From Darwin to Eternity

Evolutionary moral psychology.

Are People "Naturally" Polygamous?

The human mind evolved in a polygynous world.

Western civilization is both fascinated and repelled by the idea that a man could have more than one wife at a time. In the USA, polygamy-themed shows like Big Love and Sister Wives have attracted much attention, as has the trial of polygamist Warren Jeffs. And although polygamy is considered generally unacceptable by most Westerners, there is some uncertainty and ambiguity about its moral status. For instance, although Jeffs received a life sentence, the conviction was for having sex with underage girls, not for polygamy per se. And although Western culture remains officially monogamous, it tolerates de facto polygamy in many forms. For example, serial monogamists like Donald Trump and Larry King divorce older wives to marry younger ones, which serves to monopolize the fertile years of multiple women (the same thing that polygamy would accomplish). Celebrities like Hugh Hefner and Charlie Sheen live openly with multiple girlfriends, and various male athletes, rock stars, and actors accumulate hundreds or thousands of sexual partners.

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Does all this de facto polygamy bursting through the seams of our nominally monogamous culture tell us anything about our evolved mating psychology? To answer that, we should examine the types of small-scale societies in which nearly all of our evolution has occurred. When we do so, we find that these hunter gatherer and tribal societies have, throughout the world, historically practiced polygamy. Although most men in these societies strive for polygamy, however, only a minority can achieve it, because maintaining a large family requires an often prohibitively high degree of wealth and status. Further, because it is generally difficult to store and hoard wealth in small-scale societies, even men who do achieve polygamy can usually afford no more than two or three wives. It wasn't until the emergence of large-scale agricultural civilization, a few thousand years ago, that wealth-hoarding became possible and powerful men began accumulating large harems of hundreds or thousands of women. This pattern occurred in similar ways all over the world, as Laura Betzig describes in Despotism and Differential Reproduction. So once the ecological constraints on polygamy were lifted, high status men began accumulating many more wives than they had in small-scale societies.

Tibetan polyandry

When polygamous marriages occur in premodern societies, they are overwhelmingly likely to involve polygyny (one husband, multiple wives) as opposed to polyandry (one wife, multiple husbands). Overall, of the 1,231 cultures in the Ethnographic Atlas Codebook, 84.6 percent are classified as polygynous, 15.1 percent as monogamous, and 0.3 percent as polyandrous. Polygyny is much more common than polyandry because evolutionarily, the benefits of polygyny for men are much higher than the benefits of polyandry for women. This is true for reasons outlined in my previous post; in short, men benefit reproductively much more than women do from having multiple partners, and so they are relatively psychologically adapted to seek novel mates. By the same token, the evolutionary costs of polyandry for men are much higher than the costs of polygyny for women. If you're a man whose co-husband has impregnated your shared wife, then you have to wait a long time (at least nine months of gestation, and probably years of lactation as well) before you get another shot at reproduction. But if you're a woman whose co-wife has been impregnated by your shared husband, you can still get pregnant right away. You'll have to share your husband's parental investment with your co-wife, but at least you'll be able to reproduce. And being the co-wife of a wealthy, high status, highly desirable man may well be a better option for you than being the only wife of a less impressive husband. Although some Westerners will wonder why any woman would choose to marry polygynously rather than monogamously, many women cross-culturally nevertheless do choose to do just that.

Another contrast between polygyny and polyandry is pointed out by Miriam Zeitzen in her book Polygamy: whereas men compete intensely to attract multiple wives, and male status is positively correlated with number of wives, women don't compete to acquire multiple husbands, and female status is not related to number of husbands. Polyandrous marriages in fact tend to be initiated by the co-husbands, who are nearly always brothers, for economic and reproductive reasons (e.g. to avoid partitioning scarce arable land, or because the brothers can't find wives individually). Zeitzen also notes that these marriages involve a relatively large workload for the wife, in terms of the services--both domestic and sexual--that she is expected to provide for her husbands. (Many Western men, who might imagine that providing sexual services for multiple spouses would be a nice responsibility, will be surprised to learn that polyandrously-married women tend to find it onerous).

In light of the cross-cultural evidence, then, the question of what kind of marriage system emanates most directly from evolved human mating psychology does not appear to be a very challenging one. That system is polygyny. Most ancestral men aspired to polygyny, even if only a minority could achieve it, while many ancestral women perceived that their own interests would be better served as the co-wife of a really impressive man than as the sole wife of a lesser one. The modern human mind is composed of genetically encoded psychological adaptations for mating behavior that evolved in these ancestral polygynous environments. That's why there is so much de facto polygyny in Western culture, despite the West's attempts to abolish polygyny in favor of a marriage system that biologist Richard Alexander refers to as "socially imposed monogamy."

So the more challenging question is: Why, given the "naturalness" (I hesitate to use that term, because in an important sense everything is natural, but hopefully you'll get my meaning) of polygyny, did the West end up proscribing polygyny and prescribing monogamy? I'll answer this question in my next post.

Copyright Michael E. Price 2011. All rights reserved.

Michael Price, Ph.D., is a lecturer at the psychology department at Brunel University, West London. He is also the co-director at the Centre for Culture and Evolutionary Psychology.

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