Migration persistence models

The general push-pull framework is static in nature, in the sense that it explains only the initiation of migration but not its prevailing perpetuation. As pointed out above, migration may be initiated by a variety of reasons: individuals chas-ing higher income; households searchchas-ing to diversify risks; employers in indus-trial countries recruiting from abroad to satisfy their permanent need for labor;

inhabitants of peripheral regions moving after the core disturbed their economic structure through the penetration of their markets; or a combination of the above.

But nothing indicates that the conditions which initiate the transnational

move-ment are the same as those that perpetuate it across time and space. As suggested by Massey et al. (1993), wage differentials, relative risks, recruitment efforts, and market penetration may continue to cause people to move, but the act of migra-tion also generates condimigra-tions which themselves turn out independent causes for new migratory streams. Theories of the perpetuation of international migration are able to explain why migration to a certain destination might persist even if the initial incentive for migration has disappeared. As such, migration processes de-velop their own momentum once they have started, making additional movement in the future more likely, a process Myrdal (1958) called cumulative causation (Massey et al., 1993).

The most important source of persistence concerns the presence of social networks in the destination country, which add a dynamic perspective to the migration de-cision. Nelson (1959) was the first to emphasize the role of prior migrants in influencing settlement patterns of subsequent migrants. This type of migration is often called ‘chain migration’ because it might be seen as a self-sustaining diffu-sion process. New migrants are attracted to destinations inhabited by friends or relatives who moved there before. The larger the network of interrelated individ-uals in the host country, the lower the costs and risks of migration. Having friends and relatives in a receiving country not only reduces the psychological costs of migration (the costs involved in cutting old ties and building new ones); they also have an impact on the monetary costs of the move (family and friends can help to arrange transport and offer temporary accommodation) and the expected returns to migration (by offering assistance to find a job upon arrival). Over time this process spreads outward and motivates larger segments of the sending society to move (Hugo, 1981; Taylor, 1984; Massey and Denton, 1987; Massey, 1990a,b;

Gurak and Caces, 1992).

Other theories aimed at explaining the continuation of migration concern the

insti-tutional theory of migration, the theory of cumulative causation and the migration systems theory. The first theory argues that as migration between countries oc-curs on a larger scale, institutions step in devoted to the arrival and integration of successive waves of immigrants. As these private and voluntary organiza-tions become known to migrants, they comprise another form of social capital which reduces the cost of migration and thus increases the propensity to migrate.

This gradual build-up of institutions generates a flow of migrants which becomes more and more institutionalized and independent of the factors which initiated it (Massey et al., 1993). The theory of cumulative causation, originally devel-oped by Myrdal (1958), posits that once the migration process has started off, it changes the economic and social context in which subsequent migration decisions by individuals or households are made (Stark et al., 1986; Taylor, 1992; Massey et al., 1993). The migration systems theory, finally, can be considered a general-ization of the theories previously described. It incorporates the theory of cumu-lative causation in the sense that it takes into account changes in demographics and in the political, social and economic conditions caused by international mi-gratory moves. As such, the network theory and institutional theory of migration can be seen as two examples of how international migration modifies prevailing conditions in an international migration system. Yet, it goes even further by also considering changes in the political, demographic, economic and social context which cannot be related to international migration. Specifically, an international migration system consists of a group of receiving countries that are linked to a set of source countries by relatively intense flows and counterflows of goods, capital and migrants. As such, the countries in the migration system are not only linked by exchanges of people but also by other types of linkages (Fawcett, 1989). Kritz and Zlotnik (1992) distinguish four of them: historical, cultural, colonial and tech-nological linkages, all of which are allowed to change across time and space.

Nonetheless, although these dynamic models of international migration initially rely on very different initial assumptions compared to the traditional push-pull model of migration, the latter does not prevent these sources of persistence from being included in the empirical model. Both social networks and migration-encouraging institutions as well as the broader societal factors and linkages can easily be integrated into a push-pull framework for migration.

Although most recent utility based models of international migration control for the impact of social networks in the migration decision, Hatton (1995) goes even further by building in two dynamic components in the neoclassical model of mi-gration. The model introduces uncertainty in the migration decision and accounts for the formation of expectations about future income streams based on past in-formation. This approach has two important implications. First, both the changes and the levels of the explanatory variables enter the model separately. This makes it possible to distinguish between short-run and long-run determinants of migra-tion. Second, the estimation equation includes both the lagged dependent variable and the migrant stock. The simultaneous inclusion of these variables forms one of the major methodological issues discussed below.