Kin selection and sexual conflict drive the duration of breastfeeding

2025 · doi:10.32942/x23w8j
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Abstract

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.
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This is a Preprint and has not been peer reviewed. This is version 3 of this Preprint. You must log in to post a comment. There are no comments or no comments have been made public for this article. This is a Preprint and has not been peer reviewed. This is version 3 of this Preprint. Add a Comment You must log in to post a comment. Comments There are no comments or no comments have been made public for this article. 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 co-operation in the family. https://doi.org/10.32942/X23W8J Social and Behavioral Sciences cooperative breeding; average relatedness; bargaining power; breastfeeding cessation; patriarchy; residence patterns Published: 2025-10-24 02:14 Last Updated: 2025-11-17 10:24 CC-BY Attribution-NonCommercial 4.0 International Conflict of interest statement: None Data and Code Availability Statement: fully reproducible GitHub repository to be shared upon acceptance Language: English

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