Circumcision and other forms of male genital mutilation have always been a puzzle. The ritual mutilations can leave the man vulnerable to infection and even death. So why do some societies insist on such a risky ritual for their men?

There may be an evolutionary explanation, according to Christopher Wilson, of Cornell University in New York, US. It could function to reduce a young man's potential to father a child with an older man's wife, he says.

Sperm competition theory predicts that males will evolve ways to ensure that their sperm, and not another male's, fertilises a female's eggs. Genital mutilation, in this view, is just another way to win the sperm war.