Some people may be genetically programmed to be unfaithful to their partner, a scientist has claimed.
All in the genes?
Professor Tim Spector, of the Twin Research Unit at St Thomas' Hospital, London, says he has evidence of a genetic component to infidelity.
Focusing on women, he found that if one of a pair of twins had a history of infidelity, the chances her sister would also stray were about 55%.
In general it is estimated that just 23% of women are not faithful.
In addition, Professor Spector found the tendency for both twins to be either faithful or unfaithful was strongest in identical pairs - who have identical genes.
He stressed that genes alone did not determine whether somebody was likely to be unfaithful - much was down to social factors.
But he said it made good sense in evolutionary terms to get a good mix of genes - and for women to choose a better option if one came along.
No one gene
However, he stopped short of concluding that there was an infidelity gene.
He said: "There is unlikely to be a single gene for anything like this. But there are likely to be genes that participate in it, a number of genes working together; it might be things like risk taking or those associated with personality."
Dr Petra Boynton, a social psychologist, said it was very hard to know what element of behaviour was inherited, and what was learned.
"If as a child you see your mother cheat on your father, it comes easier for you. You copy the behaviour.
"It is not predisposed by genes, it's because you think that is what a relationship is like or that you can get away with it."