Book excerpt: At the end of the 20th century nearly all developed nations have become countries of immigration, absorbing growing numbers of immigrants not only from developed regions, byt increasingly from developing nations of the Third World. Although international migration has come to play a central role in the social, economic, and demographic dynamics of both immigrant-sending and immigrant-receiving countries, social scientist have been slow to construct a comprehensive theory to explain it. Efforts at theoretical explanation have been fragmented by disciplinary, geographic, and methodological boudaries. Worlds in Motion seeks to overcome these schisms to create a comprehensive theory of international migration for the next century. After explicating the various propositions and hypotheses of current theories, and identifying area of complementarity and conflict, the authors review empirical research emanting from each of the world's principal international migration systems: North America, Western Europe, the Gulf, Asia and the Pacific, and the Southern Cone of South America. Using data from the 1980s, levels and patterns of migration within each system are described to define their structure and organization. Specific studies are then comprehensively surveyed to evaluate the fundamental propositions of neoclassical economics, the new economics of labour migration, segmented labour market theory, world systems theory, social capital theory, and the theory of cumulative causation. The various theories are also tested by applying them to the relationship between international migration and economic development. Although certain theories seem to function more effectively in certain systems, all contain elements of truth supported by empirical research. The task of the theorist is thus to identify which theories are most effective in accounting for international migration in the world today, and what regional and national circumstances lead to a predominance of one theoretical mechanism over another. The book concludes by offering an empirically-grounded theoretical synthesis to serve as a guide for researchers and policy-makers in the 21st century.