Methodology and Assumptions for the Population Projections of the United States: 1999 to 2100

This working paper discusses the methodology and assumptions used to develop the recently released projections of the population of the United States from 1999 to 2100. The new series includes projections of the population by single year of age, sex, race, Hispanic origin, and nativity. While the basic methodology used to produce these projections is the same as in earlier Census Bureau national population projections, there have been changes, in both the time horizon and reference dates of the projections, as well as in the specific methods used to estimate population change. The extension of the series to 2100 carries the projections 20 years further into the future than any series previously issued by the Census Bureau. For the first time, projection results include a break on nativity, defined dichotomously by the presence or absence of U.S. citizenship at birth, as well as its cross-classification with other variables. Also new with this series is the projection to quarterly reference dates, allowing users to view the national population seasonally, or simply to select annual reference dates other than July 1. In addition, international migration in the new series is allowed to vary over time, remaining somewhat lower than the constant value in the previous series for the first two decades of the century, but reaching considerably higher levels than in the previous one after 2020. Fertility rates in both models are allowed to change very little over time. However, fertility rates by race and Hispanic origin are allowed to converge in the new middle series, whereas in the previous middle series they remained constant within race and origin category. Finally, the new mortality assumptions show more improvement in life expectancy for all racial and Hispanic origin groups, except the non-Hispanic White population, than did the assumptions of the previous projection series.