Demographers divide the determinants of the volume of migration into two components. The first concerns the propensity of a given type of individual to migrate, that is, the probability that someone will migrate to a different place of usual residence within a given time. The second concerns the number of individuals who are at risk of migration at each level of propensity. The volume of migration can be calculated as the sum over all levels of propensity of the products obtained by multiplying each separate propensity to migrate by the number of individuals possessing that propensity.
A large part of this chapter is devoted to analyzing variations in the individual propensity to migrate. In trying to project the future volume of immigration into or away from the United States, however, it is extremely important to be aware of the trends in world population size by region forecast for the future by demographers at the United Nations (UN). These forecasts are the necessary basis for estimating the population at risk of immigrating to the United States and emigrating from the United States. The future population at risk of immigrating is of course the population of the whole world minus the population of the United States. The future population at risk of emigrating from the United States is simply the future population of the United States. Accordingly, I begin this chapter with a discussion of the UN population forecasts.
Every other year the Population Division of the UN prepares new population projections for each nation in the world.1 The UN prepares three series for each nation: high, medium, and low. The medium projection is taken the most likely and may therefore be taken to be the forecast. Similar to projections prepared by the U.S. Bureau of the Census, the UN projections employ the cohort component method. Such a method demands assumptions concerning age-sex-specific rates of death, birth, and net immigration for each birth cohort. In making these assumptions, the UN relies heavily on the theory of the demographic transition. Therefore, in