Polyandry.

From The Atlantic:

When Taking Multiple Husbands Makes Sense

For generations, anthropologists have told their students a fairly simple story about polyandry -- the socially recognized mating of one woman to two or more males. The story has gone like this:

While we can find a cluster of roughly two dozen societies on the Tibetan plateau in which polyandry exists as a recognized form of mating, those societies count as anomalous within humankind. And because polyandry doesn't exist in most of the world, if you could jump into a time machine and head back thousands of years, you probably wouldn't find polyandry in our evolutionary history.

That's not the case, though, according to a recent paper in Human Nature co-authored by two anthropologists, Katherine Starkweather, a PhD candidate at the University of Missouri, and Raymond Hames, professor of anthropology at the University of Nebraska. While earning her masters under Hames' supervision, Starkweather undertook a careful survey of the literature, and found anthropological accounts of 53 societies outside of the "classic polyandrous" Tibetan region that recognize and allow polyandrous unions. (Disclosure: I first learned of Starkweather's project while researching a controversy involving Hames and he is now a friend.)

Indeed, according to Starkweather and Hames, anthropologists have documented social systems for polyandrous unions "among foragers in a wide variety of environments ranging from the Arctic to the tropics, and to the desert." Recognizing that at least half these groups are hunter-gatherer societies, the authors conclude that, if those groups are similar to our ancestors -- as we may reasonably suspect -- then "it is probable that polyandry has a deep human history."

Read the rest here.