Media heralds the discovery of 'infidelity gene'

“Women are more likely to cheat on their partner if they carry the ‘infidelity gene’,” reports the Mail Online. They say that this gene “only has an impact on women”.

The headline is based on a study by Finnish researchers who were interested in a long-standing evolutionary puzzle: why do some women cheat on their partners? From an evolutionary perspective, the more partners a man has the more chances to pass on their genes. But as women can only have one pregnancy at a time, the benefit of having multiple partners is less clear (in strictly evolutionary terms).

The researchers looked at more than 7,000 twins and siblings who had been in relationships for over a year. Analyses comparing identical twins with non-identical twins or siblings suggested that some of the variation in infidelity behaviour seen could be accounted for by genetics.

The researchers also found that certain variations in the gene encoding a receptor for the hormone vasopressin were more common in women who reported having sex with more than one person in the past year, than in women who had sex with just one person. This association was not found in men.

This study only found an association between variations in one gene and infidelity.

Infidelity is likely to have complex influences, and while this might include a genetic component, this is unlikely to boil down to a single “infidelity gene”.

Where did the story come from?

The study was carried out by researchers from the University of Queensland in Australia and other research centres in Australia, Sweden and Finland. Study authors were funded by the Australian Research Council and the Academy of Finland.

The study was published in the peer-reviewed journal Evolution and Human Behavior (sic).

The Mail Online’s headline oversimplifies what is likely to be an issue with complex causes.

The study does not suggest that there is a single “infidelity gene”, and the authors themselves note that their findings are tentative.

What kind of research was this?

This was a cross-sectional study looking at possible genetic reasons for infidelity.

In evolutionary terms, having sex with people who are not your partner if you are a man increases your chances of fathering more children and passing on your genes. As women can only carry one baby at a time, the evolutionary reasons why infidelity might be advantageous are less clear.

One theory is that women could increase the “genetic benefits” for their children if they become pregnant by having sex with a man who has “higher-quality genes” than their partner. However, evidence gathered from socially monogamous birds suggests this may not be the case.

Another theory is that any genetic variations which predispose men to infidelity could also predispose women to infidelity. Therefore, if these genetic variations give rise to a better chance of a man’s genes being passed on, they will also exist in women, even though there is no advantage. The researchers wanted to assess if this could be the case in humans.

The methods used in this study are commonly used to look at how much variation in a particular trait might be explained by genes in a given population, and also to look for associations with particular genetic variations. However, the results may not be representative of other populations. It is also not possible to say with certainty whether any associations identified directly cause or contribute to the outcome – particularly when talking about a complex behaviour such as infidelity.

What did the research involve?

The researchers first looked at how much of infidelity might be accounted for by genetic factors. They then looked at whether variations in two genes (oxytocin and vasopressin receptor genes) were associated with infidelity.

In their first analysis, the researchers assessed infidelity in 7,378 twins and their siblings who were in long-term relationships (married or with a steady sexual partner for at least a year). The participants were asked how many different sexual partners they had had in the past year, and those who reported having more than one partner were counted as having been unfaithful (“extrapair mating”, to use the study’s terminology).

They analysed whether infidelity behaviour was more likely to be shared by identical twins (who have the same genes) than non-identical twins or siblings (who only share half of their genes, on average). If identical twins share a behaviour more than non-identical twins or siblings, this indicates that genetics could be playing a role. Twins and siblings are considered to share their environment to a similar extent.

The researchers also looked at whether brother-sister sibling pairs tended to show the same infidelity behaviour. If they did, this might suggest that the theory that the genes affecting fidelity in men might also affect fidelity in women might be responsible for this similarity.

In the second part of their study, they looked at whether variations in the genes encoding receptors for the hormones vasopressin and oxytocin were associated with infidelity. These hormones and their receptors have been found to affect pair-bonding behaviour in voles, and to be associated with social behaviour in humans.

The researchers looked at 19 single “letter” variations and two other variations in and around the two main genes of interest: AVPR1A and OXTR. The researchers also looked at 120 variations in and around other genes.

What were the basic results?

The researchers calculated that genetics may account for around 62% of the variation in infidelity behaviour in men in the population, and 40% in women. They found that brother-sister sibling pairs were not particularly similar in their likelihood of being unfaithful to their partner. This suggested that the genes which might be affecting infidelity in men were not likely to be affecting infidelity in women.

They found that variations in the gene for the vasopressin receptor AVPR1A were associated with infidelity behaviour in women, but not men. Variations in the oxytocin receptor gene (OXTR) were not associated with infidelity behaviour in either sex.

How did the researchers interpret the results?

The researchers conclude that they “found significant genetic influences accounting for around half the variation in extrapair mating in both sexes, confirming biological underpinnings to the behaviour”. They note that the AVPR1A association they found “should be regarded as tentative until subjected to rigorous replication”.

Conclusion

This study has suggested that there may be some genetic influence on variations in infidelity behaviour. It also identified variations in a gene for AVPR1A that were linked to infidelity in women, but not men.

The limitations to this study include the following:

• The researchers note that some of their results are not very precise (have wide confidence intervals) as infidelity was relatively uncommon.
• Some of the findings relating to the two genes (AVPR1A and OXTR) differed from what might be expected, based on the findings of some other studies of related behaviours, such as social pair bonding.
• Many studies have found associations between genetic variations and complex behaviours that have not been confirmed in later studies.
• An association does not necessarily mean that one factor causes the other.

Infidelity is likely to have complex influences, and while this might include a genetic component, it is unlikely to boil down to a single “infidelity gene”, as suggested by the media.

As the authors themselves note, the findings should be considered tentative until they are confirmed.

The study may be of interest to evolutionary biologists, but it doesn’t mean that your genes are an excuse for infidelity (especially in the week after Valentine’s Day).

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.

Links To The Headlines

Women are more likely to cheat on their partner if they carry the ‘infidelity gene’, scientists discover. Mail Online, February 15 2015

Links To Science

Zietsch BP, Westberg L, Santtila P, Jem P. Genetic analysis of human extrapair mating: heritability, between-sex correlation, and receptor genes for vasopressin and oxytocin. Evolution and Human Behavior. Published online October 17 2015