Sept. 2, 2008 -- What if you could tell whether a man is husband material just by peering at his genes?

There has been speculation about the role of the hormone vasopressin in humans ever since we discovered that variations in where receptors for the hormone are expressed makes prairie voles strictly monogamous but meadow voles promiscuous; vasopressin is related to the "cuddle chemical" oxytocin. Now it seems variations in a section of the gene coding for a vasopressin receptor in people help to determine whether men are serial commitment-phobes or devoted husbands.

Hasse Walum at the Karolinska Institute in Stockholm, Sweden, and colleagues looked at the various forms of the gene coding for a vasopressin receptor in 552 Swedish people, who were all in heterosexual partnerships. The researchers also investigated the quality of their relationships.

They found that variation in a section of the gene called RS3 334 was linked to how men bond with their partners. Men can have none, one or two copies of the RS3 334 section, and the higher the number of copies, the worse men scored on a measure of pair bonding.

Not only that, men with two copies of RS3 334 were more likely to be unmarried than men with one or none, and if they were married, they were twice as likely to have a marital crisis.

Commitment phobia

Given that everyone surveyed had been in their relationship for at least five years, the team suggests that having multiple copies somehow contributes to commitment problems in men. Because the results were collected for a different study the team couldn't quiz the men on whether they were faithful, says Wallum.

It is not clear exactly how multiple copies of RS3 334 affect expression of the vasopressin receptor, and our most intimate relationships. And yet that's the most interesting question, says Thomas Insel, director of the National Institute of Mental Health in Bethesda, Maryland.

In some animals, the theory is that the brain has two "motivational" systems: one for reward, the other for social perception. In prairie voles and marmosets, receptors for the two systems sit on adjacent cells, so social activity is highly rewarding, leading to monogamy. To see if the same mechanism is at work in people will mean using tissue from post-mortems to map where vasopressin receptors lie, to see if variations are linked to the number of copies of RS3 334.

RS3 334's social effects extend beyond bonding in couples. Earlier this year, the same gene section was shown to affect signalling in people's amygdalas, linked to trust. Another study found that people with autism, which is characterised by unusual social behaviour, often have multiple copies of RS3 334.

Walum's colleague Paul Lichtenstein says the team's next task is to test how a nasal vasopressin spray affects altruism and jealousy.