Allowing both males and females in hunter-gatherer groups to choose their living companions reduces the number of family members in individual hunter-gatherer camps, a new study shows.

The results answer a longstanding mystery about why hunter-gatherer populations have evolved to comprise large numbers of unrelated individuals, especially since hunter-gatherers have shown a strong preference to live with kin.

Previously, studies have pointed to pair-bonding, or lifelong monogamous relationships in which couples go off and leave family behind, as one possible driver of hunter-gatherer groups made up of few related individuals.

Here, to better understand how hunter-gatherers came to reside in this way, M. Dyble and colleagues developed a model of their behavior - running two versions of it, one where both the husband and wife in a hunter-gatherer couple could have equal influence over where their household would reside, and another where only one sex had influence.

The researchers found that when both men and women had say over their living situation, with both husband and wife striving to maximize numbers of kin in their camp, communities were much less closely related than when only men or only women could choose. Data from two contemporary hunter-gatherer societies in Africa and the Philippines matched the model closely.

Sexual equality seems to reduce relatedness of the people in a camp at any one time, the researchers say, but, critically, it increases the number of camps where an individual has one or more kin living, paving the way for cooperation and information exchange among groups without the need for more complex drivers, like wealth or war.

The work represents an important contribution to our current understanding of human social and behavioral evolution.

Article #11: "Sex equality can explain the unique social structure of hunter-gatherer bands," by M. Dyble; G.D. Salali; N. Chaudhary; A. Page; D. Smith; J. Thompson; L. Vinicius; R. Mace; A.B. Migliano at University College London (UCL) in London, UK.