It sounds like the perfect excuse for a wandering eye — a man's tendency to be unfaithful may be influenced by his genes, research suggests.

Men who inherit a genetic variant that affects an important attachment hormone are more likely than usual to have weaker relationships and marital problems, and less likely to be married, according to the research. Their wives and girlfriends are also more likely to be less satisfied with them as partners.

While the study did not look directly at infidelity, the findings suggest that male monogamy might also be influenced by variations in a single gene. The study's authors cautioned that any effect would apply only on average, and that it was impossible to predict whether any individual would be unfaithful or a bad partner on the basis of his genes.

The gene in question affects the receptor for a hormone called vasopressin, which plays an important role in social behaviour, pair-bonding and sexual attachment. Its effects were first characterised by studies of different species of voles. Although the meadow and prairie voles are close cousins, their sexual behaviour is dramatically different.

Like most mammals, male meadow voles are highly promiscuous, but male prairie voles are monogamous.

When boy prairie vole meets girl, the two indulge in 24 to 36 hours of nearly continuous mating, which cements a bond that invariably lasts for life. When one partner dies, the survivor usually opts to remain celibate rather than find another mate.

Male prairie vole brains contain much higher levels of vasopressin than those of meadow voles. There also genetic differences in the receptor for vasopressin in the two species.

The role of the hormone in promoting attachment was proved in 2004, after scientists enhanced the gene for the vasopressin receptor in male meadow voles. This turned them from promiscuous lotharios into loyal and attentive spouses.

Vasopressin is also active in the human brain, and scientists have long wondered whether it might play a similar role in human relationships. Humans are among the less than 5 per cent of mammals that form at least partially monogamous bonds, even if our standards of fidelity are lax in comparison to prairie voles.

Scientists investigated three genetic variations in the human vasopressin receptor, and tested a database of 2,186 twins and their partners to determine which types they had inherited.

The team, led by Hasse Walum, of the Karolinska Institute, in Stockholm, then gave each couple a standard test to assess the strength of their relationship, and asked them whether they had experienced a marital crisis or threat of divorce or separation in the past year.

They found that one of the genetic variants, known as 334, had a moderately significant effect, but in men only. Men with two copies of this variant were twice as likely to have had a marital crisis, and they scored lower on the pair-bond quality index. They were less likely to be married to their partners, and their wives and girlfriends were more likely to be dissatisfied with their relationship. All the men used in the study had been in a relationship for at least five years

The results, which are published in the journal Proceedings of the National Academy of Sciences, indicate that genetic variations in vasopressin uptake may influence how men behave in their relationships. The scientists suggest tentatively that the association would be even stronger in a population that also comprised subjects who were not involved in long-term romantic relationships, and are planning to conduct further research into this.

The result “clearly does not mean that this polymorphism [genetic variation] may serve as a predictor of human pair-bonding behaviour on the individual level”, they said.

“However, by demonstrating a modest but significant influence of this gene on the studied behaviour on the group level, we have provided support for the assumption that previous studies on the influence of the gene coding for the vasopressin receptor on pair-bonding in voles are probably of relevance also for humans.”

THE BIRDS AND THE BEES

Inheritance

A study at St Thomas’ Hospital in London has suggested that 40 per cent of the variability in female infidelity is genetic in cause. Research by the same team has also pointed to a genetic role in the female orgasm: about a third of the variability in women’s ability to reach orgasm may be inherited

Oxytocin

Sometimes known as the “love hormone”, oxytocin is released in the brain after sex, childbirth and breastfeeding. It promotes trust and bonding between sexual partners and between mother and baby

Testosterone

Libido in men and women has links to levels of the hormone

Scent

There is evidence that men and women are attracted to the smell of people with a different set of genes governing a component of the immune system. Offspring that inherit two types of immune genes may have advantages

Symmetry

There is good evidence that both sexes find people with symmetrical physical features more attractive

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