A WOMAN’S genes are as likely to make her cheat on her husband or boyfriend as they are to trigger high blood pressure or depression.

A study of more than 1,600 pairs of female twins has revealed that genetic factors have as much influence over infidelity as they do over medical conditions in which their role has long been established.

The research at St Thomas’ Hospital in London puts the heritable element of female infidelity at 41 per cent — a figure on a par with traits such as hypertension, susceptibility to migraine, depression and anxiety, and age at menopause.

The findings suggest that some women have a genetic predisposition towards a roving eye, though whether they actually stray depends on environmental factors such as upbringing, opportunity and meeting the right men.

Genes, however, do not seem to affect moral attitudes towards infidelity. These are almost entirely determined by culture and environment, with even 17 per cent of women who have themselves cheated believing it to be “always wrong”.

The study led by Professor Tim Spector, director of the hospital’s twin research unit, is the first to demonstrate a firm genetic basis for infidelity.

It lends strong support to theories advanced by evolutionary psychologists such as Steven Pinker, of Harvard University, who argue that human sexual behaviour is at least partly determined by natural selection and our genes.

The results also add to a growing scientific consensus that many aspects of behaviour and biology are not set exclusively by either nature or nurture, but by a combination of the two.

In the research, which will be published next month in the journal Twin Research, Professor Spector recruited more than 1,600 pairs of female twins aged between 19 and 83, who were broadly representative of the population in social class, race and religion. Each volunteer completed an anonymous questionnaire on her sexual history and attitudes.

Twin studies are among the main tools for scientists investigating the genetic origins of disease or behaviour, as comparisons between identical (monozygotic) and non-identical (dizygotic) twins can reveal the role played by inheritance.

Identical twins share all their DNA and a childhood environment, while non-identical twins share an environment but only 50 per cent of their genes. If a trait is more commonly shared by identical than fraternal twins, it probably has a significant genetic component.

Professor Spector found that 22 per cent of the women had been unfaithful to a long-term partner, a rate that corresponds with previous research. If a woman’s identical twin had been unfaithful, she was twice as likely to have cheated herself, while non-identical twins were only 1.5 times as likely to stray if their sisters had done the same. The findings indicate that female infidelity is 41 per cent heritable. This does not mean that women have a two in five chance of inheriting these genes, or that 41 per cent of infidelity is caused by genes. Rather, 41 per cent of the variation between women’s tendency and motivation to be unfaithful, across the population, has a genetic origin.

Women who had had more sexual partners were also more likely to have been unfaithful, and this was found to be 38 per cent heritable. “The data show that genes influence female infidelity and the number of sexual partners,” Professor Spector said. “I would consider the influence quite large: it is comparable to other medical traits, such as hypertension, where a genetic component is not seriously disputed. In contrast, attitudes to infidelity were clearly non-genetic. That seems to be down to environment.”

Professor Spector’s team did not identify any particular gene that contributes to a tendency to infidelity, though they did pinpoint three regions on chromosomes 3, 7 and 20 that might harbour such genes. He believes that there are between 50 and 100 genes that contribute to a tendency to infidelity.

The researchers are now recruiting male pairs of twins to repeat the study and are expecting similar results.

The results, he said, suggest that female infidelity may have had evolutionary advantages. Women who cheated on their partners — particularly if they did it with men with “good genes” — might have had fitter offspring than those who were wholly faithful. “The fact that psychosocial traits such as number of sexual partners and infidelity appear to behave as other complex genetic traits in humans, in that they have a heritable component, lends support to evolutionary psychologists’ theories on the origins of human behaviour.”

WHAT’S IN OUR GENES?

Percentage heritability

Religious faith (Catholic, Muslim etc) 0

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