Kin selection and sexual conflict shape variation in breastfeeding duration

Breastfeeding provides substantial benefits to infants, yet mothers frequently cease breastfeeding earlier than health guidelines recommend. In cooperative breeding systems, maternal decisions such as the duration of breastfeeding are influenced not only by her own costs and benefits, but also by those of other household members, which are weighed by their relatedness to the child and the parents. We take advantage of variation in post-marital residence patterns among Tibetan communities in southwest China to examine how kinship structure shapes breastfeeding. Analysing feeding histories of 580 children across four residence types (patrilocal, matrilocal, duolocal, and neolocal), we show that average relatedness to co-resident family members is systematically associated with breastfeeding duration. Children and mothers who are more closely related to household members experience longer breastfeeding, while higher paternal relatedness is linked to earlier cessation. These patterns are consistent with conflicts of interest predicted by inclusive fitness theory: parent–offspring conflict and sexual conflict between mothers and fathers. Our findings highlight how family structure within households influences parental investment and cooperation in the family.