Source: Hermin/Shutterstock

With gay marriage now legal nationwide, many (like William Baude in the New York Times) are now wondering if legalized polygamy may be next, and some (like Fredrik Deboer in Politico) are suggesting that it should be.

Should it?

As Baude points out in his op-ed, polygamy should remain illegal because it would increase gender inequality and social instability:

"Judge Richard A. Posner of the United States Court of Appeals for the Seventh Circuit rejected a right to plural marriage because it would lead to gender imbalances if ‘the five wealthiest men have a total of 50 wives.’ Similarly, the same-sex marriage advocate Jonathan Rauch has argued that polygamy allows ‘high-status men to hoard wives’ and destabilizes society."

Note that Posner and Rauch are assuming that the most common type of legal polygamous marriage would be one husband with multiple wives (polygyny), as opposed to one wife with multiple husbands (polyandry). They’re also not considering more complex types of plural marriage (e.g. multi-male multi-female), or homosexual plural marriage. (Nor will I consider these two latter types here; they're relevant but beyond the scope of this post.)

Is it safe to assume that most polygamous heterosexual marriage would indeed involve one husband with multiple wives? Probably, as this chimes with the evidence about how people tend to mate cross-culturally. Historically, polygamy was permitted in the vast majority of cultures; in these cultures, polygyny was far more common than polyandry. Still, even within cultures that permit polygyny, it is much less common than monogamous marriage, in part because it can be difficult to attract more than one spouse, even if you'd want to. (Note also that polyandry rates may be underestimated in the anthropological record.)

Why is polygyny more common than other forms of plural marriage? Because of how humans are psychologically adapted for mating. The evolutionary reproductive benefits of having more than one spouse were higher for men than women. In 1972 [1], biologist Robert Trivers outlined the foundational reason for this: For men more than women, reproductive success is limited by number of mates. A man with many wives can produce many children per nine months, whereas a woman can usually produce only one, whether she has one husband or 100. Females certainly may obtain other kinds of reproductive benefits (like resources for their own children) from mating with multiple males, but these benefits are less straightforward than actual additional offspring. Relatedly, the reproductive costs of having more than one spouse are lower for women than for men. If a man's wife becomes pregnant by his co-husband, he'll have to wait a long time—nine months plus an inter-birth interval that in hunter-gatherer societies averages 3.25 years [2]—for his turn to reproduce. If a woman's husband impregnates her co-wife, he can immediately impregnate her, too. She may suffer other reproductive costs from having to share a husband (like receiving a smaller proportion of his resources for her own children), but these costs are less severe than not being able to reproduce at all for four years.

Because our minds were designed by these evolutionary environments, men—cross-culturally and on average—are more motivated to acquire multiple mates [3], and seem more averse to sharing a spouse, compared to women. That said, forms of polyandry are certainly observed anthropologically, and are quite common in some societies, so it would be misguided to suggest that it is "contrary to human nature." It would be more accurate to say that the evolved psychological mechanisms in men and women that lead to polygyny are activated under a wider range of environments than are the mechanisms that lead to polyandry.

So it seems reasonable to assume that if polygamy were legal, most polygamous marriage would indeed take the form of polygyny. We can also assume that given roughly equal sex ratios, polygyny could lead to the kinds of gender imbalances described above, with some men who were more attractive (in terms of overall mate value) having multiple wives, and some less-attractive males going wife-less or having to share a mate with other men.

Would such outcomes be a problem? Possibly. Many people would regard them as being unfair or exploitive of women in polygynous marriages, or to men unable to attract a wife of their own. Another convincing argument is that societies with too many unmated men tend to suffer from social instability due to intensified male-male mating competition. Given these potential problems, why would it be a good idea to legalize polygamy?

Maybe the simplest and most compelling argument in favor of legalization is that it would enhance people's freedom to choose their own mates. No one on either side of the debate suggests that it should be legal to coerce anyone into a polygamous marriage. Jonathan Rauch's comment (above) that polygyny allows "high-status men to hoard wives" suggests wives being collected like possessions, but this overlooks the fact that in many cultures, women in polygynous marriages actively choose to enter them—and the same goes for men in most polyandrous marriages. If the government prohibits people from choosing to marry polygamously, this is the equivalent of telling them that instead of marrying their preferred partner, they must instead marry someone they would otherwise not choose, or else not marry at all.

So it does seem that by prohibiting polygamy between consenting adults, we restrict people's ability to choose their own mate(s). However this doesn't mean making it legal is a good idea. Personal freedom is not the only value we should strive to maximize, of course, and there may be a greater social good served by keeping polygamy illegal. Would the potential costs of legalizing polygamy—such as reduced gender equality, increased numbers of low-status unmarried men, decreased social stability, or some other unmentioned problem—exceed the potential benefits?

Wikimedia Commons
Source: Wikimedia Commons

Feel free to share your thoughts with a comment below.

Copyright Michael E. Price 2015. All rights reserved.


  1. Trivers, R. L.  1972.  Parental investment and sexual selection.  In B. Campbell, ed. Sexual Selection and the Descent of Man, 1871-1971, Aldine-Atherton, Chicago, pp. 136-179. 
  2. Marlowe, F. W., & Berbesque, J. C. (2012). The human operational sex ratio: effects of marriage, concealed ovulation, and menopause on mate competition. Journal of Human Evolution, 63, 834-842.
  3. Schmitt, D. P. (2005). Sociosexuality from Argentina to Zimbabwe: A 48-nation study of sex, culture, and strategies of human mating. Behavioral and Brain Sciences, 28, 247-275.

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