"The correlation between violence and polygamy (strictly, polygyny — being married to more than one wife at the same time — as having more than one husband is much rarer) is not just about violence to women. It is also about violence among men. From Troy to Brigham Young, from Genghis Khan to Islamic State, there has been a tendency for nations that allow polygamous marriage to exhibit more crime and more warfare than those that do not. The cause is increased competition for mates. Polygamy results in more unmarried young men, and these commit most violence.

Even moderate polygamy can produce large imbalances. Imagine that in a village of 50 men and 50 women, two men have four wives, four men have three wives and fourteen have two wives: that leaves 30 men chasing the remaining two women. A recipe for trouble.

A fascinating 2009 paper called The Puzzle of Monogamous Marriage, by the anthropologist Joe Henrich and his colleagues, detailed the historical correlation between polygamy and crime, chillingly explaining it thus: “Faced with high levels of intra-sexual competition and little chance of obtaining even one long-term mate, unmarried, low-status men will heavily discount the future and more readily engage in risky status-elevating and sex-seeking behaviours. This will result in higher rates of murder, theft, rape, social disruption, kidnapping (especially of females), sexual slavery and prostitution.”

The authors argue that the gradual and erratic imposition over many centuries of “normative monogamy” in Europe and then much of the rest of the world was motivated largely by rulers wanting to suppress crime and violence. Or perhaps societies that suppressed polygamy proved more successful, displacing those that didn’t.

Little wonder that pastoralists exploded out of central Asia at regular intervals, if only to satisfy the need of their low-status, unmarried men to kill men and abduct women: hence Huns, Tartars, Mongols, Turks, Moghuls. In 1401 Tamerlane marched through what is now northern Iraq, sacked Baghdad and ordered his men to produce two enemy skulls each or lose their own. Women were taken as sexual slaves. Not much different from what Islamic State did near there last year."

http://www.rationaloptimist.com/blog/polygamy-fuels-violence.aspx