Parent-Offspring Conflict and Age at Weaning in British Cohort

ABSTRACT: Evolutionary theory has been applied to help understand and predict parenting decisions. Parent-offspring conflict theory posits that conflicts between mothers and infants are inevitable because they share only half of their genes. A prediction drawn from this body of theory is that infants will typically benefit from later weaning than the timing that most benefits the mother, who has other arenas in which she must allocate her energy to ensure her family has enough resources, that her other children are cared for, and that she has energy to allocate to future pregnancies. This research tested whether the timing of weaning from breastmilk, measured as the maternally reported age at which their infant last breastfed, is consistent with expectations from parent-offspring conflict theory in the UK Millennium Cohort. Cox proportional hazards models predicting age last breastfed for the entire sample, including those never breastfed (adjusted model N = 15,825), and models for the subsample who ever breastfed were carried out (adjusted model N = 9678). Many of the results were consistent with predictions, including for variables that would not logically link to early weaning without thinking from an evolutionary perspective, such as preeclampsia (HR for weaning ever-breastfed infants = 1.08; 95% CI 1.00–1.17), and birthweight (HR for each 1 kg increment = 0.90; 95% CI 0.88–0.93). Older mothers weaned their infants later (HR for each year of maternal age = 0.94; 95% CI 0.92–0.97). The findings are discussed from evolutionary and public health perspectives as an example of the utility of evolutionary thinking for understanding health-related behaviours.