Why is polygamy declining?

Human monogamy is an enduring puzzle. Among mammals we are the exception: just 3 per cent of mammals form pair bonds. Our closest relatives, chimpanzees, bonobos, orangutans and gorillas, are promiscuous, very promiscuous, territorial-polygamous and harem-polygamous respectively. Only gibbons among the apes practise monogamy and they don’t try to do it within a gregarious species.

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.

We are like birds. Penguins and parrots, like us, practise monogamy within large “urban” colonies. One likely evolutionary reason is that when it takes two to raise a baby, a male is more likely to have grandchildren if he puts a lot of effort into one brood, rather than loving and leaving lots of females. In the Pleistocene, the long helpless childhood of human beings probably rewarded diligent fathers with more offspring than callous philanderers.

You could achieve both if you were cunning. Monogamy and fidelity are not quite the same thing. Female birds generally like to stick to one mate to help them bring up the babies, but often — DNA studies reveal — sneak off and get the babies’ genes supplied by another, genetically superior male with better plumage or a more varied song. Successful hunters in human foraging societies tend to get the same result.

In primates, the threat of infanticide also seems to play a role in deciding female strategy. In many monkeys and apes, when a new male takes over a troop, the first thing he does is kill any babies to bring females back into oestrus quickly. Female gorillas, which live in small harems, suffer this fate frequently. Chimpanzees avoid the problem by a system of maximised promiscuity — where every female does her utmost to mate with every male in the group, the better to confuse paternity and thereby prevent infanticide by a new alpha male. In human beings, a horror of step-parents may go deep.

So at some point in the distant past, we developed the habit of monogamous pair bonding. Intellectuals, from Rousseau to Engels to Margaret Mead, have been tempted to speculate about a promiscuous human past not so long ago, from which marriage crystallised. Initial encounters with other civilisations based around agriculture and full of polygamy, such as in Mexico or Tahiti, at first seemed to confirm this idea, but when in the 20th century anthropologists began getting to know hunter-gatherers (supposedly the most primitive level of society), they were startled to find that monogamous marriage predominated in them. In human beings, monogamy probably goes back hundreds of thousands if not millions of years.

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.

However, the winners from a polygamous system are not just the high-status men, but also the low-status women. The peasant girl who joined the palace harem achieved safety, plentiful food and access to luxuries, while her brother languished in celibate poverty. The losers are the low-status men and the high-status women.

It makes evolutionary sense that high-status males are attractive to women, because they were in the past likely to be able to ensure the success of any children they fathered, and that men are attracted to what Amazonian Indians call “moko dude” women. (The phrase means “ripe” when used of fruit and, when used of women: “of the right age, health, genetic quality and unencumberedness likely to make them capable of producing many healthy children and grandchildren”, or, more pithily, “phwoar”.)

So how come the president of France, with the status of a monarch, cannot even get away with two women at a time? Inch by inch, from Odysseus to Figaro to Bill Clinton, Western mores have insisted on monogamy even for the powerful. Clearly the interests of high-status men and low-status women have lost out to the interests of high-status women and low-status men.

Interestingly, this trend continues, even as disapproval of divorce and cohabitation has diminished. Nobody minds much that François Hollande has never married his three “wives”. Yet that does not mean that Valérie Trierweiler is prepared to share.

The spread of Christianity, with its teachings on monogamy and female virtue, could hardly have been better designed to appeal to poor men, polygamy’s big losers. Democracy, too, seems to insist on monogamy. Between The Iliad and The Odyssey (as William Tucker points out in a fine forthcoming book called Marriage and Civilization), democracy arrives and there is a sea change from the polygamy of Agamemnon to the lovelorn fidelity of Odysseus and Penelope.

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.”

Which leads to the delightful thought that Mr Hollande’s amorous proclivities contribute to France’s economic stagnation.