Sex equality can explain the unique social structure of hunter-gatherer bands

Friends and family? Evolutionary theory stresses the importance of living with kin, not least because they share some of our genes. Nevertheless, a large-scale assessment of contemporary hunter-gatherer societies has established a consistent pattern of unrelated individuals living together. Dyble et al. used a modeling approach to suggest that a possible answer to this conundrum is that cohabitation choices are being governed equally by men and women. Science, this issue p. 796 As human societies evolved, modeling reveals that allowing both males and females to choose camp members reduces relatedness. The social organization of mobile hunter-gatherers has several derived features, including low within-camp relatedness and fluid meta-groups. Although these features have been proposed to have provided the selective context for the evolution of human hypercooperation and cumulative culture, how such a distinctive social system may have emerged remains unclear. We present an agent-based model suggesting that, even if all individuals in a community seek to live with as many kin as possible, within-camp relatedness is reduced if men and women have equal influence in selecting camp members. Our model closely approximates observed patterns of co-residence among Agta and Mbendjele BaYaka hunter-gatherers. Our results suggest that pair-bonding and increased sex egalitarianism in human evolutionary history may have had a transformative effect on human social organization.