Some women may have inherited a genetic predisposition to be unfaithful to their partners, according to a study.
In an attempt to provide fresh insight into the old debate on whether individuals are shaped by nature or nurture, scientists talked to 3,200 women - all identical or non-identical twins - about their sex lives.
They found wide variations in attitudes towards infidelity and in how many sexual partners they admitted to having. The average number of sexual partners was between four and five. Slightly more than 20 per cent admitted to being unfaithful in a stable relationship.
Professor Tim Spector, director of the Twin Research Unit at St Thomas' Hospital in London, said that about 40 per cent of the variation in faithfulness in the group was due to genes, with the rest of the variation down to environment and upbringing - nurture.
Attempts to link infidelity directly to a specific gene or set of genes failed, although the researchers said that they did manage to locate some of the traits to three of the 23 pairs of human chromosomes.
Professor Spector said: "There is not an infidelity gene, but 50 to 100 genes are important and give us the tendency to respond to our environments in different ways ... It may be important for women who commit infidelity."
The study, which was published in the journal Twin Research, suggests that a genetic predisposition towards female infidelity may have evolved because it was important in allowing women married to "low status" men surreptitiously to become pregnant by "high status" men.
"If female infidelity and number of sexual partners are under considerable genetic influence, as this study demonstrates, the logical conclusion is that these behaviours persist because they have been evolutionary advantageous for women," the researchers write in their scientific paper.
"Work in the UK has shown that human females generally have affairs with men of higher status than their husbands, perhaps illustrating an effort to mate with a genetically superior partner," they say.
Although the findings came out of a study of just 1,600 pairs of twins, Professor Spector said that he believed the conclusions could be generalised to include the wider female population of Britain because the sample was representative of the country at large in terms of class and ethnic background.
The study is one of many involving more than 10,000 twins who, since 1992, have been investigated and interviewed for a whole range of medical and psychological differences, from high blood pressure to snoring.
"Twins are the perfect natural experiment because they allow you to do what you can't do in families, that is to separate nature and nurture," Professor Spector said.
"We are taking a group of identical twins that are clones of each other, who have exactly the same environment, compared to a group of non-identical twins who are like brother and sister and share 50 per cent of their genes, and who also have the same environment," he said.
"Everything is perfectly matched in these two groups. So you compare the two, and when you do, you get a clear idea of what the genetic component to [the trait] is."
The average age of the women involved in the infidelity study was 50, with some reporting no extra-marital affairs or no sexual partners while some, at the other extreme, said that they had had more than 100 sex partners.
A quarter of the women were divorced and 98 per cent said they were heterosexual. The researchers calculated that 41 per cent of the variation in infidelity and 38 per cent of the variation in the number of sexual partners was genetic.
Interestingly, there was no genetic basis for attitudes towards infidelity. Professor Spector said that 90 per cent of the women reported having thought about being unfaithful but only one in four actually did anything about it.
"The fact that psychosocial traits such as number of sexual partners and infidelity appear to behave as other common complex genetic traits in humans ... lends support to evolutionary psychologists' theories on the origin of human behaviour," he said. "Not surprisingly, the average number of sexual partners was significantly higher among respondents who had been unfaithful, compared with those who had remained faithful - a mean of eight compared with four," he added.
"It's important that we all don't behave the same way. In terms of evolution, some people have a high tendency to have multiple partners and to mix the genes. The system would break down if there wasn't any pair bonding," he said.
Nature's influence: From God to snoring
It is not just infidelity that runs in the genes. The same scientists have found that sleepiness and even a belief in God has a strong genetic component.
In a study of 2,000 pairs of twins, Professor Tim Spector and colleagues at St Thomas' Hospital found that about 50 per cent of the differences in sleeping patterns in people they observed were due to the genes they had inherited.
Another unpublished study found that 40 per cent of the differences in spirituality or tendency to believe in God were due to nature rather than nurture, Professor Spector said.
Many sleepless nights are caused by obstructive apnoea - snoring - but the scientists also looked at "restless legs syndrome", where people experience involuntary movement or discomfort in their legs that can wake them.
"These results suggest a substantial genetic contribution ... That could be good news for people who suffer ... if the genes can be identified," Professor Spector said.
Adrian Williams, a member of Professor Spector's team and consultant in sleep disorders at St Thomas's Hospital, said: "Sleep disorders are surprisingly common and it is increasingly recognised that they can have a devastating impact on sufferers' everyday lives.
"For example, obstructive sleep apnoea affects approximately 24 per cent of men and 9 per cent of women aged between 30 and 60. It even contributes to road-traffic accidents when sufferers fall asleep at the wheel."
Genes were found to account for 42 per cent of the variation in snoring patterns, 45 per cent of variation in daytime sleepiness, 54 per cent of differences in restless legs and 60 per cent of that in involuntary leg movement.
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