Demographic Change in the United States, 1970-2050
Samuel H. Preston
This paper focuses on the future size and age composition of the population of the United States. The impending changes are viewed through the prism of population projections made by two federal agencies: the Bureau of the Census and the Social Security Administration. The successes and failures of the projection programs of these agencies is briefly evaluated, and we consider at greater length the plausibility of their most recent intermediate projection series. Our conclusion is that mortality is very likely to improve at a faster pace than they have projected. Exaggerating the impact of this factor on population size, but partially offsetting its impact on age structure, we anticipate that future fertility rates will be higher and immigration flows faster than projected by these agencies. These conclusions are based upon analyses of recent demographic change in light of broader social and economic trends.
Population projection is a mechanical exercise that demonstrates the implications of particular sets of fertility, mortality, and migration rates, combined with an initial population age structure, for future population size and composition. Age-specific mortality rates are applied in order to survive the population forward in time; age-specific fertility rates are then applied in order to project births. Immigrants and emigrants are typically added at the last stage. The procedures are logical and algebraically coherent. They have the virtue of making endogenous one of the most important determinants of demographic change, a population's age structure. Populations with larger proportions over age 50, for example, will have higher death rates and lower birth rates and growth rates, ceteris paribus.
The initial population size and age distribution are usually known with a high