There may be some truth in judgments that men make about women's sexual faithfulness based on their looks, according to researchers.

A new study suggests that just by comparing photographs of two unfamiliar women, men have some ability to accurately judge which one is more likely to cuckold them.

The findings are published in a recent issue of PLOS ONE.

As part of a PhD carried out at the University of Western Australia, Dr Samantha Leivers was looking for evidence that men had evolved an ability to assess a woman's likely faithfulness.

She got men to compare 17 pairs of photographs of unfamiliar women who were matched for age and ethnicity.

One woman in the pair reported never having had sex with someone other than her partner while in a committed relationship.

The other reported having extra-partner sex on at least two occasions.

Dr Leivers asked men to judge which woman in each pair was likely to be more faithful.

This "forced choice" task was carried out twice with different groups of men.

Dr Leivers found just by looking at women's faces, men's judgments on faithfulness were accurate 55 to 59 per cent of the time.

Given that one might expect to choose correctly 50 per cent of the time by chance, she said this level of accuracy was "statistically significant but modest".

"We don't expect them to be 100 per cent accurate when they are literally just looking at someone's face for a few seconds," Dr Leivers said.

"The fact that they're showing any accuracy from this limited information is pretty cool."

Previous research that got men and women to judge faithfulness in the opposite sex by rating a whole series of photos on a scale showed women were accurate at faithfulness judgments but men were not.

"We show for the first time that men's judgments of faithfulness from images of women can contain a kernel of truth when they are able to directly compare images in a forced choice task," Dr Leivers said.

Measures of trustworthiness

But there appears to be differences in which measures men and women use to identify faithfulness.

Unlike women in the previous studies, the men in this study used perceived trustworthiness as a gauge for faithfulness.

"If the faithful woman in the pair looked more trustworthy than the unfaithful woman, then the participant was more likely to guess that she was a faithful woman," Dr Leivers said.

Why perceived trustworthiness is used as a measure by men, but not women, to assess faithfulness is unknown.

One possibility is that the same visual cues used to make judgments about faithfulness may be used to make judgments about trustworthiness.

"For example, emotion expression has been found to influence a number of trait judgments including perceived trustworthiness," the researchers said.

So even though the faces shown to the men had neutral expressions, there may have still been perceived subtle differences in emotion expression.

"More research is needed to determine the visual cues that men use to make their judgments of faithfulness, including emotion expression and other face gesture clues," the researchers said.

Sperm competition study

Dr Leivers' research was part of a broader study on sperm competition, which is when the sperm of two or more males compete to fertilise the eggs of a particular female.

"There's been quite a bit of controversy about whether sperm competition has acted as a selective pressure in human evolution," she said.

Dr Leivers said the new findings provided some evidence that human males had evolved adaptations to prevent cuckoldry by distinguishing women who were more likely to be unfaithful in a relationship.

In previous research, she found men with less competitive sperm tend to keep a closer watch on their female partner and restrict her social interactions with other men, so-called mate-guarding behaviour.

She said sexual jealousy was another adaptation that could help men ensure their sperm succeeded over others.