Grandmothers and the Extended Family
Conjugal families are often assumed to be building blocks of human societies and the primary site of childrearing in traditional communities. Alternatively, the Grandmother Hypothesis draws attention to other relationships likely fundamental in the evolution of our lineage. Persistent ties that crosscut conjugal families are implied by our cooperative childcare, distinctive prosociality, and extraordinary operational sex ratios.
Sarah Hrdy has highlighted the universality of human cooperative rearing and linked it to selection on ancestral infants that propelled the evolution of our distinctive preferences for joint attention. If cooperative rearing in our lineage began with grandmothering, then those distinctive social preferences evolved with extended family associations and another crucial shift.
The number of males competing for fertile females is much higher in humans than in other primates because our extraordinary longevity evolved in both sexes without extending female fertility to later ages. High operational sex ratios raise male payoffs for mate guarding. Yet, unlike mate guarding in other species, mates in traditional communities do not remain in constant close proximity. Conjugal families disperse daily into parties of women and children, men often in association with other men. Men’s proprietary claims on particular women depend on acceptance of those claims by others; and ways men negotiate with other men affect the economics of child rearing.