Many experts believe that pupil dilation can reveal sexual orientation when a person looks at attractive people. Yet until now there has been no research supporting that theory.

A new study by researchers at Cornell University, published in the journal PLoS ONE, measured pupillary changes of participants watching erotic videos by using a specialized infrared lens. Pupils widened most when a person was watching a video featuring people they found attractive, which then revealed where the volunteers fell on the sexual spectrum from heterosexual to homosexual.

Researchers in the past have explored this theory by simply asking participants questions regarding their sexuality, or by physiological measures such as assessing their genital arousal. These measures were not sufficient, however, and they came with substantial problems.

Gerulf Rieger, lead author and research fellow at Cornell, explained:

“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. 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.”

This study adds a considerable amount of insight to the field of sexuality than just a novel measure. As the researchers expected, heterosexual men had strong pupillary responses to erotic videos of women, and little to men. Heterosexual women, on the other hand, showed pupillary responses to both sexes in the videos. This result confirms other research that has suggested that women and men have very different types of sexuality.

This current study also adds knowledge into the long-lasting debate on male bisexuality. Previous beliefs were that men do not identify themselves as bisexual based on their physiological sexual arousal, but on romantic and identity issues. This research showed bisexual men had substantial pupil dilations to sexual videos of both sexes, contrary to previous belief.

Ritch C. Savin-Williams, co-author and professor in Human Development at Cornell, concluded:

“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. In fact, not even a division into ‘straight,’ ‘bi,’ and ‘gay’ tells the full story. Men who identify 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.”

The researchers believe that their new measure will considerably help understand these groups better and also help recognize a wide range of sexualities that has been previously ignored.

Written by Sarah Glynn