Researchers are still puzzled and in disagreement about one of the most profound questions of demographic change: why fertility rates have fallen significantly in many societies and not in others, and why fertility transitions have been precipitous in some cases, and leisurely in others. There is a striking diversity in the timing and pace of fertility decline that is not accounted for by the standard variables relating to costs of parental time and other child-inputs, or economic development in general. In addition, the evidence suggests that fertility decline is a path-dependent and often self-enforcing process. Once a small decline is initiated, a rapid and persistent fertility transition usually follows quite irrespective of specific economic conditions, and this process spreads to neighboring regions with a similar language or culture even when they are less developed.
The observation of these diverse paths of demographic change indicates that other factors, such as the diffusion of information and the local environment with its social and cultural institutions, are important determinants of fertility change. The incorporation of these factors in theoretical explanations, however, differs widely. On the one hand, economists tend to abstract from social structure and accentuate individual fertility decisions as a reaction to variations in prices of inputs to the production of progeny. On the other hand, sociologists stress the embeddedness of fertility decisions in social institutions, norms and cultural traditions.
We argue in this book that the above perspectives can be fruitfully integrated. Demographic behavior is associated with externalities that renders the adoption of low fertility by one couple dependent on the contemporaneous fertility behavior of other community members. These externalities are found in several areas. They arise because the adoption of low fertility by some parents contributes to the erosion of traditional norms or pressures to conform. They occur because the diffusion of information is a path-dependent process and the choices of early adopters influence the