Why Polygamy is Always a Bad Idea

Many Americans these days fear that gay marriage opens the door to the legalisation of polygamy. While this is a textbook example of the slippery slope fallacy (some people who advance this argument also fear the imminent legalisation of bestiality, incest and child molestation), very few pundits seem to be able or willing to refute the argument.

During the struggle for marriage equality, the argument was often advanced that the state has no business regulating the love between two consenting adults of the same sex. But while consent is an important element, it is not the only one. The state’s interest in regulating marriage is does not lie on the individual, but on the societal level. While gay marriage has no measurable adverse effects on a society, polygamy does.

Anthropologist Joseph Henrich

In November 2011, the British Columbia Supreme Court ruled against legalising polygamy. This decision was partly based on a report by anthropologist Joseph Henrich, commissioned by the B.C. Attorney General (Henrich 2010; page numbers in this article refer to this report unless otherwise noted). In this paper, Henrich describes the societal effects of polygamy on crime and social disorder, male parental investment, and gender inequality.

Crime and social disorder

Polygamy increases crime because in polygamous societies, there will always be a group of unmarried young men who have no prospect of a partner because there aren’t enough women and they do not have sufficient money or status to have a wife. It is a well-established sociological fact that unmarried men tend to commit more murders, robberies and rapes (Pinker 2011, p. 104-105).

Male parental investment

In polygamous marriages, a sizable part of the family’s means is devoted to attracting and maintaining multiple wives. These means cannot be invested in the family’s children, leading to more than ten times higher child and infant mortality rates, higher chances of malnutrition, lower growth rates (both in weight and length) and worse overall health. It is not entirely understood why this is so, but there is no question that it is so.

Gender inequality

“Polygamy is, by its very nature, a gender issue” (Zeitzen 2008, p. 125). Several asymmetries are built into polygamy, two of which are the younger age at which polygamous women first marry (ten years earlier) and the large age gap between them and their husbands (three times larger than in monogamous societies). Women in polygamous societies are less empowered, have fewer rights, lower literacy rates and run increased risks of being sexually exploited and raped.

In some polygamous societies, “significantly elevated rates of depression, obsession-compulsion, hostility, anxiety, phobia, psychoticism, and paranoid ideation” were measured in women (p. 39). These symptoms are not uniformly present in all polygamous society, though, because they “vary with the women’s co-wife ranking” and this in turn varies across cultures. In some cultures, younger wives have more status and dominate the older ones, in other cultures it’s the other way around (p. 40).

Other effects

Henrich makes several other observations which cast polygamy in a broader light. In chapter III of his report, he traces the emergence of western monogamy from its origins in ancient Greece and Rome, to its adoption in Europe through Christianity, and ties it to increased economic equality among men. This point is reinforced by his observation that polygamous countries today have a three times lower GDP than comparable non-polygamous countries (p. 31).

Mormon polygamy as a case in point

While this article leans heavily on Henrich’s report, his results are consistent with the relevant sociological and anthropological literature. Henrich’s primary focus may be on the adverse societal effects of polygamy but in all honesty, there are very few positive effects associated with polygamy from a modern point of view.

Mormon polygamy is a case in point. In Mormonism101.com’s annotated versions of the Mormon church’s essays on Utah and Nauvoo polygamy, almost all the effects mentioned above can be seen to apply:

  • Increased numbers of unmarried young men
    (note 7 to the Utah polygamy essay);
  • Decreased age of women at first marriage
    (notes 7 and 14 to the Utah polygamy essay, note 19 to the Nauvoo polygamy essay);
  • Increased age gaps between husbands and polygamous wives
    (notes 18 and 19 to the Nauvoo polygamy essay);
  • Unequal distribution of wealth
    (note 8 to the Utah polygamy essay);
  • Gender inequality
    (note 10 to the Utah polygamy essay, notes 2, 8, 19, 21, 25, 27, 28, 31 and 33 to the Kirtland essay);
  • Polygamy as a status symbol of the ruling elite
    (note 11 to the Utah polygamy essay, notes 25 and 32 to the Nauvoo essay).

However, the adverse societal effects of polygamy are not restricted to 19th-century Mormonism. To this very day, Mormon fundamentalism offers a chilling insight into the misery and squalor that are commonly associated with polygamy. It would be a dangerous mistake to consider these the exception rather than the rule.


While there may not be a moral objection to the theoretical concept of polygamous marriage between consenting adults, historical and contemporary evidence show that there is no practical application of such a concept in a modern society. Even if the adults are consenting to the marriage (which, by the way, can be ambiguous given the cultural and familial pressures often involved in marriage practices), the detrimental effects on society in general, and women and children in particular, provide the state with a compelling and moral incentive to prohibit polygamy.