The study found that participants had up to 65% accuracy when looking at women’s faces on deciding whether they were gay or straight; and up to 57% accuracy when looking at men’s faces. With the photos flipped upside-down, the figures came in slightly lower at 61% and 53%.
The college student participants were only shown pictures of faces, and not hairstyle or clothing.
Dr Joshua Tabak, of the University of Washington said: “We were surprised participants were above-chance judging sexual orientation based on upside down photos flashed for just 50 milliseconds.
"That’s about a third of an eyeblink.”
“It may be similar to how we don’t have to think about whether someone is a man or a woman, or black or white.”
The study in science journal PLoS One suggests we may make initial gay or straight decisions without realising upon meeting a person.