The Effects of Changes in Mortality and Fertility on Age Composition

HE age composition of any population is determined wholly by the past history of its births and deaths at each age, and by the number and age of the migrants who have entered or left the population. In the 19th century and the first half of the 20th, many national populations have been characterized by quite substantial changes in birth and death rates, and, inevitably, by considerable changes in age composition. Our purpose is to analyze as generally as we can the effects of changed vital rates on populations undisturbed by migration. We cannot provide a usefully clear analysis of the general case of an arbitrary initial age distribution, arbitrary initial vital rates, and arbitrary vital rate changes. One reason that this case is too complicated is that the age distribution would typically change from its initial form even if the birth and death rates were to remain unaltered. In other words, most age distributions have unavoidable alterations built in-for example, a small cohort due to a pronounced birth deficit for a few years will remain smaller than neighboring cohorts as it becomes older and there will be a hollow moving out through