Is he gay or straight? At a glance, the key to telling might be in the way he walks.

A swing of the hips or a swaggered shoulder is enough for many casual observers to identify a man’s sexual orientation, according to a study published in the September issue of the Journal of Personality and Social Psychology.

Observers were only able to accurately guess the sexual orientation of men; with women, their guesses didn’t exceed chance. But what’s most interesting to researchers is understanding how that snap judgment can unleash a series of stereotypes — even from the most liberal-minded.

“This is important for the understanding of perception and feelings of assumptions and bias,” says lead author Kerri Johnson, an assistant professor of communications at the University of California, Los Angeles. “Once you know an individual’s sexual orientation, every else that happens is seen through that lens.”

Johnson and her colleagues attached motion sensors, like those used in the movie industry, to the hips and shoulders of eight volunteers – four men and four women, half of whom were gay. The motion sensors captured the only movements of the walkers, masking details such as clothing or hairstyles.

The researchers videotaped the volunteers walking on a treadmill at various speeds, and then played the video for 150 undergraduate observers, who were asked to determine the sexual orientation of each person.

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