The way people's pupils react when they see other people is an effective way to assess sexual orientation, according to a new study.

The reactions of study participants' pupils revealed that heterosexual men responded most to images of women and homosexual men responded most to images of men.

Additionally, researchers found that homosexual women responded most to images of women, and heterosexual women expressed arousal in response to both men and women, though they were more likely to choose to watch men.

Previous studies have shown that people's pupils widen in response to seeing others who they find attractive; the new study showed that, indeed, a person's sexuality is evident in their pupils' responses.

Results also revealed that bisexual men were attracted to both men and women, an idea that has been disputed, and that heterosexual women may be aroused by both genders, despite being straight.

“The pupil reacts very quickly, and it is unconscious, so it's a method that gives us a subconscious indicator of sexuality,” said lead study author Gerulf Rieger, a researcher at Cornell University.Sex researchers don't always want to rely on people's own reports about who they are sexually attracted to, because cultural and societal pressures can influence what people say, he explained.

The findings are detailed today (Aug. 3) in the journal PLoS ONE.

Reasons for women's arousal

Researchers asked about 300 study participants to watch 30-second videos of people of both sexes masturbating, and tracked the dilation of participants' pupils in response. The participants also watched simultaneous videos of males and females, and the researchers tracked where they spent more time looking.

The finding that heterosexual women are aroused by both genders is in line with other studies.

“The female brain is not as differentiated,” said Sandra Witelson, a professor of psychiatry at the Michael G. DeGroote School of Medicine of McMaster University. “They don’t have as strong a response for only men, as heterosexual men have for only women.”

The female tendency to be aroused by both sexes may be because of female brain composition, Witelson said.

Study researcher Ritch Savin-Williams, a psychology professor at Cornell, said women's less-distinct preference for men may be the result of a defense mechanism that evolved to protect women from forced sexual intercourse. If a woman can become aroused with any sort of sexual simulation, the lubrication that results can protect her from injuries.

Bisexual men

The pupils of men in the study who identified themselves as bisexual responded similarly to videos of males and females, confirming that bisexuality truly exists in nature, Savin-Williams said.

This fact has been disputed because of past research suggesting that men who say they are bisexual actually respond only to men, in measurements of genital responses, he said. Some have suggested that being bisexual is not a true state of sexuality, and is instead a sign of someone transitioning to accept himself as being gay.

“I was surprised that the pupil tells us something more in line with what the people tell us, which is not what the penises tell us,” Rieger said. In general, studies measuring genital responses are trickier, because such responses vary greatly between people, and are difficult to prompt naturally in a lab setting.

Studies on genital responses from Northwestern University researchers have also confirmed the possibility of male bisexuality, Rieger said.

Overall, the study results show that pupil dilation tests can be used to assess overarching trends in sexuality in a large population, not necessarily the sexual orientation of individuals, Savin-Williams said.

However, the researchers said pupil responses could possibly be used in the future for more specific measurements, like understanding the sexuality of someone on trial for a sex crime, Savin-Williams said.

Pass it on: Pupils dilate to indicate sexual interest.

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