T HE migration of Puerto Ricans to the continental United States (primarily to the New York City region) represents a most striking and important example of population redistribution. The purpose of this study is not only to chronicle some of its important aspects, but also to make use of a fruitful method for relating migration to economic incentives. In it we treat population from the point of view of its function as labor supply; consequently this is a study of net migration. The movement of population from Puerto Rico to the United States mainland provides us with an unusual natural experiment something which is very hard to find in the social sciences in that both the source and receiving areas of movement are readily definable, and information is available over a series of years. Perhaps it is crude to think of the United States and Puerto Rico as points which are receiving and source areas of migration, respectively, but a study of the geographical distribution of the origins and destinations of migrants must follow this preliminary analysis. Of course, since Puerto Rico is politically a part of the United States there are no legal barriers to hinder the flow of population. The theoretical framework which forms the basis of this examination is straightforward. It assumes that migrants change homes in order to improve their lifetime earning prospects; against the expected gains from moving must be balanced various pecuniary and non-pecuniary costs. Examples of the pecuniary costs readily come to mind. Among the most important are transportation and the loss of earnings to migrants while they are travelling and initially unemployed in the receiving country. Nonpecuniary costs involve, among other things, the psychological discomforts of changing social environments. We should expect that changes in the difference between the gain and cost of moving will affect the quantity of migration, and, over the length of time considered in this paper, that the pecuniary factors have fluctuated more widely and systematically than the non-pecuniary. Therefore, a discussion of the migration primarily in economic terms should prove fruitful. The basic model we have used is inspired by a simple empirical observation: in the short run, relative hourly wage rates in the source and receiving areas appear to have but little effect on population flows, but employment rates or job availabilities have an important effect. Consider a potential migrant.' He will probably derive some notion of the gain to him from moving by observing the interregional pattern of wage rates over some length of time, and he is unlikely to be influenced greatly by day-to-day, or even year-to-year fluctuations in these rates. But having decided that the longrun gains outweigh the long-run costs, the potential migrant must decide exactly when to quit his old job and board plane or boat. This will probably depend largely on the difficulty of financing the trip. It is not surprising that, especially when funds for moving are scarce, fluctuations in the short-run cost of migration will be closely associated with fluctuations in migration, itself.2