Why is polygamy declining? -- Matt Ridley

http://www.rationaloptimist.com/blog/why-is-polygamy-declining.aspx

Yet we are clearly monogamous by instinct as well as by tradition. Even in societies that allow polygamy, most people are in one-partner couples. Free-love communes always, without exception, collapse because people will insist on falling in love with particular individuals. This pairing tendency would baffle a bonobo, where sexual jealousy is apparently unknown.

Polygamy, in this reading, was mainly an aberration of the last 10,000 years caused by agriculture, which allowed the accumulation of huge surpluses, which powerful men translated into prodigious sexual rewards. Herding societies in particular became highly polygamous, causing people with names such as Attila, Ghenghis or Tamerlane to conquer other lands so as to supply women to their sex-starved followers: polygamy and violence tend to go together.

In a recent paper entitled "The puzzle of monogamous marriage", three American anthropologists argue that this trend is partly explained by competition between societies. To be economically successful, modern nations had to suppress violence within themselves.
This was incompatible with rulers grabbing all the best girls: "In suppressing intrasexual competition and reducing the size of the pool of unmarried men, normative monogamy reduces crime rates, including rape, murder, assault, robbery and fraud, as well as decreasing personal abuses . . . By shifting male efforts from seeking wives to paternal investment, normative monogamy increases savings, child investment and economic productivity."