Why fertility changes.

Many people think that lowered mortality and lowered fertility following socioeconomic development is the classic definition of demographic transition. Demographers are not, however, sure. Demographic transition was completed in most industrial countries over the period 1870-1930, while contemporary transitions are currently underway in many developing countries. Considerable controversy exists over the causes of these changes. Although new data and empirical analyses of historical and contemporary fertility declines have weakened the standard theory of demographic transition, no definitive, all-encompassing theory of fertility change exists. Empirical evidence on the origins, speed, and correlates of fertility declines in different historical and geographical settings points to the existence of more diversity than a simple theory of fertility change can predict. The author considers the measurement of fertility and fertility change and summarizes the recent evidence on fertility levels and trends in historical and contemporary populations. The core of the essay consists of a comparison of classical and contemporary theories of fertility transitions and a discussion of unresolved issues in current research. An alternative model of demographic transition is suggested.