Researchers from The University of Western Australia have found a kernel of truth in the idea that unfaithfulness can be judged just by looking at a stranger's face.
And it is women who have the knack, according to Winthrop Professor Gillian Rhodes, Winthrop Professor Leigh Simmons and researcher Grace Morley, whose article on their research was published today in the Royal Society journal Biology Letters.
The trio asked study participants to look at the faces of unfamiliar people for three seconds and then rate them for traits, including faithfulness and trustworthiness.
The people whose faces were rated had already told researchers in an anonymous questionnaire whether they had a history of cheating on their own partners or poaching a partner off another person in the past.
Professor Simmons, Director of UWA's Centre for Evolutionary Biology, said female participants had a much higher strike rate than their male counterparts in accurately choosing whether strangers of the opposite sex were likely to be cheaters. Women got it wrong 38% of the time compared with men who chose wrongly 77% of the time.
"What was really surprising was that women were able to do that above chance," Professor Simmons said.
"They were able to look at a face and rate it for faithfulness or unfaithfulness. There was a correlation between their ratings for faithfulness and the actual behaviour of the individuals they were rating. Now men couldn't do that, or the relationship was much weaker."
He said the gap may be due to any number of reasons.
"It may be that women have evolved the greater ability to make accurate assessments than men because the costs of making mistakes, for women, are greater," Professor Simmons said.
"On the other hand, the men were making mistakes a lot. One potential reason could be that males of most animals, including humans, tend to be less discriminating of their partners because they have less to lose if that partner is unfaithful.
"Obviously they could end up being cuckolded in human terms, but they don't have all the physiological costs of gestation, child bearing and child rearing that women have. And males have greater opportunities for reproducing with other individuals."
The authors said their research provided the first evidence that impressions of unfaithfulness made from the faces of opposite-sex strangers contain a kernel of truth.
"Previous studies have focused on accuracy of impressions from samples of behaviour," they wrote.
"Our results demonstrate that accurate judgements of unfaithfulness can be made from the face alone, in the absence of behavioural cues."
The authors also found a high correlation between attractiveness and perceptions of trustworthiness, with more attractive people judged more likely to be trustworthy.
"There might be some sort of attractiveness halo effect going on there," Professor Simmons said.
"What was also really interesting is there was no correlation between peoples' rating of sexual faithfulness and trustworthiness. So they're obviously very different tasks and people are looking for different things."
He said even though women might have a radar more finely attuned to unfaithfulness, there was more to relationships than first impressions.
"We don't go into relationships based just on those sorts of visual cues. We have long periods of courtships and we get to know individuals and learn more about them than your first impressions might convey. Nevertheless we make errors even then."