Humans generally form long-term pair bonds in which both partners invest important resources in order to raise offspring. In this context, sexual unfaithfulness can be incredibly costly, which is why humans may have evolved the ability to judge trustworthiness in the opposite-sex.

A new study found that women have above-chance accuracy in judging unfaithfulness in men, but men did not show the same inclination when judging women’s faces. The participants were more likely to judge a man’s face as unfaithful if it had more masculine features, such as a wider face The findings suggest that this ability may have evolved in order to spot cheating men and same-sex poachers.

Credit: Pixabay.

Credit: Pixabay.

The face of infidelity

Although no one likes to be judged, the truth is that we’re all guilty of making value judgments about other people. For instance, think about first impressions: we often instantly decide if we like someone or not, and then use this information to interact with that person in the future or avoid them. Sometimes, we make these trait judgments based on a person’s face. Previously studies have shown that untrustworthy-looking individuals are less likely to receive cooperation in economic game theory settings. They’re also more likely to be judged as guilty in a simulated court setting despite evidence to the contrary — good thing normal people don’t run courts. But that’s not the point. The point is that humans place great emphasis on trustworthiness and have adapted to reading trustworthiness cues in other people, particularly their faces.

Am important area of trustworthiness is sexual unfaithfulness, which has important reproductive costs. Today, infidelity is cited as the number one cause of divorce across 160 different cultures.

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Evolutionary theories predict that since the cost of unfaithfulness is high, humans should have evolved strategies that prevent sexual infidelity. One such strategy might be determining infidelity by reading social cues. There’s also an incentive to detect “poachers” of the same sex. According to the new study, 70% of people across more than 50 cultures say they’ve made an attempt to poach someone else’s partner and 60% claim they were successful.

Researchers at the University of Western Australia recruited 1,500 men and women who were shown pictures of 101 men and 88 women, before they were asked if they had ever been unfaithful to their partners. Each participant had to rank these faces on a scale of 1 to 10, where one is “not at all likely to be unfaithful” and ten is “extremely likely.”

The results were somewhat unexpected. The findings suggest that men were able to spot poachers among other men or unfaithful men. Women were able to spot unfaithful men with above-chance accuracy but were not able to accurately spot female poachers. So the findings suggest that the female sex is somewhat inscrutable to both men and women.

“Taken together, both men and women showed above-chance accuracy for men’s faces but not women’s faces. Therefore, perceived unfaithfulness may indeed contain some kernel of truth in male faces,” the scientists wrote in the journal Royal Society Open Science.

According to the researchers, the differences in infidelity detection can be boiled down to perceived masculinity. Men who possessed more masculine traits, such as a square jaw, were more likely to be classed as unfaithful.

The researchers, however, caution people not to jump to conclusions. Men’s features may betray infidelity but it is still difficult to spot a cheater just by looking at an individual’s face. That would simply be prejudice, not some science-backed judgment.

“If we are to rely solely on our first impressions to detect cheaters/poachers, then we will make substantial errors,” said Yong Zhi Foo, lead author of the new study and a psychologist at the University of Western Australia.

“Our results must not be taken to mean that first impressions can be used in any everyday situations,” he added.