The University of Washington study suggested that most people possess an in-built ability, called gaydar, which helps them judge whether someone is gay or straight by just looking at the individual's facial features.
In tests, the researchers found that gaydar works on an unconscious level and is more accurate when directed at women.
People use a combination of clues from individual facial features and from the way those features fit together to make snap judgements about sexual orientation, said study researcher Joshua Tabak.
"We may be doing this so efficiently that we may not even have to try to make this judgement," Tabak told LiveScience.
In the study, published in the journal PLoS ONE, over 100 college students were invited to view about 96 photos of young men and women -- half of whom who identified as gay. The black and white photos showed only the person's face; no hairstyles, facial hair, glasses or makeup to give clues.
The participants were asked to make a snap judgement of whether someone was gay or straight by looking at a photo of their face for 50 milliseconds, less than the blink of an eye.
More than half of the time, they guessed right.
For women's faces, participants were 65 per cent accurate in judging sexual orientation, while for men's faces, accuracy dropped to 57 per cent but was still statistically better than chance, the researchers said.
"Why this is we can only speculate," Tabak said. "It's really interesting to speculate that there may be this ironic effect that because we're more familiar with the concept of gay men, maybe we're more liberal with labelling a man gay."
When looking at upside-down faces, people were still able to guess their sexual orientation correctly at rates better than chance -- although not quite as accurately as when the faces were right-side up, the researchers found.
That suggests both facial features (which can be processed in upside-down and right-side-up photos) and facial configuration provide hints into orientation, they said.
But, it remains to be seen how or if people use "gaydar" in real life, when they have more to go on than a glimpse of a photograph, Tabak said.
