Since childhood, we’ve been told not to judge a book by its cover. But new research conducted at the University of Western Australia and published in PLOS One suggests that men have some clues about a woman’s faithfulness to her partner based on her appearance alone.

The study investigated whether sexual faithfulness could be accurately judged by viewing opposite-sex strangers’ appearance.

“Our results provide the first evidence that such judgments can contain a kernel of truth,” wrote evolutionary psychologist Samantha Leivers and her colleagues, Leigh W. Simmons and Gillian Rhodes, in their study.

“It is striking that men were able to show any accuracy from images alone after only a brief presentation, considering that accuracy in faithfulness judgments made from behavioral information is relatively poor,” they said.

In two experiments, the researchers found when heterosexual men were presented with images of two women in a forced choice task, they tended to choose the more faithful woman. However, the male participants showed no accuracy in rating individual women, suggesting men’s ability to detect a cheater based solely on her appearance is rather weak.

The researchers obtained photographs of 34 heterosexual women between 20 and 42, along with information about whether they had cheated on a romantic partner. The 34 models were grouped into 17 pairs, with one woman who reported cheating on a partner two or more times and the other reporting having never cheated on a partner.

The 43 participants in the experiment were shown the pairs of women on a computer screen, and asked to choose which woman was more faithful. The “unfaithful” woman appeared on the left of the screen for nine trials and on the right for the other eight trials.

Half of the participants judged the women’s faithfulness based on full person images, while the other half viewed only the women’s faces.

Another 29 participants viewed individual images of the women and were asked to rate the women based on their perceived trustworthiness. Fifteen other participants rated the women based on attractiveness.

The researchers found a relationship between perceived trustworthiness of the models and accurate judgments of faithfulness, but there was no relationship between the model’s rated attractiveness and faithfulness.

The researchers conducted a second experiment with a new sample of 60 male participants to see if they could replicate their initial findings.

“When asked to choose the more faithful women from pairs of images, men chose the woman who had not reported any [cheating behavior] significantly above chance level,” Leivers and her colleagues wrote. “Accuracy of judgments from faces had a moderate-large effect size and was repeatable across two experiments using different samples of men.”

The researchers also noted some important caveats in their study.

“Although men’s judgments contained a kernel of truth when they selected the more faithful of two women in a forced choice task, they showed no accuracy in rating individual women. Their judgments of faithfulness did not correlate with the self-reported [cheating] behavior of the models,”

“Furthermore, our results also suggest that the ability to accurately judge faithfulness at above-chance levels in a forced choice task does not generalize to all faces, being easier for some pairs of women than others.”