POPULATION MOMENTUM EXPRESSES POPULATION AGING

Population momentum and population aging occur when an initially growi ng population experiences a reduction in fertility to replacement leve l. Conceptually and empirically, momentum and aging express the same c hange, albeit on different scales. Fundamentally, they are two manifes tations of the underlying process of demographic transformation. We co nsider three measures of aging over the transition to stationarity: th e increase in mean population age, the decrease in the proportion unde r age 30, and the increase in the proportion over age 65. The three me asures of aging are highly correlated though the relationship to momen tum is weakest for the increase in the proportion over age 65. We find that momentum is linearly related to aging. In both model and actual populations, a one-year increase in mean age translates into about 4.5 % more population growth. The population below age 30 does not grow ov er the transition to stationarity, and the ratio of initial to ultimat e proportions under age 30 is virtually identical to momentum.