Relative cohort size: Source of a unifying theory of global fertility transition?

Using United Nations estimates of age structure and vital rates for 184 cou ntries at five-year intervals from 1950 through 1995, this article demonstr ates how changes in relative cohort size appear to have affected patterns o f fertility across countries since 1950-not just in developed countries, bu t perhaps even more importantly in developing countries as they pass throug h the demographic transition.The increase in relative cohort size (defined as the proportion of males aged 15-24 relative to males aged 25-59), which occurs as a result of declining mortality rates among infants, children, an d young adults during the demographic transition, appears to act as the mec hanism that determines when the fertility portion of the transition begins. As hypothesized by Richard Easterlin, the increasing proportion of young a dults generates a downward pressure on young men's relative wages (or on th e size of landholdings passed on from parent to child), which in turn cause s young adults to accept a tradeoff between family size and material wellbe ing, setting in motion a "cascade" or "snowball" effect in which total fert ility rates tumble as social norms regarding acceptable family sizes begin to change.