Polyandry occurs when, for whatever reason, a society emerges where women are permitted to marry more than one man at the same time. For decades, anthropologists believed that it was extremely rare. But new evidence suggests it's quite common in human history, occurring in many different cultures, though it never became as popular as polygyny (one man with multiple wives) or monogamy (two people in an exclusive relationship). Over at The Atlantic, Alice Dreger calls our attention to a paper published last year which chronicles the history of polyandry, exploring the social circumstances where tends to happen.

Writes Dreger:

Rather than treating polyandry as a mystery to be explained away, [authors] Starkweather and Hames suggest polyandry constitutes a variation on the common, evolutionarily-adaptive phenomenon of pair-bonding — a variation that sometimes emerges in response to environmental conditions.

What kind of environmental conditions? Well, "classical polyandry" in Asia has allowed families in areas of scarce farmable land to hold agricultural estates together. The marriage of all brothers in a family to the same wife allows plots of family-owned land to remain intact and undivided."

In other cultures, it appears that a man may arrange a second husband (again, frequently his brother) for his wife because he knows that, when he must be absent, the second husband will protect his wife — and thus his interests. And if she gets impregnated while Husband #1 is gone, it will be by someone of whom he has approved in advance. Anthropologists have recorded this kind of situation among certain cultures among the Inuit (the people formerly called Eskimos).

Then there's the "father effect" demonstrated by Penn State's Stephen Beckerman and his colleagues in their study of the Bari people of Venezuela. The Bari have a system for recognizing two living men as both being fathers of a single child. Becerkman's group found that children understood to have two fathers are significantly more likely to survive to age 15 than children with only one — hence the term "father effect."