THE WORLD IS agonising over the cruel and self-destructive behaviour of certain young men. Why did Jude cheat on Sienna with the nanny? Why did Brad leave Jennifer for Angelina? The answer is provided by science.

When a female laboratory mouse is placed in a cage with a male, he pounces on her and bonks her a few times. But once the male is satiated, any further approaches from her are rejected. But should a new female — indistinguishable in our eyes from the first — be introduced, the male will pounce on her until, satiated again, he will reject further advances both from her and the first one. So the cycle continues, with the male interested only in new females.

This phenomenon is known as the Coolidge effect, after an episode from Calvin Coolidge’s 1924 reelection campaign. The President and his wife, Grace, visited a chicken farm, touring in separate parties. When Grace reached the farm’s prize rooster, the farm manager explained that the cock, er, covered 20 times a day. Impressed, Grace asked the manager to “tell that to the President, when he comes round”.

When Coolidge was told, he asked “same hen every time?” “Oh no, Mr President,” the manager replied, “different hen every time.” “Tell that to Mrs Coolidge,” the President said.

Because we human beings are promiscuous we enjoy such stories but, actually, the males of some mice species are faithful. Male prairie voles are; but their relatives, the montane voles, are not. It was Professor Tom Insel, of Atlanta, Georgia, who discovered that fidelity is mediated by brain receptors for a hormone called vasopressin. Monogamous male prairie voles have the receptors but the unfaithful montanes do not.

Vasopressin is the hormone for bliss. When animals, human or otherwise, have orgasms — or when they take cocaine and other addictive drugs — they release vasopressin into the brain. If those animals have the vasopressin receptor, the joy of sex is so magnified that they fall in love with their partners (or their dealer). But animals that are deficient in the receptor are also deficient in bliss. Perhaps we should feel sorry for Jude and Brad.

To arm women against other Judes and Brads, can we provide a DNA test? Elizabeth Hammock and Larry Young, of the Atlanta team, reported in Science magazine that the difference between the genes of the prairie and montane voles lies not in the DNA for the receptor itself but in a piece of “junk” DNA.

Most of our DNA seems to have no purpose. It is so-called junk DNA that clogs the chromosomes to no apparent end. But in a paper to be published later this month in Genome Research Adam Siepel and David Haussler, of the University of California, reveal that large stretches of junk DNA across a huge diversity of plants and animals are surprisingly similar. When lengths of DNA across species are similar, it is because they are too important to change over evolutionary time; that is, they have a function.

Increasingly, we now know that junk DNA is not junk but, rather, that it regulates the genes in the functional part of the chromosomes. The Atlanta group has therefore discovered a stretch of DNA that appears to be junk but which, in fact, regulates male promiscuity. The best guide, incidentally, to the emerging role of junk DNA is Matt Ridley’s book Nature via Nurture.

But perhaps the most intriguing aspect of this research is the existence of the faithful prairie vole. The universal myth is that males are programmed to be promiscuous for reasons that we deem obvious, but many species do pair-bond for life, abhorring infidelity for the mutual society, help and comfort that one animal ought to have of the other. And the evolutionary inevitability of male chauvinism hardly explains the behaviour of the female black widow spider or praying mantis, both of which eat the male post-coitally. And it’s the male seahorse that rears the offspring.

One evolutionary reason for faithful pair-bonding is that — if they can be confident in their paternity — fathers will help to raise their offspring. The father who spurns promiscuity for childcare is, therefore, making a rational investment in the reproduction of his own genes. But his wife and all the other males must be above suspicion, which rules out Jude and Brad.

Terence Kealey is Vice-Chancellor of the University of Buckingham

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