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."

Rather than treating polyandry as a mystery to be explained away, 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."

Two fathers? As odd as it can sound to those of us who know of human development as the one-egg-meets-one-sperm story, some cultures maintain the idea that fetuses develop in the womb as the result of multiple contributions of semen over the course of a pregnancy. In cultural systems of what Beckerman has named "partible paternity," two men can be socially recognized as legitimate fathers of a single child. Starkweather and Hames call this a form of "informal polyandry," because while the two fathers may not be both formally married to and living with the mother in all cases, the society around them officially recognizes both men as legitimate mates to the mother, and father to her child.

What all these polyandrous situations—classical and non-classical, formal and informal—have in common is that they are all socially recognized systems in which women may openly have multiple mates simultaneously. Women in such systems are not "cheating" by any stretch of the imagination, nor are the men being cuckolded. The systems are socially sanctioned. But this does not mean that the women are in control of the arrangements; in many of the cultures Starkweather and Hames reviewed, the first husband functions as the decider when it comes to resource distribution and acceptance of additional male mates.