Kin selection and sexual conflict drive the duration of breastfeeding

Whilst breastfeeding is highly beneficial to infants, mothers frequently do not breastfeed for as long as health guidelines recommend. Here, we show this discrepancy is arising from multiple individuals being involved in offspring care. The support of other family members is modifying the opportunity costs of breastfeeding. We test whether predictions from inclusive fitness theory can help explain the duration of breastfeeding by using demographic data and causally validated, Bayesian Cox linear regression models. We exploit the fact that different residence patterns generate variation in the relatedness within households, and hence different patterns of cooperation and conflict. We analyze the feeding histories of 580 Tibetan children born post-2010 from 5 regions in southwest China with 4 distinct post-marital residence patterns: Patrilocal, Matrilocal, Duolocal, and Neolocal. Our findings reveal that relatedness to the co-resident family of the child, the mother and the father are associated with breastfeeding duration, but in different ways, where child’s and mother’s average relatedness to the household is positively associated with prolonged breastfeeding duration but father’s average relatedness to the rest of the household is associated with shorter breastfeeding duration. Both parent-offspring conflict and sexual conflict between parents are shaping patterns of breastfeeding in ways predicted by inclusive fitness theories of parental investment. Our research highlights the importance of the relatedness of co-resident family members in understanding breastfeeding behaviour. Patriarchal norms of high fertility may be directly mediated by patrilineal relatives curtailing the duration of breastfeeding.