In 2012, researchers from the University of Western Australia found that straight women could judge the faithfulness of a potential mate by looks alone. And straight men, they found, could not.
But a recent study out of the same university suggests otherwise. It might just be that men need the information presented differently.
Evolutionarily speaking, the ability to spot faithfulness in a potential mate is an advantage, even more so for men than for women. If a man's mate has sex outside the relationship, he's less likely to pass on his genetic material and may end up raising genetically unrelated offspring.
In that 2012 study, published in the journal Biology Letters, 34 heterosexual men and 34 heterosexual women looked at successive photographs of opposite-sex faces, rating how faithful each model seemed. When researchers compared the ratings with the self-reported faithfulness of the models, they found that while the women judged with some accuracy, the men weren't accurate at all.
Evolutionary biologist Samantha Leivers, along with colleagues Leigh W. Simmons and Gillian Rhodes, explored whether men's accuracy might improve if the photos were presented in a different format. Their results, published in the journal PLOS ONE in August 2015, suggest there's some hope for men who want to assess a woman's penchant for monogamy without even talking to her.
Leivers and colleagues gathered 34 women of similar age and ethnicity to pose for photos wearing no makeup and neutral facial expressions. The models answered confidential surveys about whether and how many times they had cheated when in a committed relationship.
Forty-three men then assessed the women's faithfulness based on their photos. First, the men viewed the photos one at a time, rating each woman on a faithfulness scale. Next, they viewed the photos in 17 sets of two and were asked to select the more faithful woman from each pair.
When the men rated the women individually, they exhibited no statistically measurable accuracy, says Leivers in an email. The results were the same as those of the earlier study.
But when they had to choose the more faithful woman from a pair of photos, they were accurate 55 percent of the time.
Leivers repeated the "force-choice task" with a second group of men. They averaged 59 percent accuracy.
The findings suggest that men can, with some degree of accuracy, identify a faithful woman by physical appearance alone, but only in an oppositional, "A or B" scenario.
The study doesn't explore why, but Leivers thinks it could be that "by having people compare stimuli and forcing them to make a dichotomous choice, it can tease out subtle differences in perception and preferences that a ratings task might not."
"For instance," she explains, "in a ratings task where you have to rate Brad Pitt and Chris Hemsworth for attractiveness, you might give them both the top score. But if you have to compare them in a forced choice task and you are forced to choose the one you find the most attractive, it might reveal a preference that would never have been detected in a ratings task."
Leivers denies any connection between her findings and how to spot a faithful (or unfaithful) woman at, say, a bar on a Saturday night.