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Immigration can play a critical role in determining the future size of the U.S. population. Within the population balancing equation, population growth could rise substantially if fertility or net immigration rises or if mortality declines. The rate of fertility, which has been below the replacement level for almost 20 years, shows little evidence of reviving sufficiently to alter the course of population growth. Our mortality assumptions incorporate mortality declines over the projection period, which will lead to higher levels of natural increase and population growth. Population growth will be even higher if there is an even greater improvement in life expectancy.18
That leaves immigration as the most likely factor acting to spur population growth in the coming decades. Apart from the additions its net numbers make to the population, immigration, because it alters the age and racial/ethnic composition of the population, influences the rates of fertility and mortality that are the basic components of population change.
Demographic change has three main effects: on population size, on population composition, and on population growth. We draw attention to changes in the national population size and growth in later sections of this chapter.19 Age composition is one of the most important compositional effects, because immigration alters relative numbers of the school-age population, young adults, and the elderly.
Table 3.3 lists our projections of the future size of the American population under five alternative immigration assumptions. It is important to remember that this country's population will continue to grow well into the next century, even if net immigration was immediately eliminated forever. Under the assumption of zero immigration, the population of the United States will grow slowly, reaching a peak of 311 million in about 2035.20 Thereafter, it will very slowly decline, because, as the baby-boom cohorts age, deaths will outweigh births.
Table 3.3 also demonstrates that population growth will be significantly higher under any of our scenarios of positive levels of immigration. According to
The sensitivity of the overall population results to alternative fertility or mortality assumptions are described in Appendix 3.B.
We do not present projections for states or other subnational population groups. The Census Bureau makes population projections for states; however, their projections do not separately display different immigration assumptions. Most states also prepare population projections, although they do not appear to examine separately the effect of different U.S. immigration assumptions.
The projections reported here differ from but are close to those prepared by the U.S. Bureau of the Census (1996a). The Census Bureau's median series projection begins with the same 1995 base population of 263 million. The Census Bureau's projected population for 2050 is 394 million, 7 million or 2 percent greater than this projection. Because we make similar immigration, emigration, and mortality assumptions, the reason for the difference is that our projections assume overall lower fertility levels. We make separate fertility assumptions by immigrant generation, including the notion that fertility decreases with greater generational residence in the United States. The Census Bureau's projections do not distinguish fertility levels by the native- and the foreign-born.