Infidelity happens in most species, even those with tight social bonds such as the swift fox (Vulpes velox), gibbons - and humans. Is it because mating outside the pair, as psychologists call it – or cheating, as we know it – can be exciting and physically rewarding? Or is there an underlying evolutionary drive whereby sex outside a relationship could potentially give you more offspring if you are a man, or better-quality children if you’re a woman? Studies of non-paternity show that up to 9% of children may be brought up by men who don’t actually know they are not the biological fathers – but in studies where men say they are certain of their paternity, the rates fall to under 2%.

Last week, a study in Evolution and Human Behavior of 7,737 Finnish twins and siblings who had been in relationships for at least a year found that nearly 10% of men and 6.4% of women had been unfaithful. By comparing rates between identical and non-identical twins and siblings (considered to share the same environment) the researchers concluded that genetics accounted for 62% of the variation in men and for 40% in women. The researchers did a second study, looking specifically for variations in specific genes to see if they were associated with increased rates of infidelity. For women, a variation in the AVPR1A gene (a gene that codes for the receptor for vasopressin) was more common among those who had been unfaithful. This gene is associated with mating behaviour in vole studies, and in human research with marital difficulties. There was no gene found to be associated with men’s infidelity. Studies do report people claiming to be surprised they have cheated. So could our genes be a subconscious influence on whether we stay monogamous or not?

The solution

No one has bottomed out an evolutionary drive for infidelity. Sure, men can disseminate their DNA, but for women, there is no evidence that having sex outside of a monogamous relationship creates a better-quality child. The only evidence, from unfaithful monogamous birds, is that such offspring are actually punier. Infidelity is a complex behaviour and while Brendan Zietsch, the lead author of this paper, is confident that genetic effects on behaviour are “virtually universal and highly replicable”, he says he is cautious about the findings on AVPR1A, as further studies often fail to replicate the effects of single genes. He would also not “bet his house” on the robustness of the actual percentages for the role of genetics in infidelity. So where does that leave us? In the land of common sense. A genetic association does not equal destiny. As Zietsch says: “Our genes don’t put us on auto pilot.”