JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. OVER THE PAST 30 YEARS, imrnigration has emerged as a major force throughout the world. In traditional immigrant-receiving societies such as Australia, Canada, and the United States, the volume of immigration has grown and its composition has shifted decisively away from Europe, the historically dominant source, toward Asia, Africa, and Latin America. In Europe, meanwhile, countries that for centuries had been sending out migrants were suddenly transformed into immigrant-receiving societies. After 1945, virtually all countries in Western Europe began to attract significant numbers of workers from abroad. Although the migrants were initially drawn mainly from southern Europe, by the late 1960s they mostly came from developing countries in Africa, Asia, the Caribbean, and the Middle East. By the 1980s even countries in southern Europe-Italy, Spain, and Portugal-which only a decade before had been sending migrants to wealthier countries in the north, began to import workers from Africa, Asia, and the Middle East. At the same time, Japan-with its low and still declining birth rate, its aging population, and its high standard of living-found itself turning increasingly to migrants from poorer countries in Asia and even South America to satisfy its labor needs. Most of the world's developed countries have become diverse, multiethnic societies, and those that have not reached this state are moving decisively in that direction. The emergence of international migration as a basic structural feature of nearly all industrialized countries testifies to the strength and coherence of the POPULATION AND DEVELOPMENT REVIEW 19, NO. 3 (SEPTEMBER 1993) 431 At present, there is no single, coherent theory of international migration, only a fragmented set of theories that have developed largely in isolation from one another, sometimes but not always segmented by disciplinary boundaries. Current patterns and trends in immigration, however, suggest that a full understanding of contemporary migratory processes will not be achieved by relying on the tools of one discipline alone, or by focusing on a single level of analysis. Rather, their complex, multifaceted nature requires a sophisticated theory that incorporates a variety of perspectives, levels, and assumptions. The purpose of this article is to explicate and integrate the leading contemporary theories of international migration. We begin by examining models that describe the initiation of international movement and then consider theories that account for why transnational population flows persist across space and time. Rather than favoring one theory over another a priori, we seek to understand each model on its own terms in order to illuminate key assumptions and hypotheses. Only after each theory has been considered separately do we compare and contrast the different conceptual frameworks to reveal areas of logical inconsistency and substantive disagreement. In undertaking this exercise, we seek to provide a sound basis for evaluating the models empirically, and to lay the groundwork for constructing an accurate and comprehensive theory of international migration for the twenty-first century. A variety of theoretical models has been proposed to explain why international migration begins, and although each ultimately seeks to explain the same thing, they employ radically different concepts, assumptions, and frames of reference. Neoclassical economics focuses on differentials in wages and employment conditions between countries, and on migration costs; it generally conceives of movement as an individual decision for income maximization. The "new economics of migration," in contrast, considers conditions in a variety of markets, not just labor markets. It views migration as a household decision taken to minimize risks to family income or to overcome capital constraints on family production activities. Dual labor market theory and world systems theory generally ignore such micro-level decision processes, focusing instead on forces operating at much higher levels of aggregation. The former links immigration to the structural requirements of modern industrial economies, while the latter sees immigration as a natural consequence of economic globalization and market penetration across national boundaries. Given the fact that theories conceptualize causal processes at such different levels of analysis-the individual, the household, the national, and the international-they cannot be assumed, a priori, to be inherently incompatible. It is quite possible, for example, that individuals act to maximize income while families minirnize risk, and that the context within which both decisions are made is shaped by structural forces operating at the national and international levels. Nonetheless, the various models reflect different research objectives, focuses, interests, and ways of decomposing an enormously complex subject into analytically manageable parts; and a firm basis for judging their consistency requires that the inner logic, propositions, assumptions, and hypotheses of each theory be clearly specified and well-understood. Probably the oldest and best-known theory of international migration was developed originally to explain labor migration in the process of economic development (Lewis, 1954; Ranis and Fei, 1961; Harris and Todaro, 1970; Todaro, 1976) . According to this theory and its extensions, international migration, like its internal counterpart, is caused by geographic differences in the supply of and demand for labor. Countries with a large endowment of labor relative to capital have a low equilibrium market wage, while countries with a limited endowment of labor relative to capital are characterized by a high market wage, as depicted graphically by the familiar interaction of labor supply and demand curves. The resulting differential in wages causes workers from the lowwage country to move to the high-wage country. As a result of this movement, the supply of labor decreases and wages rise in the capital-poor country, while the supply of labor increases and wages fall in the capital-rich country, leading, at equilibrium, to an international wage differential that reflects only the costs of international movement, pecuniary and psychic. Mirroring the flow of workers from labor-abundant to labor-scarce countries is a flow of investment capital from capital-rich to capital-poor countries. The relative scarcity of capital in poor countries yields a rate of return that is high by international standards, thereby attracting investment. The movement of capital also includes human capital, with highly skilled workers moving from capital-rich to capital-poor countries in order to reap high returns on their skills in a human capital-scarce environment, leading to a parallel movement of managers, technicians, and other skilled workers. The international flow of labor, therefore, must be kept conceptually distinct from the associated international flow of human capital. Even in the most aggregated macro-level models, the heterogeneity of inmmigrants along skill lines must be clearly recognized. The simple and compelling explanation of international migration offered by neoclassical macroeconomics has strongly shaped public thinking and has provided the intellectual basis for much inmmigration policy. 1 The international migration of workers is caused by differences in wage rates between countries. 2 The elimination of wage differentials will end the movement of labor, and migration will not occur in the absence of such differentials. 3 International flows of human capital-that is, highly skilled workersrespond to differences in the rate of return to human capital, which may be different from the overall wage rate, yielding a distinct pattern of migration that may be opposite that of unskilled workers. 4 Labor markets are the primary mechanisms by which international flows of labor are induced; other kinds of markets do not have important effects on international migration. 5 The way for governments to control migration flows is to regulate or influence labor markets in sending and/or receiving countries. Corresponding to the macroeconomic model is a microeconomic model of individual choice (Sjaastad, 1962; Todaro, 1969 Todaro, , 1976 Todaro, , 1989 Todaro and Maruszko, 1987). In this scheme, individual rational actors decide to migrate because a cost-benefit calculation leads them to expect a positive net return, usually monetary, from movement. International migration is conceptualized as a form of investment in human capital. People choose to move to where they can be most productive, given their skills; but before they can capture the higher wages associated with greater labor productivity they must undertake certain investments, which include the material costs of traveling, the costs of maintenance while moving and looking for work, the effort involved in learning a new language and culture, the difficulty experienced in adapting to a new labor market, and the psychological costs of cutting old ties and forging new ones. Potential migrants estimate the costs and benefits of moving to alternative international locations and migrate to where the expected discounted net returns are greatest over some time horizon (Borjas, 1990) . Net returns in each future period are estimated by taking the observed earnings corresponding to the individual's skills in the destination country and multiplying these by the probability of obtaining a job there (and for illegal migrants the likelihood of being able to avoid deportation) to obtain "expected destination earnings." These expected earnings are then subtracted from those expected in the community of origin (observed earnings there multiplied by the probability of employment) and the difference is summed over a time horizon from 0 to n, discounted by a factor that reflects the greater utility of money earned in the present than in the future. From this integrated difference the estimated costs are subtracted to yield the expected net return to migration. This decisionmaking process is summarized analytically by the following equation: ( 1) where ER(0) is the expected net return to migration calculated just before departure at time 0; t is time; P1 (t) is the probability of avoiding deportation from the area of destination (1.0 for legal migrants and <1.0 for undocumented niigrants); P2(t) is the probability of employment at the destination; Yd(t) is earnings if employed at the place of destination; P3(t) is the probability of employment in the community of origin; Yo(t) is earnings if employed in the community of origin; r is the discount factor; and C(O) is the sum total of the costs of movement (including psychological costs). If the quantity ER(0) is positive for some potential destination, the rational actor migrates; if it is negative the actor stays; and if it is zero, the actor is indifferent between moving and staying. In theory, a potential migrant goes to where the expected net returns to migration are greatest, leading to several important conclusions that differ slightly from the earlier macroeconomic formulations: 1 International movement stems from international differentials in both earnings and employment rates, whose product deternines expected earnings (the prior model, in contrast, assumed full employment). 2 Individual human capital characteristics that increase the likely rate of remuneration or the probability of employment in the destination relative to the sending country (e.g., education, experience, training, language skills) will increase the likelihood of international movement, other things being equal. 3 Individual characteristics, social conditions, or technologies that lower migration costs increase the net returns to migration and, hence, raise the probability of international movement. 4 Because of 2 and 3, individuals within the same country can display very different proclivities to migrate. 5 Aggregate migration flows between countries are simple sums of individual moves undertaken on the basis of individual cost-benefit calculations. 6 International movement does not occur in the absence of differences in earnings and/or employment rates between countries. Migration occurs until expected earnings (the product of earnings and employment rates) have been equalized internationally (net of the costs of movement), and movement does not stop until this product has been equalized. 7 The size of the differential in expected returns determines the size of the international flow of migrants between countries. 8 Migration decisions stem from disequilibria or discontinuities between labor markets; other markets do not directly influence the decision to migrate. 9 If conditions in receiving countries are psychologically attractive to prospective migrants, migration costs may be negative. In this case, a negative earnings differential may be necessary to halt migration between countries.
Network theory has been widely used to explain international migration REF .
Theories of International Migration: A Review and Appraisal
Population and Development Review
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