While the eyes may be the window to the soul, pupil dilation can reveal a person's sexual orientation, finds a new Cornell study. Pupils were found to widen most when study participants watched erotic videos of people they found attractive, thereby revealing where they were on the sexual spectrum from heterosexual to homosexual.

Although there was a popular belief that sexual orientation can be revealed by pupil dilation to attractive people, there was no scientific evidence until now. For the first time, researchers at Cornell used a specialized infrared lens to measure pupillary changes to participants watching erotic videos.

The findings are published Aug. 3 in the journal PloS ONE.

Previous research explored these mechanisms either by simply asking people about their sexuality or by using such physiological measures as assessing their genital arousal. These methods, however, come with substantial problems.

"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," said Gerulf Rieger, lead author and Cornell postdoctoral associate, who conducted the study with Ritch C. Savin-Williams, professor of human development and director of the Sex and Gender Lab at Cornell.

"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," said Rieger. "This will give us a much better understanding how sexuality is expressed across the planet."

The new study adds considerably to the field of sexuality research than merely a novel measure, say the authors. As expected, heterosexual men showed strong pupillary responses to sexual videos of women, and little to men; heterosexual women, however, showed pupillary responses to both sexes. This result confirms previous research suggesting that women have a different type of sexuality than men.

Moreover, the new study feeds into a long-lasting debate on male bisexuality. Previous notions were that most bisexual men do not base their sexual identity on their physiological sexual arousal but on romantic and identity issues. Contrary to this claim, bisexual men in the new study showed substantial pupil dilations to sexual videos of both men and women.

"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," says Savin-Williams. "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 are confident that their new measure will aid in understanding these groups better and point to a range of sexualities that has been ignored in previous research.

The research was funded by the American Institutes of Bisexuality and U.S. Department of Agriculture. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript.