As societies modernize, they go through what has become known as "the demographic transition;" couples begin to limit the size of their families. Models to explain this change assume that reproductive behavior is either under individual control or under social control. The evidence that social influence plays a role in the control of reproduction is strong, but the models cannot adequately explain why the development of small family norms always accompanies modernization. We suggest that the widening of social networks, which has been found to occur with modernization, is sufficient to explain the change in reproductive norms if it is assumed that (a) advice and comment on reproduction that passes among kin is more likely to encourage the creation of families than that which passes among nonkin and (b) this advice and comment influence the social norms induced from the communications. This would, through a process of cultural evolution, lead to the development of norms that make it increasingly difficult to have large families.
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