a major preoccupation of social scientists, particularly the massive economic transformation labeled “industrialization,” and the concomitant shift in settlement patterns toward greater concentration of populations in towns and large urban centers. Industrialization and urbanization, it was argued, resulted in a substantial increase in the costs of rearing children and a decrease in the benefits children conferred on older generations (Thompson, 1929; Davis, 1945; Notestein, 1945, 1953). By mid-century, arguments along these lines held sway as the dominant explanation for fertility decline in the West. Some scholars also emphasized the importance of mortality decline as a precondition (Davis, 1963), and in the emerging field of social history there was an interest in the causal contribution of changing notions about family life (Aries, 1962, 1980; Caldwell, 1982) and declining adherence to long-dominant religious and ethical systems (akin to the secularization argument rearticulated by Lesthaeghe and collaborators in the 1980s [Lesthaeghe, 1983; Lesthaeghe and Surkyn, 1988]).

By the time rigorous quantitative research was initiated on the European fertility decline and the emerging declines in Asia and Latin America, the presumption was that variables such as modes of production, urbanization, and levels of schooling, themselves indicators of basic economic and social structural changes that had taken place in these societies, would largely account for the decline in fertility. Hence it came as some surprise when researchers associated with the European Fertility Project at Princeton discovered that the empirical associations between the standard battery of economic and social indicators and fertility decline in fact were rather modest in strength (van de Walle and Knodel, 1967; Knodel and van de Walle, 1979; Watkins, 1986). In hindsight, this may have been a mistaken conclusion, drawn from aggregate-level studies that were incapable of detecting the many linkages at the household level between social and economic change and demographic change (see, e.g., Kertzer and Hogan, 1989). In any case, confronted with these findings from the Princeton project, scholars turned to other explanations to augment, or even to supplant, the dominant theoretical framework in which the primary causal forces underlying fertility decline were mortality decline and the paradigmatic economic and social changes that occurred in Europe in the nineteenth and early twentieth centuries.

One set of alternative explanations that came to the fore has usually been collected under the label “diffusion.” As we shall note below, the arguments classified as “diffusion theories” vary somewhat in their emphasis, and particularly in what they regard as the unique causal contribution of diffusion theory. What unites them is an overarching model of social change in which attitudes and behaviors become more prevalent in a population through their spread from some individuals to others,