A new study has found that the amount our pupils dilate can reveal our sexual orientation, and that women are just as likely to be aroused by images of women as they are of images of men.
Researchers at Cornell University in New York found people's pupils dilate most when looking at a sexual image they find attractive.
The researchers recruited 325 people to watch erotic videos and measured their pupil dilation using an infrared lens to reveal whether they were heterosexual, bisexual or homosexual.
They found heterosexual male participants had the strongest pupil dilation when they viewed sexual videos of women, but had little response to videos of men.
However, heterosexual women responded to videos featuring both men and women, which the authors say proves men and women have different types of sexuality.
Bisexual participants showed equal pupil dilations for videos of 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," study co-author Ritch Savin-Williams said in a media 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."
In the past, researchers have explored people's sexuality by asking them questions or measuring their genital arousal. These methods have obvious limitations and mean big sections of the world cannot be studied due to cultural or age restrictions.
Now the researchers hope this non-invasive pupil screening could open up sexuality studies to broader populations.
"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 lead author Gerulf Rieger.
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"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 of how sexuality is expressed across the planet."
The study was published in the journal PloS ONE.