. In China the species appears to have been present until at least 300,000 years ago. Its presence in Europe is far less certain. We don't know whether H. erectus reached Europe. But since the assortment of animal fossils found at Dmanisi closely resembles the forest and grassland fauna of southern Europe from the same time, an early human settlement of that region is at least a possibility. The likely route into Asia ran through northern Africa and across the Sinai Peninsula, when environmental changes meant suitable habitats and food sources stretched that far.

The dispersal of Homo sapiens is much harder to date. Fossils show that the fossil record indicates repeated Homo sapiens presence in Eurasia from Marine Isotope Stage (MIS) 8 (300–243 ka) onwards, which points to earlier and intermittent movements. Genetic evidence, however, places the decisive expansion later: over 90% of non-African ancestry is derived from a single late dispersal during MIS4 (71–57 ka), with the ancestors of Sahul populations arriving in MIS3. Estimates for an earlier departure around 125,000 years ago, proposed on the basis of southern Asian archaeology, remain in the literature, so the chronology is still debated.

Contact with archaic populations was part of this process. Neanderthal interbreeding is older than the main dispersal: analysis of a Neanderthal leg bone found in Germany indicates breeding between early Homo sapiens and Neanderthals around 270,000 years ago. The later and more consequential event is recorded in the genomes of people outside Africa. Genomic data suggest that this Neanderthal inheritance came from a single interbreeding event between Homo sapiens and Neanderthals somewhere between 50,500 and 43,500 years ago, most likely in Eurasia. Genomes of present-day Papuans and Aboriginal Australians also carry a further layer of archaic ancestry: a further 5% of archaic human DNA with more local origins – the results of interbreeding in Southeast Asia and perhaps even in Sahul itself. Denisovans appear in this record as well, since the latest findings show that this population also interbred with other human species such as Neanderthals and Denisovans as they spread out.

Reaching Sahul, the combined landmass of Australia, New Guinea and Tasmania during periods of low sea level, required sea crossings. Even with lowered sea levels around 60,000 years ago, that second group must have crossed at least 60 miles (100km) of open sea to reach Sahul – some of the earliest evidence we have for human seafaring. The arrival date is contested. Madjedbebe, a rock shelter in northern Australia, has an OSL chronology that places the oldest occupation at 65,000 years ago, but critics have argued that the OSL determinations were affected by termite activity. Genetic timelines pull the estimate toward later dates, and the earliest Australians, they argue, likely didn't arrive until closer to 50,000 years ago.

Explanations for these movements rest on climate, glacial cycles and resource-following, and each is tested against different evidence. The most studied climatic mechanism is the cycle of "Green Sahara" periods. Compilations of continental and marine paleoenvironmental records from within and around North Africa enable identification of over 230 GSPs within the last 8 million years. These humid phases recurred every 21,000 years and were determined by changes in Earth's orbital precession. Glacial ice sheets, by contrast, suppressed the monsoon, so the humid periods did not occur during the ice ages. Evidence for a green corridor during the Homo erectus exit comes from carbon isotopes in Saharan sediments, which show that the Sahara, precisely in the period when the first Homo erectus migrated, was greener than at any other time in the 4.5 million year period we studied. Speleothem records provide independent support for this picture. Their interpretation depends on the chronology of each record, and the North African sites that might test the corridor directly are in poor stratigraphic context.

Prey-following is the least direct explanation. Remains of sabre-toothed cats at Dmanisi suggest that carnivores moved with the same resources. These specialised carnivores lacked the teeth to strip a carcass clean of its meat, so might have provided scavenging opportunities for early humans following them out of Africa. Its support is inferential, based on co-occurrence in one site rather than on repeated patterns across many sites.

Farming, seafaring and pastoralism

The Neolithic spread and the demic–cultural debate

The spread of farming from its early centres in the Near East is usually described through two competing mechanisms. The first one, demic diffusion, refers to the dispersal of farming populations. The second one, cultural diffusion, refers to the incorporation of hunter–gatherers into the farming communities (via interbreeding and/or acculturation). Early archaeological work in Europe, including Ammerman and Cavalli-Sforza's estimate of a front moving at roughly 1 km per year, was read as favouring demic spread, though they suggested that there had also been some degree of cultural diffusion.

Ancient genomes have shifted the balance in European cases. Early farmers from Germany, Hungary and Spain carry a Near Eastern Anatolian component of more than 90 percent, with the remainder traceable to local hunter-gatherers, and this pattern has led many geneticists to treat demic movement as the dominant route in Europe. Archaeological modelling has, however, not been fully reconciled with this view. One synthesis that integrates both mechanisms concludes that cultural transmission accounts for about 40 percent of the spread rate in Europe, which leaves the demic share substantially larger but not exclusive. Regional studies reinforce the point that mechanisms differed: some areas show slow, cultural-dominated spread, and others faster, population-driven fronts.

Outside Europe the pattern can differ. A study of the transition to herding in southern Africa estimated a spread rate of about 2.4 km per year, roughly twice the European rate, and concluded that cultural diffusion was the main mechanism there. The authors linked this to the fact that the end state was herding rather than mixed farming and stockbreeding, which suggests that the primacy of population movement is not a general law of Neolithic change.

Seagoing vessels, pastoralism and the colonization of new environments

From roughly 4000 to 3000 BCE, two innovations expanded the reach of food-producing communities: seagoing boats and pastoral herding on grasslands. Pastoral nomadism allowed mobile herders to exploit steppe and savanna ecologies that were unsuitable for permanent cultivation, and the herding economy, which relies on moving animals across seasonal pastures, made long-distance movement a routine part of life rather than an exceptional event. Island colonization depended on watercraft capable of carrying people, plants and animals across open water. The debate over these processes is largely about whether movement was carried by small founding groups, by integration with local foragers, or by a mixture of both, and the archaeological record for seafaring is thinner than for settled farming.

Austronesian expansion

The Austronesian dispersal is the clearest case of a language family spreading with farming and boats across a vast area. The linguistic evidence places the proto-Austronesian homeland in Taiwan: most Austronesianists today agree that proto-Austronesian was indeed spoken in Taiwan some 5,500 years ago. The model known as "out-of-Taiwan" links this to a large-scale expansion of rice farmers that began in South China roughly 5,000 to 6,000 years ago and then moved into the Philippines and the wider Island Southeast Asia. In its eastward phase, the Lapita culture first appears in the Bismarck Archipelago some 3400 years before present and rapidly spreads into the previously uninhabited islands of Remote Oceania, reaching Tonga and Samoa by about 2900 BP.

The model has been challenged. Pottery finds in northern Luzon, dated to around 3,500 years ago, have been taken to suggest that the pre-Lapita tradition was not located in Taiwan, and some archaeologists now place the stamped-pottery origin in Southeast Asia. Genetic dates for the Out-of-Taiwan movement range from about 4,000 to 8,000 years ago, and ancient genomes from Remote Oceania show ties to Taiwanese Austronesian groups. Competing accounts, such as the multidirectional maritime trading networks proposed by Solheim, also remain in discussion.

Bantu expansion

The Bantu expansion is the second major language-linked dispersal of this period. It began in West Central Africa, and the Bantu expansion that swept out of West Central Africa beginning ∼5,000 y ago is one of the most influential cultural events of its kind. Phylogenetic work on the Bantu languages suggests that migrants did not spread randomly: migrating Bantu-speaking populations did not expand from their ancestral homeland in a "random walk" but, rather, followed emerging savannah corridors, with rainforest habitats repeatedly imposing temporal barriers to movement. Genetic studies largely support a demic model, with serial founder events along the route. The spread also absorbed or displaced earlier hunter-gatherer and pastoralist groups, and the degree of replacement versus assimilation varies by region.

Explanations for the acceleration of the expansion have included climate-driven changes in the forest. Some accounts point to the climate-induced destruction of the rainforest in West Central Africa around 2,500 years ago as a boost to the spread, but the driving forces remain debated.

The drying of the Sahara

Environmental change has been proposed as a common trigger for several of these movements. The Sahara was a wetter, greener landscape during the African Humid Period, and its drying during the mid-Holocene is widely cited as a push factor for herding and farming populations moving south and east. The role of this change is contested. Some researchers see it as a decisive pressure that pushed pastoralists into new regions, while others argue that the shift was gradual, that people had already adapted to changing conditions, and that the archaeological chronology does not always match a simple environmental trigger. Assessing climate as a cause therefore requires dating the environmental shift, the settlement sequence, and the genetic and linguistic evidence together.

Early states, empires and the Silk Road

Movement organized by states was among the earliest large-scale forms of migration. In Mesopotamia, forced relocation became a routine tool of rule. The Neo-Assyrian Empire moved conquered populations on a large scale. After it took the northern kingdom of Israel in 722 BCE, many residents of Samaria were deported to other Assyrian provinces, and people from other conquered regions were settled in their place. Assyrian royal inscriptions and administrative texts show that the deportations targeted urban elites, craftsmen, specialists and scholars, and that families often moved together. A 1979 estimate by Bustenay Oded put the number of people transferred over roughly 250 years at about 4.4 million, but the figure depends on reconstructed records and is debated. The Neo-Babylonian king Nebuchadnezzar II later deported Judean elites to Babylon in 597 and 586 BCE. The Achaemenid Persian kings also relocated groups; Herodotus reports that Darius I moved Paeonian communities to Phrygia in Anatolia.

Other ancient states relied on similar methods. New Kingdom Egypt brought captives and laborers from Nubia and the Levant into its territory to work on temples, estates and state projects. In the Roman Empire, Trajan settled colonists in Dacia after the conquest of 106 CE, and after the Bar Kokhba revolt of 132–135 CE, many Jewish captives were sold into slavery and dispersed through the Mediterranean. The Han dynasty used relocation both inside and at its frontiers. Han rulers moved wealthy families from across the realm to the capital region, which weakened local power bases. In the Hexi Corridor, after the Han drove the Xiongnu out of the area in the second century BCE, the state placed soldiers, convicts and farmers in garrison and military-agricultural colonies, a practice known as tuntian.

Merchant diasporas and the Silk Road

Long-distance trade created its own migration networks. The term "Silk Road" was coined in the nineteenth century for a web of routes that linked China, Central Asia, Persia, India and the Mediterranean, and it carried goods, people, religions and technologies rather than a single road. Among the most important intermediaries were the Sogdians, an Iranian-speaking people from oasis cities such as Samarkand and Bukhara. From the fifth to the seventh century, they were the principal long-distance caravan merchants in Central Asia, and from the fifth century onward a steady stream of Sogdian migrants settled in the towns of northern China. Some rose to official positions, and Chinese sources refer to Sogdian community leaders with the title sabao, meaning caravan chief. A bundle of letters written in Sogdian around 313 CE and found near Dunhuang in 1907 shows merchant agents and family networks stretching from Samarkand to Luoyang. Diasporic communities worked as interpreters, bankers and brokers, and their multilingual skills made it possible to conduct trade between groups that otherwise had little trust, including Chinese officials and steppe powers. Such diasporas became a model for merchant communities elsewhere in Eurasia.

Nomadic and steppe empires

The Mongol expansion of the thirteenth century was the largest example of steppe-based mobility. Mongol armies took the Central Asian cities of Bukhara, Samarkand and Urgench between 1219 and 1221, and the destruction and enslavement that followed drove artisans, merchants and peasants from Khorasan and other regions into exile or remote refuges. The Mongols also treated skilled people as a resource to be moved. Craftsmen were conscripted in large numbers and relocated to new centers of production, and after conquest rulers often seized experts and forced them to follow the khans to their capitals. When Ögedei Khan founded Karakorum, he brought in thousands of artisans and architects from across Eurasia. Three weaving communities were transferred from Central Asia and Persia to China to produce a prized gold brocade. Not all movement was coerced: Marco Polo and other foreigners entered Mongol service voluntarily as envoys, translators and merchants. The earlier Xiongnu, Turkic and Mongol confederations had likewise exchanged goods with sedentary neighbors and controlled sections of the trade routes, so the Mongol case shows how nomadic power and urban production became entangled.

Religious mobility

Religious networks were often built on the same routes. Buddhism spread into China partly through Central Asian monks and translators. The Kuchean monk Kumarajiva (344–413) worked at Chang'an, where his translations shaped Chinese Buddhist scripture, while the pilgrim Faxian (c. 337–422) travelled to India in search of texts and returned by sea. Xuanzang made his journey to India in the seventh century and brought back a large collection of manuscripts. Nestorian Christians were recorded in Tang China from 635 CE onward, and their presence is commemorated in the Xi'an stele of 781. Islam spread along the same corridors after the Arab conquests of Central Asia in the seventh and eighth centuries, and the hajj drew pilgrims from across Africa and Asia to Mecca, with Ibn Battuta's journeys in the fourteenth century showing how far such routes reached. Under Mongol rule, Franciscan missionaries such as John of Montecorvino established churches in Khanbaliq, the Mongol capital in present-day Beijing. Across these examples, religious travel, trade and state power frequently reinforced one another.

Early modern colonization and forced migration

From the early sixteenth century, the Iberian crowns of Castile and Portugal founded permanent settlements in the Americas, and England, France and the Dutch Republic followed in the seventeenth century. In the Americas, the sharp decline of Indigenous populations, driven largely by Old World epidemics, created labor shortages and opened land to newcomers. The result was a new demographic order of European, African and Indigenous populations, with mixed-descent groups forming in many colonies. In Asia and much of Africa, European expansion took a different form. Portuguese, Dutch and other companies mainly held fortified trading posts such as Goa, Batavia and the Cape, and European settler communities stayed small until the nineteenth century.

The transatlantic slave trade was the largest coerced movement of this period. Estimates of the number of people who were embarked from Africa cluster around 12.5 million, a figure built largely from shipping records, though the number who survived the crossing was lower. Approximately 1.8 million African people are thought to have died during the passage, accounting for about 15–25 percent of those who were taken from Africa. Destinations were uneven. Brazil imported over 5.5 million enslaved Africans, almost half of the total number of forced migrants. The non-Spanish Caribbean absorbed over 4.7 million; Spanish America just under 1.6 million; and mainland North America took an estimated 472,000. In the earliest phase, the Iberian powers were responsible for 99% of the nearly 630,000 kidnapped Africans trafficked from 1501 to 1625. Later, other European states joined, and almost 12 million kidnapped Africans were trafficked from 1625 to 1867. Most captives ended up in plantation agriculture, growing sugar, tobacco and other crops for export. Scholars also note that uncertainty remains: there are no complete records and estimates vary from a few millions to 100,000,000 people when the broader toll of deaths in African interior regions is included.

Once slavery was abolished in the British colonies in 1834, planters sought replacement labor under contract. From 1834 to the end of the WWI, Britain had transported about 2 million Indian indentured workers to 19 colonies including Fiji, Mauritius, Ceylon, Trinidad, Guyana, Malaysia, Uganda, Kenya and South Africa. Chinese workers went to the Americas, the Pacific and Southeast Asia. In Cuba, plantations recruited 138,000 indentured laborers from China between 1847 and 1873, and in the Dutch plantations of Sumatra, over 300,000 indentured Chinese laborers migrated between 1879 and 1933. Recruitment was frequently coercive. Recruiters were paid for each person they signed, which encouraged misrepresentation about work, wages and destination, and in some places kidnapping and forcible transport, especially in China. Mortality on the voyages and on the plantations was high. Some historians have described the system as a continuation of slavery under another name, while others emphasize that contracts existed and workers were paid, so the debate over how free these migrants were remains active. Taken together, one estimate places the 3.7 million contractual workers, mostly from India and China but also from Africa, Indochina, Java, Japan, and Melanesia, who migrated throughout and beyond the colonial plantation world between the 1830s and 1920s.

Rulers also moved populations deliberately to fill land they considered underused. In the 1760s, the Spanish Crown brought Catholic settlers from central Europe to the depopulated Sierra Morena, and Catherine II of Russia invited German farmers to settle the Volga region from 1763. Habsburg authorities resettled Germans and other groups in the Banat and the Danube basin after the wars with the Ottomans. These state-sponsored schemes were intended to secure frontiers and restore tax-paying populations, and they often produced colonies with distinct languages and religious identities that lasted for generations.

Religious policy also produced large-scale expulsions. On 9 April 1609, Philip III of Spain decreed the removal of the descendants of Spain's Muslim population who had been forced to convert to Christianity. The estimate usually given is 275,000 and 300,000 Moriscos, but modern historians have challenged how completely the order was carried out, and one scholar suggests that around 40% avoided expulsion altogether and tens of thousands of those expelled managed to return. Most of those who left settled in North Africa, where they adapted quickly to new circumstances. In France, Louis XIV's revocation of the Edict of Nantes in 1685 drove a substantial Protestant emigration to Prussia, the Dutch Republic, England and North America, and the expulsion of Protestants from the Archbishopric of Salzburg in 1731–32 sent roughly 20,000 people northward to Prussian territories. These movements show how confessional conflict in Europe could turn into population transfers, with destination states often competing to attract skilled artisans and farmers.

Industrial-era mass migration

Rural-to-urban movement in Britain and continental Europe

The Industrial Revolution concentrated employment in mills, ironworks, mines and workshops, and many of the new jobs were in towns. Rural labourers who lost work or land, or who could not earn a living from small holdings, moved to places such as Manchester, Leeds, Birmingham and Glasgow, where textile and metal industries were expanding. By the mid-nineteenth century, the census of Britain recorded more people living in towns than in the countryside. Similar flows reached the coal and steel districts of the Ruhr and the expanding capitals of continental Europe, including Berlin. Many early urban migrants were young and unmarried, and many moved in stages, first to a nearby market town and then to a larger city. Overcrowded housing, poor sanitation and recurrent cholera epidemics made the urban settings of this period harsh, but the wages on offer kept drawing people in. Rapid growth outran the building of water, sewerage and transport systems, a problem that later became central to urbanization debates.

Transoceanic migration from Europe

Between the 1840s and the First World War, tens of millions of Europeans crossed the Atlantic and other oceans. Most went to the United States, with large flows to Canada, Argentina and Brazil, and smaller but significant movements to Australia, South Africa and New Zealand. Estimates of the total to the Americas in this period range from about 50 to 60 million, but the figures for individual destinations are more firmly established; roughly 35 million Europeans are commonly associated with the United States flow. Steamships transformed the journey. Sailing ships had taken weeks and suffered high mortality, while steamers shortened crossings, made departures predictable and allowed year-round service. Economic historians note that the replacement of sail by steam sharply reduced the time spent at sea and the variability of voyage durations, and that the total cost of crossing, once foregone earnings were counted, fell substantially. Railways completed the system: by the late 1880s, most of the regions supplying American immigrants were linked by rail to major ports such as Hamburg, Bremen, Liverpool and Hull. Between 1900 and 1914, roughly eleven million European-born migrants made about nineteen million ocean crossings to the United States, counting repeated voyages. The composition of the flow also changed. After about 1890, a growing share of migrants to the United States came from southern and eastern Europe, travelling in steerage on steamers. The flow largely ended with the outbreak of war in 1914 and with new immigration restrictions in the United States and South Africa.

The Great Migration within the United States

The United States experienced one of the largest internal movements of the period when Black Americans left the rural South. Between about 1910 and 1970, roughly six million people moved from the South to cities in the North, Midwest and West. The movement is commonly divided into two phases. The first, from about 1915 to 1930, involved more than 1.5 million people, many of them moving from southern towns and cities rather than directly from the countryside. The second, from 1940 to 1970, involved a further 4.5 million or more and drew more heavily on rural populations. Destinations included Chicago, Detroit, Cleveland, Philadelphia and New York, and later Los Angeles, Oakland and Seattle. Wartime industrial demand, segregation and the Jim Crow system, sharecropping debt and violence all contributed to the decision to leave. Migration was shaped by networks: relatives and neighbours who had already settled in a particular northern city provided housing, information and sometimes money, so that migrants from one southern district often ended up in the same destination. Detroit's population rose from about 41,000 in 1910 to over 120,000 in 1920, and New York's Black population more than doubled in the 1910s. Estimates of the total differ by source and by how the period is bounded, and some scholars put the total above seven million. By 1970, about half of Black Americans lived in the South, compared with roughly 90 percent in 1910.

Indentured labour and commercial networks from China and India

After the abolition of slavery in the British Empire in 1833, and in later decades in other European colonies, plantation owners in the Caribbean, the Indian Ocean, the Pacific and elsewhere sought replacement labour in Asia. Between 1834 and 1920 about 1.5 million indentured labourers, roughly 85 percent of them from India, were sent to British colonies such as Mauritius, the British West Indies and Natal. Roughly 750,000 Chinese workers went to destinations including Malaya, Sumatra, Cuba, the British West Indies and Réunion. In Mauritius, more than 450,000 Indian workers had been exported by 1922, but only about 160,000 returned home. Chinese labour flowed to Cuba, where some 138,000 indentured workers arrived between 1847 and 1873, and to the tobacco plantations of Deli in eastern Sumatra, where over 300,000 Chinese indentured workers were recorded between 1879 and 1933. Recruitment often relied on deception and debt, and the contracts tied workers to plantations for years. Resistance, mortality and criticism from abolitionists led to regulation, and the government of India banned indentured emigration in 1916. Alongside these contract systems ran commercial migration. Chinese emigrants to the United States used the credit-ticket system, in which they borrowed the fare and repaid it from wages, and the construction of the transcontinental railroad from 1863 drew on Chinese labour. Federal legislation against contract labour followed in 1862, and California's Anti-Coolie Act later that year.

Transport costs and their effects

Across all these movements, the fall in transport costs was decisive. Steamships, rail links to ports and the opening of the Suez Canal in 1869 reduced both the price and the duration of long journeys, making it feasible for poorer households to send one member abroad and for sojourners to make repeated trips. Lower costs also widened the pool of people who could travel, though the cost of a fare remained high relative to the income of the poorest, which is why agents, credit systems and employers often financed the journey.

Typology: internal and international, voluntary and forced

Migration is classified along several axes at once: whether the move crosses a state boundary, whether it is chosen freely, how long it lasts, and what kind of places it connects. These axes do not map neatly onto one another, so a single movement can belong to several categories at once.

Internal and international flows

Internal migration takes place within one state, and international migration crosses a state boundary. The distinction matters because the boundary determines which laws apply, who counts as a foreigner, and which records a government keeps. Within a country, geographers and demographers often sort moves by the type of place of origin and destination: rural-rural (for example, settlement of frontier lands), rural-urban (the principal flow into industrial and service cities), urban-urban (movement between metropolitan areas, often for professional or corporate employment), and urban-rural (return to villages, or retirement and lifestyle moves away from cities). Rural-urban movement has historically dominated, and urban-rural flows are usually smaller but become more visible in countries where housing costs or congestion push people outward.

Voluntary and forced movement

Voluntary migration is usually understood as movement chosen after weighing available options, typically for work, family, study or a better living environment. Forced migration includes flight from armed conflict, persecution, state-directed expulsion and population transfers. Between these poles lies a substantial gray zone. People fleeing famine, drought, flooding or economic collapse often have little real alternative, yet they may not meet the legal criteria for refugee status, which under the 1951 Convention turn on persecution for specific grounds. Consequently, the label applied to a movement can determine access to protection, and many agencies now refer to "mixed flows" to describe groups that include both economic migrants and people in need of protection.

Temporary, circular and commuting movement

Temporary labor migration covers workers who take up employment abroad or in another region for a defined period, such as contract workers, seasonal agricultural laborers, or trainees. Circular migration describes repeated back-and-forth movement, in which a person returns home periodically rather than settling. Daily or weekly commuting involves regular travel to work without a change of residence, and it can cross borders in regions where labor markets and housing markets are integrated. These forms are often excluded from definitions of migration in statistics because the person's usual home does not change, even though they may be significant for remittances, household finances and local labor supply.

Overlap and statistical classification

The categories blur in practice. A worker may commute for several months, then take a seasonal contract, and eventually settle permanently. Statistical agencies therefore rely on explicit thresholds rather than on the subjective intentions of migrants. The 1998 United Nations recommendations distinguished short-term migrants, who stay at least three months but less than a year, from long-term migrants, who stay for a year or more. Many agencies later found that the "short-term" label was misleading, because a stay of this length does not change a person's usual residence, and counting such movers as migrants inflates totals. The revised framework endorsed by the UN Statistical Commission in 2021 therefore separates changes of residence, which form the resident migrant population, from temporary mobility, which is measured separately. Countries also differ in the residence criterion they apply, with some using the "most of the last 12 months" rule (at least six months and one day) and others using the "at least 12 months" rule. Under the second rule a person may be counted in neither country if two states apply different thresholds to a stay split between them. Exceptions for holidays, business trips, medical treatment and religious pilgrimage further complicate comparisons. Agencies such as national statistics offices combine census responses, population registers, border and visa records and surveys, and they often adjust flows for people who leave earlier than their visa or permit would suggest. Refugees who remain in a host country for years are usually counted as migrants in the end, even though they were not originally expected to stay. These conventions are a matter of measurement rather than of how people themselves experience their moves.

Causes: economic, family and educational motives

Economic push and pull

Economic explanations start from the gap between what a person can earn at home and what labor markets elsewhere offer. Wage differentials, the number of available jobs, and pressure on farmland or local resources in crowded rural areas push people out, while expanding employment in cities and abroad pulls them in. Neoclassical economics anticipates income-maximizing migrants who expect to earn higher wages at the destination. A second line of reasoning, often linked to household decision-making, describes people who move to spread risk. New economics predicts risk-diversifying migrants who seek to complement earnings that are at risk in the place of origin. Both models treat economic motives as central, but they differ on whether the decision belongs to the individual or to the family that pools income and shares losses.

Family reunification and marriage

Family ties are among the largest channels of permanent entry. In 2024, nearly half (44%) of new permanent migrants to the OECD moved for family-related reasons – either to reunite with family already in the host country (34%) or as accompanying family members of labour migrants (10%). The share is especially high in some destinations: In the United States, nearly three‑quarters of new permanent immigrants arrived for family reasons. Immigration law usually separates two situations. Family formation covers cases in which a resident marries a foreigner and sponsors that individual for admission or for status change, whereas family reunification refers to the later admission of relatives who already exist. The distinction matters for eligibility rules, though the boundary between marriage migration and other forms of family migration is not always clear.

Students, skilled workers and transfers within firms

Education has become a major motive for crossing borders. The number of students going abroad for higher education has more than tripled over the past two decades, from 2.1 million in 2000 to nearly 7.3 million in 2023. Even so, internationally mobile students still account for only 3 per cent of the global higher education cohort. Flows are concentrated: Seven countries (United States, United Kingdom, Australia, Germany, Canada, the Russian Federation and France) continue to host half of all international students. Origin patterns have also shifted. The number of Chinese students dipped to just over one million (or about 14% of the total) while the number of Indian students climbed to an estimated 1.3 million, representing nearly 19% of the total in 2024. Student mobility often feeds into later skilled employment, and skilled migration and intra-company transfers operate through work permits and employer sponsorship, which connect these flows to human capital and to the rules of each destination.

Networks, information and the cost of moving

Movement is rarely a set of isolated decisions. Migration network theory holds that migration is cumulatively caused as a result of reduced social, economic, and emotional costs of migration pursuant to the formation of migration networks. Earlier arrivals help later ones. Already settled migrants function as “bridgeheads” reducing the risks and costs of subsequent migration. Relatives and acquaintances abroad also supply information on housing, jobs and legal routes, and social networks are an important determinant of the choice of destination for migrants as well. Because the cost of a move depends heavily on these ties, a family or community with contacts in one city may face a very different set of options from one without them.

Network effects are not automatic. Studies of Mexico–United States migration suggest that the emergence of cumulative causation can be curtailed by conditions in the origin and destination communities. Some streams slow or stop altogether, demonstrating that these self-sustaining processes are also constrained. Networks, in other words, lower barriers for some movers while leaving others, particularly those without contacts, to bear the full cost of leaving. Research on these mechanisms draws on sociology and economics alike, and the balance between them is still debated.

Forced migration, refugees and displaced persons

Forced migration describes movements in which people leave home because of persecution, armed conflict, generalized violence, human rights violations or disasters, and the decision to leave is constrained rather than freely chosen. International frameworks separate several legal categories. Under the 1951 Convention, eligibility for refugee status depends on a rights-based test: a well-founded fear of persecution, which may be based on race, religion, nationality, political opinion or membership in a particular social group. An asylum-seeker is a person whose claim to that status has not yet been decided. An internally displaced person (IDP) has been forced from home but remains inside the borders of their own state. The UN Guiding Principles on Internal Displacement (1998) describe this group, but IDPs have no separate treaty status, which is why they fall outside the refugee regime.

The 1951 Convention, adopted at a Geneva conference in July 1951, was drafted in the shadow of World War II. The 1967 Protocol removes these geographic and time-based limitations, expanding the Convention to apply universally. Before that change, the treaty applied only to people displaced by events before 1 January 1951, and in practice largely to Europe. The Protocol entered into force on 4 October 1967. The UN refugee agency began work in January 1951 with a mandate to provide international protection and to seek lasting solutions for refugees. Its work rests on the Convention and Protocol, but it does not hold a veto over state admissions, so its operational reach depends on the cooperation of governments in sovereign states.

Current figures show the scale. At the end of 2025 there were 41.6 million refugees, and 68.7 million people displaced within their own country due to conflict or violence. Taken together, the total forcibly displaced population was 117.8 million, the first decline in a decade. The 2024 baseline is reported differently across sources, with one summary giving 122 million and another giving 123.2 million. Figures for individual crises also diverge. UNHCR reports that Sudan has 9.1 million people still displaced within the country, while a summary of the 2025 report gives 14.4 million internally displaced there. Such discrepancies stem from differing counting methods and reference dates, and readers should check which source a given figure comes from.

Persecution and armed conflict remain the dominant causes, and state-driven expulsions have a long history. The Rohingya are a clear modern case: approximately 1.2 million are hosted as refugees in Bangladesh, having fled persecution in Myanmar. Earlier episodes were often organized by treaty or policy. The 1923 Greek–Turkish exchange under the Lausanne settlement, which followed the collapse of the Ottoman Empire, uprooted roughly 1.2 million Orthodox Christians from Turkey and about 400,000 Muslims from Greece. The 1947 Partition of India and the expulsion of ethnic Germans from Central and Eastern Europe after 1945 each displaced millions, and the wars in the former Yugoslavia in the 1990s made ethnic cleansing a central instrument of territorial control.

Durable solutions are the framework for ending displacement. UNHCR recognizes three: voluntary repatriation, local integration in the country of asylum, and resettlement to a third state. All three are regarded as durable because they promise an end to refugees' suffering and their need for international protection. Voluntary repatriation is the option most often favored: about 4.4 million refugees returned to their countries of origin in 2025, although many returns have occurred under adverse circumstances and to areas that remain fragile. An evaluation of UNHCR's repatriation programmes argued that the usual binary view of voluntariness misrepresents refugees' choices, which are more about a scale of imperfect options in an environment of constrained choice.

Local integration and resettlement have been less available. Only a small fraction of refugees receives opportunities for resettlement, and integration is often impeded by socio-political resistance and weak policies. Resettlement was once more central: after the 1956 Hungarian revolution, nearly 200,000 refugees were resettled. Protracted displacement shows the cost of delay. For decades, more than 100,000 refugees from Bhutan were living in camps in southeastern Nepal until 2006, when a large-scale resettlement programme began. Camps are meant as temporary shelter, but in many cases they become long-term settings in which refugees lack the legal rights and economic opportunities that durable solutions are designed to restore.

Environmental drivers and climate-related mobility

Environmental change shapes human movement through two broad channels. Sudden-onset hazards, including floods, storms, cyclones and wildfires, destroy homes, roads and basic services within hours or days. Many displaced households return within weeks or months to rebuild, but repeated disasters or damage severe enough to make a place unliveable can turn a temporary evacuation into a permanent move. The scale is large: according to figures compiled by the Internal Displacement Monitoring Centre, disasters produced about 265 million new internal displacements between 2008 and 2018, or roughly 24 million a year. Drought adds to this total in some years. In 2022 more than one million Somalis were displaced by drought, mostly within Somalia.

Slow-onset processes, such as drought, desertification, land degradation, salinization and sea-level rise, work differently. They wear down the resources on which livelihoods depend: crops fail, pasture thins, groundwater turns saline and fields lose fertility. Because the decline is gradual, people often try to adapt in place for years before moving, and the moves that follow may be spread out and difficult to attribute to a single event. Pastoralists in Ethiopia and Uganda have travelled hundreds of kilometres after pasture deteriorated, and on low-lying Pacific islands such as Tuvalu, saltwater intrusion and rising seas are expected to add to outward movement. The two categories also interact. Sea-level rise combined with storms produces storm surges, and drought can trigger acute food insecurity that prompts sudden flight.

The available evidence points to a pattern of short distances and internal movement. Studies using census microdata from 72 countries for 1960 to 2016 found that environmental stress, especially drought and aridity, tended to produce moves within national borders, and that these moves were mostly short in distance. Household surveys point the same way while also showing that climate is rarely the only reason for leaving. In El Salvador, Guatemala and Honduras, only about 6 percent of migrant-sending households cited climate or environmental reasons for emigrating, and a 2022 survey in Central Africa found about 5 percent of migrants gave environmental reasons. In practice, climatic stress usually works through economic channels, such as lost income, higher food prices and competition over land, water or work, so the environmental component is often hard to separate from other motives. Long-distance and cross-border moves are the exception, and many environmentally driven movements are temporary.

Immobility is an important part of the picture. Those with the fewest financial resources, social networks or legal options are often the least able to leave, and they may face the greatest exposure to hazards. Migration-policy analysts commonly distinguish between people who choose to move, people who are forced to move, people who choose to stay, and people who are trapped because they cannot leave. Climate stress can reduce overall mobility, and studies that track populations through remote sensing and mobile-phone data have documented both evacuation and a lack of departure in the same disaster. Staying put, then, is itself an adaptive decision, sometimes made under severe constraint.

The terminology used for these movements remains contested. The phrase "climate refugee" is widely used in public debate but has no legal meaning. The 1951 Refugee Convention defines a refugee by reference to five grounds: race, religion, nationality, political opinion and membership of a particular social group. Climate change is not among them, and people fleeing famine or natural disaster qualify only if they also face persecution on one of those grounds. Some scholars argue that the Convention should be amended, while others prefer complementary protection through human rights law. A decision by the UN Human Rights Committee on a deportation case from New Zealand did not grant relief, but it affirmed that states must respect the principle of non-refoulement when climate-related harm threatens the right to life. Regional instruments, including the 1969 OAU Convention in Africa and the 1984 Cartagena Declaration in Latin America, already extend protection beyond the 1951 definition. The 2018 Global Compact for Safe, Orderly and Regular Migration, which is not legally binding, calls for cooperation on planned relocation and visa options for people compelled to leave because of slow-onset change, but it does not create a status for them. Researchers and agencies therefore tend to use broader descriptive terms such as climate migrant, environmentally displaced person or climate mobility, because these capture internal, temporary, and immobile outcomes that the legal category does not reach.

Theories and models of migration

Theories of migration have developed from several starting points: regularities in census data, the cost-benefit calculations of individuals, the economic strategies of households, the social networks that link places, and the long-run relationship between migration and development. None of these accounts fully explains every flow, and they are often used in combination.

Ravenstein's laws and the distance principle

Ernst Georg Ravenstein, an Anglo-German geographer, published studies of internal and international migration in 1876, 1885 and 1889, drawing mainly on British census returns for 1871 and 1881. His "laws of migration" are empirical generalizations rather than a formal theory. Most migrants move only short distances. Growing towns absorb people from their immediate surroundings, and the vacated places are filled by people from farther away. Each migration stream tends to produce a weaker counter-stream. Long-distance migrants usually head for major centres of commerce and industry, and economic motives are the main cause of movement. The list is not stable: the three papers contain different wordings, and later commentators have counted anywhere from seven to eleven laws. Ravenstein's distance observation later became the basis of the gravity model, in which flows between two places rise with their population sizes and fall with the distance separating them. Critics have called his propositions descriptive and tied to their historical setting, but they set the agenda for much later research.

Push-pull and neoclassical wage models

Push-pull frameworks, popularized in the 1960s, explain migration by contrasting conditions at the origin, which push people away (low wages, unemployment, land shortage, political repression), with conditions at the destination, which pull them in. Intervening obstacles such as distance, cost and immigration rules, along with personal circumstances, also shape the outcome. Neoclassical models give these forces a economic form. A person compares expected lifetime earnings in each location, net of the costs of moving, and migrates when the balance is positive. The Harris–Todaro model added the probability of finding employment, which explained why rural workers might migrate to cities even when urban unemployment was high. These models predict that migration responds to wage gaps, but they have difficulty explaining why migration often increases as incomes rise at the origin, and why many people with the means to move do not do so.

The new economics of labor migration

The new economics of labor migration, developed in the 1980s by Oded Stark and David Bloom and others, shifts the unit of decision from the individual to the household. Where insurance, credit and futures markets are weak or absent, a household may send one or more members to work elsewhere in order to diversify its sources of income against crop failure, unemployment or price shocks. Remittances then act as insurance for the household that stays behind and as capital for investment. This approach implies that migration can occur even without large wage differentials, and that relative deprivation within a village, measured against neighbours who already receive remittances, can itself encourage departure. Stark's related work argued that risk avoidance, rather than risk seeking, is a major factor in rural-to-urban migration. Critics note that the approach fits settings with incomplete markets better than settings where they function well, and that it tends to assume cooperative households with shared interests.

Networks, migration systems and cumulative causation

Once migration begins, it tends to sustain itself. Earlier migrants lower the costs of travel, housing and job search for newcomers, and they transmit information about opportunities. Douglas Massey and colleagues described this as cumulative causation: each movement changes the social and economic context in which later decisions are made, so that migration becomes more likely over time even if the original causes weaken. Migration-systems theory extends this idea by examining the links between particular pairs or clusters of countries, which are often created by trade, colonial ties, investment, or cultural connections. Networks help migrants, but they can also channel people through brokers and intermediaries who charge fees, a feature that shapes which destinations become accessible.

World-systems and mobility transition

World-systems theory, associated with Immanuel Wallerstein, interprets migration as a consequence of the incorporation of peripheral regions into a global capitalist economy. Land expropriation, the spread of commodity production, and the unequal terms of trade displace workers, who then move along the lines of core-periphery relations. This perspective emphasizes structural power rather than individual choice, and it has been criticized for economic determinism. Wilbur Zelinsky's 1971 "Hypothesis of the Mobility Transition" linked migration to modernization in a manner parallel to the demographic transition. Zelinsky identified five phases, moving from traditional societies with little movement, through early transitional societies dominated by rural-to-urban and international migration, to later phases in which urban-to-urban and suburban moves predominate and mobility eventually declines. He is often credited with proposing an inverted U-shaped relation between emigration and development. Critics, including Richard Skeldon, have argued that the model is unilinear and based on the historical experience of Western Europe, and the upward segment of the curve remains difficult for neoclassical accounts to explain.

Contemporary scale and patterns

Size and measurement

The United Nations Population Division estimated that the number of international migrants worldwide stood at 304 million in 2024, a figure that has nearly doubled since 1990, when there were an estimated 154 million. Relative to the world's population, the change has been much smaller: the share of people living outside their country of birth rose from 2.9 per cent in 1990 to 3.7 per cent in 2024. Absolute growth has therefore outpaced the growth of the global population only modestly, a point that matters when interpreting headlines about "record" migration.

The UN counts migrants mainly through birthplace. When data on country of birth are available, they are given precedence because such data reflect a change in the country of residence more accurately than do data on country of citizenship. Citizenship is used only when birthplace data are missing. The resulting figure is a stock, a count of people residing abroad at a given time, and it does not distinguish between motives or legal status. Estimates are revised as new censuses and registers become available; for the 2024 edition, full reassessment was conducted for 60 countries with new data from censuses or population registers, which is one reason earlier totals, such as 275 million international migrants in 2020, are not directly comparable with the 2024 series.

Regional and national patterns

Migrants are unevenly distributed. Europe hosted more international migrants than any other region with 94 million in 2024. Northern America was home to the second largest number with 61 million, followed by the region of Northern Africa and Western Asia with 54 million. Taken together, Europe and North America accounted for about 51 percent of all international migrants. Asia hosted 92.2 million international migrants at mid-year 2024, but because of its population size the migrant share there is much smaller. The regional shares of population differ sharply: international migrants represented 22 per cent of Oceania's population, 16 per cent in North America, and 13 per cent in Europe, while Asia and Africa were both at 1.9 per cent.

At the national level, the United States hosted more international migrants than any other country in 2024, with 52.4 million. Germany, Saudi Arabia, the United Kingdom and France followed with roughly 17 million, 14 million, 12 million and nine million migrants respectively. The labor-importing Gulf economies stand out for their proportions: Qatar and the UAE have the highest share of migrants in their residents, with over 75 percent of residents being migrants.

Most movement is also short-distance. At mid-year 2024, more than half of all international migrants (53%) migrated within their same region of origin. The pattern varies by region: 86 per cent of all international migrants in Africa originated from the same region, while nearly half (48%) of all international migrants in Europe originated from the same region. Origin countries are more dispersed than destinations; India was the source of 18 million migrants in 2024, followed by China and Mexico, which each sent 12 million citizens abroad.

Forced displacement and its measurement

Forced displacement is counted separately, by UNHCR and related bodies, and the totals are not interchangeable with the UN migrant stock. UNHCR reported that at the end of 2024, 123.2 million people worldwide were forcibly displaced. Its later report revised the end-2024 figure to 122 million people, and the total for end-2025 was 117.8 million recorded worldwide, the first decrease in a decade.

The categories also diverge in scope. Many displaced people never cross a border. The Internal Displacement Monitoring Centre counted 83.4 million people remaining displaced within their countries at the end of 2024, and UNHCR's count of internally displaced people differs from IDMC's: IDMC reports 73.5 million IDPs while UNHCR reports 68.1 million conflict-affected IDPs at end-2024. Among those who do cross borders, there were 42.7 million refugees globally by the end of 2024, and most people fleeing conflict and persecution remain near their countries of origin. Conflicts such as those in Sudan, Ukraine and Afghanistan illustrate the concentration: the number of displaced people from Sudan climbed to 14.3 million, and a total of 8.8 million Ukrainians had been forcibly displaced by the end of the year.

Urbanization and internal migration

Movement from the countryside to towns is one of the main ways cities grow, though it is rarely the only one. The proximate components of urbanisation include the balance of internal migration flows between rural and urban areas, the balance of births and deaths across rural and urban areas. Between 2000 and 2010, migration accounted for slightly less than half of the world's urban population growth, but migration only accounts for about one third of urban population growth in sub-Saharan Africa. Where settlements are reclassified as urban, the figures change: in China and Thailand, rural-to-urban migration combined with reclassification is credited with 80 percent of urban growth.

Regional patterns differ. In China, an estimate attributes 38.1% of the 420 million population increase in the PRC's urban population between 1978 and 2007 to rural migrant workers. Africa followed another course. One estimate puts natural growth within towns at as high as 76% in the 1990s, while in some African countries rural-urban migration still produced about 60% of urban growth. Such estimates are fragile, because the continued absence of sufficient and timely data on rural–urban migration flows in many countries makes it hard to conclude that the slow urbanisation in Africa and Asia can be attributed to a single factor.

China's household registration system, the hukou, shows how state rules can shape internal movement. Established in 1958, China's household registration system initially assigned a rural (agricultural) or urban (non-agricultural) status to every citizen. The distinction was dropped in 2014, but the place of registration still determines access to state provisions. Because a registration is determined by a person's place of birth and transferring it to another city is difficult, migrants working in cities without local registration have been ineligible for subsidized public housing, must pay significantly higher fees to enroll their children in urban public schools (if they are admitted at all), cannot access urban healthcare facilities on the same terms as registered residents. The scale is large: internal migrant workers were estimated at nearly 269 million in 2013. A 2014 reform aimed to let around 100 million people register where they lived by 2020, yet one assessment found that the reforms were far less transformative than they appeared, since the distinction between urban and rural residents was maintained through the continued linkage of social service access to place of registration rather than place of residence. Easing access appears to matter for movement itself: a study of a reform across 283 Chinese cities found that improved access to local registration substantially increases migration from rural areas. Access to services also has health effects; a Shanghai hospital study found a greater likelihood of high-risk pregnancies among internal migrant women who moved from other parts of China to Shanghai, though this is an observational finding.

Rapid urban growth has been accompanied by large numbers living in informal settlements. In 2024, about 1.16 billion urban residents – almost one in four – lived in slums or informal settlements, lacking access to safe water, sanitation, secure housing or adequate living space. The regional picture is uneven: the share declined slightly, from 25.4% to 24.2% between 2014 and 2020, while the total number of slum dwellers kept rising, and UN-Habitat later reported that the share increased from 22.7 per cent to 24.9 per cent between 2020 and 2024. In sub-Saharan Africa, the slum population rising from 133 million in 2000 to 266 million by 2022. Projections without policy change suggest that the number could reach 2.9 billion by 2050, concentrated in Central and Southern Asia and sub-Saharan Africa. Planners face the need for integrated, data-driven urban planning that expands affordable housing, sustainable transport and public spaces while curbing inefficient urban sprawl, along with basic services such as water and sanitation that often lag behind arrivals.

Internal movement is rarely one-directional. Many rural-urban migrants keep land, a house or a registration in their home villages, return at harvest time or for the Lunar New Year, and send money home. Remittances from urban earnings commonly pay for children's schooling, medical costs, house repairs and farm inputs, which can reduce a household's dependence on a single harvest. The same movement often leaves children and older people behind in villages, creating demand for rural schools and health services that may be thin. Return migration, when people retire, lose urban jobs or simply go back, brings back savings, skills and ideas, and it complicates official counts, since a move is often recorded only when residence changes permanently.

Economic effects on origin and destination societies

Wages and employment in receiving economies

Simple labor-supply models predict that an inflow of workers lowers the wages of native workers whose skills most resemble those of the newcomers, while raising the pay of workers whose skills complement them. Empirical work has generally found smaller aggregate effects than this model implies. A meta-analysis covering 88 studies published between 1985 and 2023 reports that estimates vary widely but are quantitatively small. Earlier syntheses found that employment effects on native-born workers tend to be smaller than wage effects, and that impacts are generally smaller in the United States than in other countries studied.

The result depends heavily on how labor markets are defined. The national skill-cell approach compares education-experience groups and therefore measures relative effects, while spatial approaches estimate the total effect of a regional inflow, including complementarity across skill groups and adjustments by employers and workers. Some European studies find that natives move into more skilled and better-paid occupations when immigrant inflows rise, with greater labor-market flexibility associated with this upgrading. Negative effects tend to concentrate on the least-skilled natives and on specific sectors. Short-run displacement, for example in studies of refugee arrivals in Turkey, has been linked to competition with low-skilled native workers, while longer-run outcomes often differ. The productivity effects of immigration are commonly understood to depend on the skill mix of migrants, the structure of the host economy and the adaptability of its labor-market institutions. Some analyses also find that the economic gains from highly skilled migrants can be smaller than expected, partly because of skill mismatch and credential non-recognition.

Remittances and origin economies

Remittances are among the largest financial flows to low- and middle-income countries (LMICs). World Bank estimates put remittances to these countries at about 685 billion US dollars in 2024, up from roughly 656 billion in 2023, although the figures are revised over time and informal transfers are not fully captured. Over the past two decades, remittances to LMICs have exceeded official development assistance, and in recent years they have also surpassed foreign direct investment, which has declined since the 2010s. India, Mexico and China are the largest recipients in absolute terms, but in several small economies remittances account for a very large share of GDP, which makes them important for financing current-account and fiscal gaps.

At the household level, transfers are mainly spent on food, housing, health care and schooling, and a smaller share goes into small businesses, land and other investment. Because they are private and not subject to the same conditions as aid, remittances can smooth household consumption after shocks and, in some cases, finance children's education. However, they are sensitive to economic conditions in destination countries: the 2020 downturn produced a fall in flows to LMICs, and rising transfer costs have been shown to reduce the real income that migrants can send home. Dependence on remittances also exposes recipient economies to shocks in a small number of host countries.

Brain drain and brain gain

The emigration of skilled workers from poorer countries has been studied as a potential constraint on development. By 2000, the OECD area hosted about 20 million high-skilled immigrants, and about two-thirds of them came from developing and transition economies. Emigration rates of high-skilled workers exceed those of low-skilled workers in nearly all countries, and the largest rates occur in small, poor tropical countries, where more than 80 percent of the highly educated population in some cases lives abroad.

The overall balance is contested. One strand of research finds that the net effect of brain drain is negative where the emigration rate of the highly educated exceeds about 20 percent or where tertiary education is widespread, and that this condition applies to a large share of developing countries. Another strand finds that the income-maximizing level of skilled emigration is usually positive for developing countries, but that the majority of poor, small countries have emigration rates above that level. Case studies show diverse outcomes: the contribution of Indian engineers and information-technology professionals to India's growth is widely recognized, while the medical brain drain from sub-Saharan Africa has been regarded as harmful to health systems. Possible channels for brain gain include return migration, diaspora networks, technology transfer and higher investment in education in anticipation of migration. A 2025 review in Science argued that expanded migration opportunities can raise the stock of educated workers at home, although the magnitude of this effect varies by country. Evidence from Uruguay indicates that the largest gains from emigration accrue to the migrants themselves.

Fiscal effects and labor shortages

Fiscal consequences depend on the age, education and employment of migrants, the timing of their arrival and the rules governing access to public services and benefits. Migrants arriving at working age can contribute to pension and health financing in societies with low birth rates and rising old-age dependency ratios, while reception, language training and integration costs fall mainly in the early years. Estimates of net fiscal impact differ according to whether they use static or dynamic accounting and whether they include the costs of the migrants' children.

Labor shortages are often most acute in health care, long-term elderly care, construction, agriculture, food processing and transport, and in some high-income countries migrant workers fill a large share of these roles. Temporary worker schemes and bilateral labor agreements are used to match such demand, although they raise questions about worker mobility, protection and the transfer of skills. The overall economic effect of migrant labor on productivity and growth remains contested, and results vary with the skill profile of the migrants, the level of sectoral demand and the institutional setting of the host economy.

Cultural, linguistic and social change

Language contact and the formation of pidgins and creoles

Migration brings people with unrelated languages into close and lasting contact, and the new communities that result often generate new ways of speaking. A pidgin or creole is a language variety that arises when groups without a shared language need to communicate. The usual distinction is functional: a pidgin is a reduced medium for trade and basic exchange, and it becomes a creole only when children grow up using it as their first language, which allows its grammar and vocabulary to expand. Pidgins evolved as simple means of communication between speakers of different languages, while creoles emerged when pidgins became the mother tongue of certain communities.

The settings that produced these languages were typically shaped by migration and coerced labor. Caribbean plantations were a major site of creole formation, where enslaved people from dozens of different language backgrounds had to develop shared communication. Scholars also distinguish the setting: creoles typically developed in plantation settlement colonies, while pidgins emerged in transient trade colonies or whaling ships. Later cases follow the same pattern. Hawaiian Creole developed on Hawaii's sugar plantations in the 19th century, where workers from different backgrounds needed to communicate. In the Antiguan case, the pidgin underwent the process of creolization as it acquired native speakers in the late seventeenth and early eighteenth centuries, and children there used the creole while their parents still used the pidgin. Contact with migrant and military populations also shaped Tok Pisin, which has roots in contact with English-speaking colonial and military personnel.

Not all of this is settled. Some linguists challenge the older view that creoles always evolved from pidgins, and they point to the different sociohistorical routes by which the two formed. Expanded pidgins, such as Nigerian and Cameroon Pidgin Englishes, display grammatical complexity similar to creoles despite their different evolutionary trajectories.

Language shift and intergenerational change

Immigrant languages usually decline across generations, though the speed of that decline varies. Some researchers have argued that immigrant languages disappear through mother-tongue shift within three generations when immigrants or their immediate descendants do not teach their non-English language to the next generation. Alba and colleagues examined whether this three-generation model still fits the descendants of recent immigrants. Their model holds that the second generation faces pressures to assimilate and often adopts the dominant language more readily. The third generation, unless exposed to structured maintenance efforts, typically loses meaningful proficiency in the heritage language.

Maintenance is not automatic, and the evidence points to several supporting conditions. Heritage-language retention was particularly noticeable if the participants attended school with fellow community members or frequently visited their ancestral country of origin. A Canadian study found that participants who didn't attend bilingual schools obtained lower scores than those who did attend. Family practices matter as well; one linguist argues that immigrant families lose heritage languages faster simply due to limited opportunities to speak those languages. The claim that Spanish in the United States will follow the older pattern has itself been contested, and studies in Southern California were designed to test the assertion that Spanish is unlikely to go the way of other immigrant languages in the United States and succumb to English language dominance across the generations.

Identity, integration and assimilation

Assimilation and integration are often treated as separate processes. Assimilation refers to a gradual movement toward the majority culture and language, while integration describes participation in host institutions that may coexist with retained origin identities. Second-generation individuals often face particular tensions here. Researchers describe them as perpetual insiders and outsiders in the lands they are meant to call "home." Identity is therefore frequently transnational and hybrid rather than a choice between two cultures. Studies of diaspora musicians describe how migrants rediscover their musical culture of origin and reinterpret it from a new, cosmopolitan perspective. In one account, music helps these communities construct an "inner homeland" to identify with, while living abroad in constant balance (or contradiction) between cultures.

Diffusion of religion, cuisine, music and other cultural forms

Diasporas and transnational networks are important channels through which religion, food, music and other cultural forms move. Cuisine is a clear example. Diasporic food is complex and fluid, shaped by the tensions between their original and new countries and cultures. Such cuisines also evolve, incorporating new hybrid forms and characteristics. Host societies then become consumers as well, and a boom in restaurants of cuisines of the migrant communities grew in number. Some dishes move beyond ethnic enclaves altogether; for example, couscous has appeared on the menus of non-ethnic restaurants and as an optional accompaniment instead of rice, potatoes or pasta in popular restaurants.

Religious practice follows similar networks of trade, pilgrimage and missionary activity, such as those linking Buddhism, Islam and Christianity to particular migrant communities and routes. Language, music and food often serve as markers of diaspora identity, which is why they are central to cultural continuity in host countries.

Public responses to diversity

Host societies have responded to cultural and linguistic diversity in different ways. Common measures include language instruction for newcomers, heritage-language programs in schools, and rules on access to citizenship, which in many states include language requirements. The balance between these measures and the expectations placed on migrants remains a matter of public debate, and the outcomes depend heavily on local policy, school resources and the composition of each immigrant population.

Governance, law and international frameworks

State sovereignty and its limits

Under the prevailing international order, states decide who may enter their territory, how borders are controlled, when a non-citizen may be expelled, and who may be naturalized. The UN Global Compact for Migration (2018) restates this principle, describing its own commitments as respecting each state's sovereign right to determine who enters and stays. Human rights law sets the main limits on that discretion. The most important is non-refoulement, the ban on returning a person to a place where they face serious threats to life or freedom. UNHCR describes this principle as part of customary international law, which binds all states whether or not they are parties to the refugee treaties. Other treaties add parallel protections, such as the Convention against Torture, which applies a similar ban where there is a risk of torture.

The Universal Declaration of Human Rights

The Universal Declaration of Human Rights, adopted by the UN General Assembly in 1948, is not a legally binding treaty, but it is widely treated as the reference point for human rights standards. Article 13 affirms freedom of movement and residence within each state's borders and the right to leave any country and return to one's own. Article 14 affirms the right to seek and enjoy asylum from persecution, with an exclusion for prosecutions arising from non-political crimes or from acts contrary to the purposes and principles of the United Nations. The wording of Article 14 is deliberately limited. It does not give an individual the right to be granted asylum, a compromise between states that treated asylum as part of territorial sovereignty and states that wanted an individual entitlement. Many of the Declaration's drafters had seen governments turn away Jewish refugees before the Holocaust, and that experience shaped the inclusion of these provisions.

The 1951 Refugee Convention and its 1967 Protocol

The 1951 Refugee Convention defines a refugee as a person who is unable or unwilling to return to their country of origin because of a well-founded fear of persecution on grounds of race, religion, nationality, membership of a particular social group or political opinion. Its original scope was narrow: it covered people fleeing events before 1 January 1951 and, in practice, within Europe. The 1967 Protocol removed these temporal and geographic limits, which gave the Convention universal application. The core provision is Article 33, which contains the non-refoulement principle. Other articles prohibit penalizing refugees for irregular entry (Article 31), restrict expulsion to strictly defined grounds (Article 32), and set minimum standards for access to work, housing, education and documentation. The Convention also excludes certain categories, such as people who have committed war crimes or crimes against humanity, from refugee status. The Convention is supervised through the mandate of the United Nations refugee agency, UNHCR, which has responsibility for promoting and supervising its application.

The Global Compact for Safe, Orderly and Regular Migration

The Global Compact for Safe, Orderly and Regular Migration was negotiated among states under UN auspices and is described as the first intergovernmentally negotiated agreement covering all dimensions of international migration. It was endorsed by the General Assembly on 19 December 2018 and sets out 23 objectives covering the whole migration cycle. The Compact is explicitly non-binding. Its implementation therefore depends on what member states choose to do with it. Several states voted against its endorsement, and some states that did not support it have since joined its follow-up process. Because it creates no enforceable obligations, the Compact works as a shared policy framework and a source of norms rather than as a source of individual rights that courts can enforce.

Visa regimes, labor agreements and free-movement zones

Most day-to-day regulation of cross-border movement takes place through national and bilateral instruments. Visa regimes determine which nationals need prior authorization to enter a given country, and they are among the most direct tools of admission policy. Bilateral labor agreements between sending and receiving states can set recruitment procedures and conditions of employment for temporary workers. At the other end of the spectrum, free-movement zones remove many internal border controls. The Schengen area is the best-known example, and EU freedom of movement gives citizens of member states the right to live and work across the Union. These arrangements show that sovereignty can be exercised collectively, through pooling and limiting border controls, as well as individually.

Open questions and scholarly debates

Timing and routes of early Homo sapiens dispersals

Most researchers accept an African origin for Homo sapiens, but the date and route of the first departures remain disputed. Fossil and genetic dating of the main exit point to roughly 65,000 years ago, The dating of most out-of-Africa fossils, and of the split between Eurasian and African populations based on mitochondrial and whole-genome data point to a major exit ~65k years ago. Earlier movements are also documented. Some groups reached the Levant well before this main exit, and a handful of lineages belonging to Homo sapiens, for example, migrated out of Africa approximately 70,000 to 100,000 years ago, only to become extinct. Whether such early groups count as failed dispersals or as the first stage of a longer process is still argued over.

The South Asian leg is especially contested. Many researchers have proposed a dispersal from East Africa between 60,000 and 40,000 years ago, but one review judges the supporting evidence very weak, particularly the lack of human skeletal evidence between the Levant and Borneo before 40 ka, and argues that the population history may be more complex than the single-wave model allows. Genetic work adds another layer: a study of eastern African mitochondrial lineages suggests that it was eastern Africa, not the south, that witnessed the first major demographic expansion, ~70–60 ka, which led to the peopling of the rest of the world. The origin population within Africa is therefore still a matter of inference rather than direct evidence.

Interbreeding with archaic populations

Genetic studies have shown that archaic humans contributed to the ancestry of present-day people. Analyses of a draft Neanderthal genome estimated that Neanderthals made a 1–4% contribution to the gene pool of modern non-African populations, and later work refined this to around 2–3 percent, with interbreeding dated to about ~40 and ~54 kya. Denisovan admixture is also documented, most strongly in Oceanian populations. Open questions concern how many such episodes occurred, where, and with what consequences. Recent work suggests that gene flow ran in both directions; one study estimates that as a result of this interbreeding, 2%–3% of non-African AMH genomes are derived from Neanderthals. Estimates of how much archaic DNA was introduced vary with the method used, and the functional effects of many introgressed variants are not fully resolved. Interpretations also differ on how much these exchanges shaped the overall genetic picture. Most of the genome of present-day humans is still regarded as African in origin, and replacement without any interbreeding is now considered unlikely.

The relative weight of climate, economic and social causes

Explanations for early and historical expansions compete rather than sit in a settled hierarchy. Palaeoclimate studies emphasize environmental windows. One analysis of rainfall requirements for hunter-gatherers concludes that the estimated timings and routes of potential contact with Eurasia are compatible with archaeological and genetic evidence of human expansions out of Africa, highlighting the key role of palaeoclimate variability for modern human dispersals. Other researchers stress social and technological factors. One proposal holds that a mitochondrial signal of a dispersal from southern Africa shortly before the main expansion could have carried cultural elements that enabled the later movement. Another finds that a notable expansion in human niche breadth within Africa precedes this later dispersal. These accounts are not mutually exclusive, but the evidence rarely isolates one cause. Similar uncertainty applies to historical expansions such as the Bantu or Mongol movements, where climate, economic opportunity, state formation and military organization interact, and where written sources are unevenly distributed.

Data gaps and inconsistent definitions

Evidence about past and present migration is uneven, and this affects the conclusions drawn from it. Archaeological and genetic records are shaped by preservation, sampling and political access; in some regions, scholars note that more physical evidence is needed from key areas in Eurasia such as Iran, Afghanistan and Pakistan, but politics is currently making this difficult. For recent periods, the problem is different. Undocumented migrants are, by definition, absent from registers, permit systems and censuses, so their numbers are usually estimated indirectly. National and international bodies also differ in how they define a migrant, a refugee or a foreign-born resident, and in whether they count by place of birth, citizenship, or length of stay. These differences produce figures that cannot be compared directly, and they complicate debates about forced displacement, labor flows and the size of diasporas.

Economic and social effects of immigration

Research on the labor-market effects of immigration has produced findings that vary by skill level, local context and method. The 1980 Mariel boatlift, which brought 125,000 mainly low-skill Cuban immigrants arrived in Miami in 1980, has become a central test case. An early analysis by David Card found no measurable wage or employment effect on non-Cuban workers in Miami. A 2015 reanalysis by George Borjas argued that Card's comparison had obscured a substantial wage decline for less-educated workers, and the estimated drop for high school dropouts in Miami dropped dramatically, by 10 to 30 percent (depending on which control "placebo" groups is employed). Subsequent studies disputed this result, and some attribute the disagreement to changing sample composition in the survey data, particularly the racial mix of workers. The dispute has not fully ended. One review concludes that the evidence from the Mariel Boatlift does not offer a compelling reason to revise the consensus among research economists that the effect of immigration on native wages is small, while the same authors acknowledge that this is fundamentally different from proving that the Mariel Boatlift did not reduce wages for that group. Broader studies of refugee inflows show similar disagreement, with some finding the impact of immigration on average native-born workers remains small and inconsistent, and others reporting larger negative effects. Fiscal, cultural and social outcomes are contested in comparable ways, and the findings are best read as ongoing research rather than settled conclusions.

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