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However, it should be noted that immigration involves primarily a redistribution of the world's population, not an absolute increase. Indeed, since the fertility of immigrants tends to decline after they come to the United States, total world population will be slightly lower. The potential for negative environmental effects (congestion and the like) must then be primarily local. From a world perspective, (negative) environmental effects in the United States may be counterbalanced by possible (positive) effects in the sending countries that are losing population. Total consumption by immigrants will typically be higher in the United States than in the places they left (which, after all, is one reason they immigrated). But efforts to abate environmental effects at any given level of consumption may also be higher in the United States. A weighting of the factors should enter into an evaluation of the environmental effects of immigration.
Age and Sex Composition
Even with zero net migration, a population's age and sex composition will shift if the characteristics of the immigrants and emigrants differ. The general effect of migration on the population structure over the past decades can be illustrated by comparing the actual age and sex compositions in 1995 with what they would have been in the absence of migration since 1950 (see Table 3.5).25 It can be seen that the effect of net migration has been noteworthy over this period in terms of both the population and sex composition.
In the absence of migration since 1950, the U.S. population would have numbered about 225 million in 1995, about 14 percent fewer than the actual 263 million. However, the impact of migration on sex composition would have been trivial: the actual population distribution has slightly more males and slightly fewer females than the hypothetical population with no migration.
Immigration would, however, have influenced the age composition for both males and females. In general, migration would have made the population younger, adding to the proportion of the population aged 10 to 39 years and reducing the proportion who are aged 50 to 79.
Sex Composition
As our illustration just suggested, the impact of alternative immigration
25
The data (more readily available for immigrants than for emigrants) suggest that immigrants are more concentrated than emigrants in the young adult ages, 15 to 29 years. Emigrants tend to include more children and older adults, reflecting a tendency for emigrants to be families with children. Nevertheless, the general age and sex distributions are similar for immigrants and emigrants. The zero-migration compositions were obtained by applying appropriate cohort survival values to the 1950 U.S. population (who would have been 45 years and older in 1995) and to the age-zero population (the annual number of observed births) in each subsequent year to obtain a complete set of estimates for 1995.
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