Polygamy is so deeply detested in our society that Mormons like Mit Romney face an uphill task in getting elected even though the Mormons abandoned multiple marriage over a century ago (breakaway sects notwithstanding). Writing in defense of an anti-polygamy statute, in 1878, the U.S. supreme court noted that "Polygamy has always been odious among the northern and western nations of Europe..." They meant that it was not civilized. Why is our part of the world so hostile to polygamy?

Plural marriage is surprisingly common, and popular, elsewhere. In poorer countries about a sixth of women share their husbands with 55 percent doing so in Benin (1) . In developed countries, serial polygamy is common where a man divorces his first wife and marries another. (Serial polyandry is less common). Contemporary fascination with polygamy of the "odious" type spawned a successful soap opera, Big Love.
Polygamy for man and beast

Some level of polygamy is observed in virtually every society even if covertly. Polygamy is accepted throughout a wide swath of the inhabited globe - the green band in the map. Why does this form of marriage persists around the equator while it has been largely abandoned, or rejected, elsewhere? Fortunately, we have much to learn from other species whose breeding systems vary from monogamy in some environments to polygyny (one male several females) in others.

Why do some male animals get to monopolize several females? There are three basic reasons. First, there may be a scarcity of adult males. Second, some males may have much better genes than others which is particularly important for populations where there is a heavy load of diseases and parasites to which resistance is genetically heritable. Third, females do better by sharing a mate who defends a good territory (with plenty of food and cover) than they would by opting to be the single mate in a bad territory. Do humans choose polygamy for similar reasons?
Why some countries prefer polygamy

My research on 32 countries where polygamy is practiced by at least 5% of married women yielded answers (1). Polygamy increased where there was a scarcity of males in the population. Countries having a heavy infectious disease load had many more polygamous marriages. Women there evidently shopped for highly disease-resistant (i.e., physically attractive) men to father their offspring. They also care more about physical attractiveness and have a higher sex drive (2). Moreover, there were more polygamous wives in countries where men could monopolize wealth whether in terms of money or arable land (analogous to animal territories). (My results corroborated earlier research).

Astoundingly, much the same explanations apply to polygamous marriage for humans as to polygyny for other species. Contrary to popular assumptions, multiple marriage has nothing to do with poverty, backwardness, or oppression of women (e.g., acceptance of wife-beating). Indeed, Mormon women got the vote in Utah, in 1871, half a century before other American women.

Why the developed world hates polygamy

Polygamy works well in underdeveloped tropical countries but not so well for developed high-latitude countries. Why? At least three factors are critical. First, instead of a scarcity of males, developed countries have an excess thanks to better public health that saves more males than females. Second, colder winters made it impossible for mothers to raise children without substantial help from their husbands. Third, developed countries are highly urbanized and it is very difficult to raise large families in cities because children are such a drain on finances that it is impossible for most men to support multiple families. In agricultural societies, by contrast, kids contribute to the family economy by working.

It is amusing to reflect that in attacking Mormon polygamy late in the nineteenth century, the U.S. Supreme Court, was repudiating their own Biblical heritage. They conveniently forgot the beloved patriarchs of the Old Testament and their many wives (which neighbors were told not to covet 3). Let's not be so hard on Isaac, David, Solomon, and their ilk. For they inhabited the polygamous green zone, lived on their herds instead of urban commerce, and knew nothing about scientific medicine - placing a premium on disease-resistant genes.

1. Barber, N. (2008). Explaining cross-national differences in polygyny intensity. Cross- Cultural Research, 42, 103-117.

2. Barber, N. (2008). Cross-cultural variation in the motivation for uncommitted sex: The role of disease and social risks. Evolutionary Psychology, 6(2): 217-228.

3. Nussbaum, M. (2008, 05). Debating polygamy. University of Chicago Law School Blog.

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