After seeing faces for less than a blink of an eye, college students have accuracy greater than mere chance in judging others' sexual orientation. Their "gaydar" persisted even when they saw the photos upside-down, and gay versus straight judgments were more accurate for women's faces than for men's.

The findings, published May 16 in the open-access online journal PLoS ONE, suggest that we unconsciously make gay and straight distinctions.

"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," said lead author Joshua Tabak, a psychology graduate student at the University of Washington. "This information confronts us in everyday life."

Tabak says that our ability to spontaneously assess sexual orientation based on observation or instinct conflicts with the assertion that if people just kept their sexual orientation to themselves then no one else would know and discrimination wouldn't exist, an argument frequently used by opponents of anti-discrimination policies for lesbian, gay and bisexual people.

In the study, 129 college students viewed 96 photos each of young adult men and women who identified themselves as gay or straight. Concerned that facial hair, glasses, makeup and piercings might provide easy clues, the researchers only used photos of people who did not have such embellishments. They cropped the grayscale photos so that only faces, not hairstyles, were visible.

For women's faces, participants were 65 percent accurate in telling the difference between gay and straight faces when the photos flashed on a computer screen. Even when the faces were flipped upside down, participants were 61 percent accurate in telling the two apart.

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At 57 percent accuracy, they had a harder time differentiating gay men from straight men. The participants' accuracy slipped to 53 percent -- still statistically above chance -- when the men's faces appeared upside down.

The difference in accuracy for men's and women's faces was driven by more false alarm errors with men's faces -- that is, a higher rate of mistaking straight men's faces as gay.

This may be because participants are more familiar with the concept of gay men than with lesbians, so they may have been more liberal in judging men's faces as gay, Tabak suspects. Another possibility is that the difference between gay and straight women is simply more noticeable than the difference between gay and straight men, Tabak said.

He was surprised that participants were above-chance judging sexual orientation based on upside down photos flashed for just 50 milliseconds, about a third the time of an eyeblink.

Don't think you have gaydar? You're not alone. Tabak says that in his experiments there are "always a small number of people with no ability to distinguish gay and straight faces."

It's unclear why some have better gaydar than others, since studies have only tested this aptitude in college students. Tabak speculates that "people from older generations or different cultures who may not have grown up knowing they were interacting with gay people" may be less accurate in making gay versus straight judgments.

Vivian Zayas at Cornell University is the other author of the paper. Funding was provided by Cornell University and the National Science Foundation.