Kin influences on fertility: a theoretical framework tested with a review of the literature

Far from being an asocial process, reproductive decision-making in humans is affected by family and other social networks. However, the direction and strength of kin effects is inconsistent across studies. Explaining this variation requires a better understanding of what various fertility outcomes mean functionally and of each individual’s strategic interests. Fertility metrics such as age at first birth, interbirth intervals, and total number of births are not as clearly related to a woman’s fitness as say, child survival rates. Furthermore, these outcomes are functionally distinct, and decisions about them can be made independently of one another. Additionally complicating matters, woman and her kin do not always have the same strategic considerations. In this paper, we develop a framework for comparing various accounts of why kin affect fertility. We test these possibilities by compiling and analyzing a database of kin effects on fertility from 130 articles from pre- and post-demographic transition settings. Focusing on how parents and in-laws might affect different fertility outcomes, we find that parents are more likely to have anti-natal effects on a woman’s total fertility, and age at first birth, while in-laws are more consistently pro-natal. However, both parents and in-laws tend to shorten inter-birth intervals.