Measuring population aging as a function of fertility, mortality, and migration.

Researchers often describe population aging with measures such as proportion of population under 15 years of age, proportion of population 65 years and over, the aged/child ratio (a ratio of the first two measures), median age, and mean age. Investigators of cross-cultural gerontology, too, use such measures to compare population aging between populations. While insights from comparing only the values of an aging measure between two populations may be helpful, results from such comparison can be misleading due to the influences of fertility, mortality, and migration. To show the effects of these demographic processes on measures of population aging, rates of change in five measures of population aging as a function of fertility, mortality, and migration are developed. These rates of change are estimated for various stable populations, for stable populations disequilibrated by fertility and mortality declines and by net migration, and for the population in Japan, 1988-1989. The findings demonstrate that the five aging measures, in general, do not give consistent rate-of-change estimates; they also suggest that directly comparing values of aging measures without considering the levels and patterns of fertility, mortality, and migration will lead to misleading conclusions.