Relative Cohort Size: Source of a Unifying Theory of Global Fertility Transition

Using United Nations estimates of age structure and vital rates for nearly 200 nations at five-year intervals from 1950 through 1995, this paper demonstrates how changes in relative cohort size appear to have affected patterns of fertility across nations since 1950--not just in developed countries, but perhaps even more importantly in countries as they pass through the demographic transition. The increase in relative cohort size (defined as the proportion of the population aged 15 to 24 relative to that aged 25 to 59) which occurs as a result of declining mortality rates among children and young adults during the demographic transition, appears to act as the mechanism of transmission which determines when the fertility portion of the transition begins. As hypothesized by Richard Easterlin, the increasing proportion of young adults would generate a downward pressure on young men's relative wages, which in turn causes young adults to accept a trade-off between family size and material well-being, setting in motion a "cascade" or "snowball" effect in which total fertility rates tumble as social norms regarding acceptable family sizes begin to change. Thus relative cohort size can be thought of as the mechanism which prevents excessive rates of population change--reducing ferility when previous high rates, in combination with low mortality rates, have caused relative cohort size to increase, and increasing fertility when previous low rates have caused relative cohort size to decline.