(CBS News) A person's pupils may reveal their sexual orientation, a new study finds.
Researchers at Cornell University used a specialized infrared lens to measure changes in pupil size while subjects watched erotic videos. The results revealed that for the most part, people's eye changes when seeing an image of a certain gender corresponded with their stated sexual orientation.
Previous studies that tried to determine a subject's sexuality measured genital arousal, which included invasive methods that they often found uncomfortable. Pupils have been shown to dilate - or widen - when people find something sexually attractive, according to the study authors.
"We wanted to find an alternative measure that would be an automatic indication of sexual orientation, but without being as invasive as previous measures. Pupillary responses are exactly that," Gerulf Rieger, lead author and research fellow at Cornell's college of human ecology, said in the press release. "With this new technology we are able to explore sexual orientation of people who would never participate in a study on genital arousal, such as people from traditional cultures. This will give us a much better understanding how sexuality is expressed across the planet."
The study was published online August 3 in PLoS One.
The scientists asked 325 subjects - 165 men and 160 women - who were between the ages of 20 to 35 and were open about their sexuality to sit and view photos and videos of men and women. Participants were first asked to rate their sexuality on a 7-point scale from "exclusively straight" to "bisexual" to "elusively gay/lesbian." In order to have a pupil size that served as a control for comparison, they showed the subjects pictures of the gender they were not attracted to as well as videos that should not illicit any sexual response, such as people talking about the weather. Responses were then compared with observed pupil dilation when seeing the images of a sexual nature.
The scientists found that heterosexual men's pupils dilated more when they saw women in sexual situations and not as much when they were shown men. The opposite effect was observed for homosexual men.
However, when it came to heterosexual women, they were more likely to have pupil reactions to sexual scenarios involving either gender. This doesn't mean however that most women are bisexual, Rieger pointed out to ABC News, noting that women are more likely to respond to anything sexual. He believes this is due to evolutionary reasons. In the past, many female primates were forced to engage in "brutal and painful" sex and the females adaptively disengaged their physical feelings from their emotional feelings to protect themselves.
"Their body is not connected to their mind, which is very different from guys," he added to ABC News.
The study also found measurable signs of pupils that suggested men too can be attracted to both genders. Men who identified themselves as bisexual had equal pupil response to sexual stimuli of men and women. Men who said they were "mostly straight" had more pupil dilation when viewing men than straight men, but much less dilation than men who identified themselves as bisexual or gay men.
"We can now finally argue that a flexible sexual desire is not simply restricted to women - some men have it, too, and it is reflected in their pupils," Dr. Ritch C. Savin-Williams, study co-author and professor in human development at Cornell., said in the press release. "In fact, not even a division into 'straight,' 'bi,' and 'gay' tells the full story. Men who identity as 'mostly straight' really exist both in their identity and their pupil response; they are more aroused to males than straight men, but much less so than both bisexual and gay men," Savin-Williams notes.
The researchers hope that their study will help better people's understanding of a broader range of sexualities.
Recent research also suggests that looking at a person's face may provide cues to their sexual orientation. The study, also published in PLoS One, showed that men and women could correctly judge a person's sexual orientation by looking at a picture of them.