The evolutionary demography of the family: a review of the effects of kin on child mortality and female fertility rates

Rebecca Sear, London School of Economics and Political Science (LSE)

The effects of kin on demographic outcomes is a relatively new field of enquiry, but one which has been attracting recent interest. Much of this research stems from evolutionary demographers’ interest in the role of kin in shaping the evolution of human life history patterns. Here I draw together and review the growing body of empirical research on the impact of kin on demographic outcomes, specifically child survival rates and female fertility patterns. The evidence for the impact of kin on child survival is clear: in almost every study which has looked for the effect of family members on child survival, at least one family member (apart from the mother) has an impact on child survival rates. Grandmothers appear to be particularly useful in improving child survival, though maternal grandmothers may be more consistently beneficial than paternal. Older sibling ‘helpers-at-the-nest’ also appear to improve child survival rates. Fathers have surprisingly little effect on child survival, with only a third of studies showing any beneficial effects. Research on the effects of kin on fertility outcomes is less well developed, but there are some studies which have also examined the effects of kin on women’s reproductive rate and age at first birth. Here, male and affinal kin appear to be important: mothers-in-law seem to increase a woman’s rate of giving birth; fathers are found to lower a woman’s age at first birth. Overall, this review suggests that while kin effects on demographic outcomes are extremely common, exactly what role kin play in women’s reproductive lives varies between categories of kin, and is also dependent on ecological conditions. It further highlights that kin interactions may be characterised as competitive rather than cooperative, as in certain cases, kin effects may not necessarily be beneficial.