Modern contraception in research on fertility transitions is increasingly seen as an innovation that is associated with considerable uncertainty about costs, benefits and potential side effects. Markets that effectively provide relevant information to reduce this uncertainty are often missing, and media may fail to convey the complex set of ideas and procedures involved in the adoption of fertility control (see Chapter 1). Women's attitudes towards a new contraceptive alternative, and their willingness to adopt modern birth control are therefore shaped by local culture, social influences, and particularly the local availability of contraceptive information. As a consequence, even when women are aware of modern methods, their assessments about health implications, convenience of use, and contraceptive effectiveness are determined by friends' and neighbors' experience through social learning.
A puzzling finding regarding this diffusion of knowledge about contraceptive methods through social networks is the lack of convergence in women's behavior. The salient regional diversity of contraceptive use in the European demographic transition and the analogous variety in contraceptive practices across villages in developing countries are not fully explained by differences in socioeconomic conditions (e.g., Coale and Watkins 1986 ; Entwisle et al. 1996 ; Potter 1999 ; Watkins 1990). Furthermore, ethnic groups and social strata frequently exhibit very different adoption levels of birth control despite comparable environments, geographic proximity, and overlapping social networks (e.g., Lesthaeghe 1977 ; Livi-Bacci 1986). This divergence is more than an immaterial cultural aspect of the demographic transition. Because the incentives to adopt fertility control in these regions or social groups are similar, this persistent diversity indicates inefficient diffusion. Women who rely on social networks in their evaluation of available alternatives