An adaptive mating strategy found in less than four percent of mammals which forms the foundation for the nuclear family.

Though sometimes assumed to be a uniquely human pattern, long-term mate attachment is found in about 3% of mammals, including beavers, marmosets, titi monkeys, gibbons, the California deer mouse, prairie voles, and pine voles. Most mammals, however, even if they are social, are either promiscuous or seasonal in their mating.

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Drawing on historical and ethnographic data, evolutionary biologists Laura Fortunato and Marco Archetti argue that when there are multiple wives each with children and hence multiple heirs, transferring resources to all heirs results in a depletion of their fitness value; for example, the patches of land to bequeath get smaller and smaller, and less able to support the families that depend on the land. A man might select one particular wife whose children inherit all the wealth, but this makes for competition among offspring, and is generally an unstable solution. In these conditions, a more stable strategy for enhancing the well-being of one’s own offspring would be to have one wife, be sure of the paternity of the offspring, and invest heavily in the welfare only of her children.

- Patricia Churchland

Braintrust