Sex-Specific Parent-Offspring Conflict in Humans

In humans, male offspring are, on average, larger at birth, weaning, and independence compared to female offspring. We argue that an underappreciated consequence of the greater cost of rearing males is the potential for greater mother-son conflict. We review evidence for this hypothesis during pregnancy, nursing, and postweaning childhood. Male offspring have higher likelihood of implantation failure, stillbirth, preterm birth, and early life mortality, and mothers carrying sons have increased risk of numerous pregnancy complications. One interpretation of these data is that differences in sex-specific growth strategies put male offspring in greater conflict with their mothers, and this contributes to adverse pregnancy outcomes and male vulnerability. During the nursing period, sons consume more milk than daughters; however, evidence for greater weaning conflict is lacking, potentially because it has not been sought in natural fertility populations. During postweaning childhood there appears to be greater behavioral conflict, including lethal violence, between parents and sons, at least in industrialized Western nations. Research on parent-offspring conflict has frequently ignored offspring sex, implicitly assuming male and female offspring are equivalent. However, consideration of sex differences in the magnitude of parent-offspring conflict, mediated by maternal condition, may add explanatory power and have implications for human health.