ences. The ''theory of demographic transition" has become the overriding paradigm to explain how fertility changes. Demographic transition describes the societal shift from high fertility and high mortality (pre-transition) to low fertility and low mortality (post-transition). According to the classic demographic literature, demographic transition is caused by socioeconomic development and modernization. In addition, a decline in mortality precedes the drop in fertility (Notestein, 1945, 1953). The empirical record, however, appears to refute the simplest statements of demographic transition theory. For example, changes in reproductive behavior often have only been loosely correlated with economic, social, or cultural change, which tend to occur at different paces (Cleland, 1985). In Thailand, for example, Knodel and colleagues (1987) document how change in reproductive behavior and attitudes permeated almost every segment of Thai society within a period of approximately 15 years. The Committee on Population documents how changes in fertility and contraceptive use in Kenya cut across social, economic, ethnic, and geographic boundaries (Brass and Jolly, 1993).

The inability of demographic transition theory to accurately predict the timing or pace of actual fertility transitions has generated debate about the relative importance of a set of factors that contribute to fertility decline within particular structural or cultural contexts. Fertility declines are undoubtedly linked to social, economic, political, and cultural changes, but the nature and specific combination of each of these factors varies from one society to another.

The growing frustration within the field at the lack of predictive power of demographic transition theory has been the catalyst for researchers to reexamine the contribution of diffusion theory to the determination of the timing and pace of fertility transition. The essential idea behind diffusion theory is that social interaction is a key mechanism through which the adoption of new technologies, ideas, and behaviors takes place. However, there are many different views on how diffusion should be defined, as explained in this report.

In the late 1970s and early 1980s, diffusion theory attracted the interest of several demographers as evidence mounted that theories relying on individual rational decision making in response to economic or structural change could not fully explain the observed fertility transitions in many areas of the world. While both mortality declines and structural and economic changes remain important elements of explanations for fertility declines, the results of two major research efforts completed in the mid-1980s—the Princeton European Fertility Project and the World Fertility Survey—caused certain researchers to conclude that structural and economic changes alone provide an incomplete explanation (see Bongaarts and Watkins, 1996; Cleland and Wilson, 1987; Coale and Watkins, 1986).

Some researchers viewed the findings of the Princeton European Fertility Project as providing major support for diffusion theory explanations of fertility change (Knodel and van de Walle, 1979). This project analyzed aggregate historical demographic data from the time of the fertility transition in Europe (ap-



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