CIRCUMCISION and other forms of male-genital mutilation are commonplace in many societies around the world. The origin of these practices, however, puzzles anthropologists and evolutionary biologists. They wonder what benefit they could bring, especially given the obvious risks of infection and reduced fertility.

Explanations have ranged from the pragmatic (a ritual that marks the beginning of adulthood and bonds men together) to the Freudian (having something to do with the pain of the separation from the mother). However Christopher Wilson, a neurobiologist at Cornell University, has a different idea. In a recent paper in Evolution and Human Behavior he suggests that male-genital mutilations are actually intended to prevent younger men from fathering children with older men's wives.

Dr Wilson takes his cue from sperm-competition theory, which suggests that males of promiscuous primate species have evolved features that maximise their own sperm's chances of fertilising an egg they might have to compete for. These features include large testicles which produce more sperm, and morphologically complex penises. Males of monogamous primate species, on the other hand, have smaller testicles and simpler penises. Human genitals are somewhere in between, perhaps reflecting the fact that people generally form pair bonds, but are susceptible to occasional bouts of promiscuity.

Some forms of genital mutilation have obvious effects on fertility. For instance, several African and Micronesian societies practice testicular ablation—the crushing or cutting off of one testicle. Some Australian aborigines engage in subincision, which exposes part of the urethra and thus causes sperm to leak out of the base of the penis. Circumcision does not have quite such clear-cut effects. But there are several ways it may affect fertility: most obviously, the lack of a foreskin could make insertion, ejaculation or both take longer. Perhaps long enough that an illicit quickie will not always reach fruition.

Older men are in a position to form alliances with younger men—passing on knowledge, lending them political support and giving them access to weapons. By insisting that the young undergo genital mutilation of some form as a quid pro quo, an older married man can seek to ensure that even if he is cuckolded, he will still be the father of his wives' children. Of course, the older man has probably undergone genital mutilation too, and seen his own fertility reduced. But that, if anything, increases his incentive to make certain that the young bucks are similarly handicapped. And if all the older men in a society conclude this is a good thing, it will rapidly become a socially enforced norm.

To test this theory, Dr Wilson made several predictions. Among them, he suggested that mutilation is more likely to be practised in polygynous societies (since a man with several wives is more vulnerable to cuckoldry), and is especially likely in those polygynous societies where a man's co-wives live in separate households from their husband. It should also take place in a public ceremony watched by other men, to avoid cheating or free-riding. And there should be a strong stigma against men who refuse it.

To test his predictions, Dr Wilson looked at a database of 186 pre-industrial societies. Some 48% of the highly polygynous ones practised a form of male-genital mutilation, and the number rose to 63% when co-wives kept separate households. By contrast, only 14% of monogamous societies practised mutilation. Moreover, and also as predicted, the mutilations were almost always carried out in public, often as part of a coming-of-age ceremony at puberty, with strong stigma attached to unmutilated men.

Dr Wilson's paper does not definitely prove that sexual competition is at the root of male-genital mutilation. But it does provide a plausible explanation for a puzzling practice. It is not likely, however, to have much effect on attitudes toward circumcision. The men who enforce and undergo the rituals are no more aware of the underlying evolutionary motivations than of why their testicles are the size they are. Those who engage in the practice for religious reasons will surely continue to do so. Otherwise, most of the Western world has already largely abandoned routine neonatal circumcision, which is seen as an outdated and unfortunate medical fad.

The exceptions are America, where more than half of newborn boys are still circumcised, and Africa, where circumcision helps to stop the transmission of HIV, the AIDS-causing virus. There, infection really is a far greater threat to the number of children a man might have than the loss of his foreskin.