Reflections on the Evolution of Human Sex Differences: Social Selection and the Evolution of Competition Among Women
Darwin’s (1871) sexual selection, in particular male–male competition over mates and female choice of mating partners, has successfully guided research on sex differences across hundreds of species, including our own. One consequence of the success of sexual selection has been a relative indifference to other pressures that can result in the evolution of sex differences. In recent years, female–female competition over resources other than mates has captured the attention of evolutionary biologists. We illustrate how this form of competition, termed social selection, has operated to create sex differences in several nonhuman species and then apply it to female–female competition in humans. We propose that a combination arranged marriages, which will lessen direct competition for marriage partners, and polygyny created a unique social context within which female–female competition evolved, specifically competition among co-wives for access to resources controlled by their husband and competitive promotion of their children’s future reproductive prospects vis-à-vis the prospects of the children of co-wives. Competition among co-wives is more subtle—termed relational aggression—than that among men and involves the subtle manipulation of social relationships and psychological harassment of co-wives and their children. We review evidence related to the psychological and reproductive costs of conflicts with co-wives and argue that this competition contributed to girls’ and women’s advantages over boys and men for cognitive competencies, such as sensitivity to facial expressions, that support relational aggression.