3 emigration statistics since 1957 and the indirect estimates that have been made are not entirely reliable. Even less is known about illegal immigration in the United States and elsewhere. Estimates of the stock of illegal immigrants in the United States have varied from 1.5 million to 12 million in the past few years. Although there is no supporting evidence for figures in the upper half of that range, even the most careful estimates range from about two to four million. There is even less of a consensus on the magnitude of net annual flows of illegal migrants. Illegal migration is also a major phenomenon in some countries in Latin America, Asia, Europe, and Africa. Measurement is further complicated by the existence in many areas of nomadic tribes who pay little attention to international boundaries. Measuring international migration is a major problem in parts of Africa where international borders have historically had little significance.

Finally, even in situations in which the measurement of international migration is reasonably accurate, statistics are rarely broken down by age group and only sometimes broken down by the sex of the migrants. All of the above considerations make it difficult to measure recent levels of international migration with any degree of accuracy or to provide a sound basis for the projection of future international migration flows. International migration is the most poorly documented component of recent trends in population growth, and in many cases it is exceedingly difficult to predict future trends. Future flows depend on economic, social and political circumstances in the countries of origin and destination, as well as changes in immigration and emigration laws in both sending and receiving countries, all of which are difficult to predict in their own right.

Therefore, estimates and projections of net international migration require an unusual degree of subjective decision making and the results do not inspire a high degree of confidence. Fortunately, for the majority of countries, the precise magnitude of international migration makes little difference for the results of population projections. Because international migration is a negligible component of overall population growth in these countries, the projections can be relied on even when there is a fairly wide margin of error in the net international migration assumptions. There are only a few countries for which the net international migration statistics will make a substantial difference.

# OVERALL ASSESSMENT OF INTERNATIONAL MIGRATION ASSUMPTIONS

For long-term projections of international migration, it is plausible to assume that net migration for each country will eventually approach zero. That assumption, however, is not reasonable in the short run. With only eleven years remaining between now and the year 2000, it is not plausible to assume that migration to the major receiving countries (the United States, Canada, and Australia) will drop rapidly during the years covered by the World Bank's projections. In fact, for some countries, even the assumption of constant migration throughout the period (which has been adopted by the United Nations for countries with a long history of international migration) should be considered conservative.