A divorced friend asked a woman at a party if she would like to have dinner one night. Her reply: "Would that involve sexual intercourse?" This anecdote came to mind as I browsed a paper by Hasse Walum, a Swedish biologist who has found that he can partly predict the future health of a marriage from a single mutation in a gene on chromosome 12. The gene is the vasopressin receptor, the mutation is called "334" and it is fairly rare: 60 per cent of us do not have it.
But if you inherit one or two copies of the mutation (and live in Sweden), you are more likely to have experienced a crisis in your marriage, and you are less likely to be married – half as likely in the case of those with two copies of the mutation.
This is no great surprise. For a start, Walum was merely confirming that human beings are much the same as voles, which – genetically speaking – we are. Tom Insel, Sue Carter and Larry Young have shown with their studies of prairie voles – which are unusual in the vole world for their monogamous sexual partnerships – that their mating exclusivity is due to a 428-letter sequence upstream of their vasopressin receptor gene. Transplant that sequence into a male mouse or a meadow vole before birth and the creature becomes less promiscuous, more aggressive towards rivals and more attentive to its mate.
Young even knows why this works. The sequence causes the gene to be expressed in a different part of the brain, where it links with a dopamine "reward" system, much the same as the one activated during drug addiction. And since vasopressin is a hormone released during sex, in effect the male vole gets addicted to his sexual partner. That's what love is – an addiction.
Vasopressin also makes male voles more attentive towards their pups. This may be true in human beings, too. Jamaican men have more vasopress in their blood when they have very young children than when they do not. The hormone is very similar to oxytocin, which encourages people to trust each other in laboratory experiments using real money, implying it is a key ingredient in pair bonding.
Mind you, the latest study to investigate the effect found that the 334 mutation did not predict marital status among the Ariaal people of northern Kenya, a small tribe of cattle- and camel-herders. But then the Ariaal practise polygamy, so having the mutation might make a polygamous relationship work better.
Most people's immediate reaction to the vasopressin story, even when only told the vole version, is to wonder whether it will lead to women testing their dates for faithful genes before agreeing to go out with them, which is why I was reminded of the startling question my friend received.
Here is how it might work. While being chatted up by a man, the woman surreptitiously slips into her pocket a napkin with which he wiped his mouth, and nips to the toilet. While there, she takes out her iPhone, clicks on the gene sequencer app, feeds the sample into the slot and awaits the result, texted from the Love-Lab<sup>TM</sup> in Utah: "congratulations – your prospective sexual partner has 0 copies of the 334 mutation. He will make a faithful husband."
This imaginary scene is not without its problems. First, even allowing for a rapid fall in cost and ease of gene sequencing, by the time she comes out of the toilet, he will be long gone, or chatting up her friend. Second, even if he has no 334 mutations, this hardly proves that he will be faithful: it is merely a statistical nudge. Third, what if she's after a fling anyway and the last thing she wants is a lovelorn obsessive? My observation of women is that they often warm to rakes.
Frankly, the information the gene test can give her is far less interesting than the information she can get the old-fashioned way: does he make good conversation/has he got a good job/is he good looking? And she is equipped with a highly sophisticated, automatic detection device for these features that is completely free and requires no internet connection. It's called a brain.
Yes, scientists will gradually pin down with some precision a genetic reason why some men find it easier to stay faithful and attentive. But the practical applications of this discovery will be virtually nil.