A new study has shown that enlarged pupils can determine a person's sexuality and can reveal whether the person is straight, gay, bisexual or somewhere in between.

According to the researchers, pupil dilation can be used along with self-reports and other physiological tests to assess the sexuality of a person. Researchers say this method could be used to determine sexuality without any invasive measurements.

Sexuality is generally determined with the help of self-reports or exams that involve measuring genital changes (circumference of penis in men and changes in the blood flow to vagina in women) in response to sexual arousal.

"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, research fellow at Cornell University and lead author of the study.

Because pupil dilation occurs in all humans, this method can be used to study sexuality in various cultures across the world.

"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," said Rieger.

For the study, 325 participants who were straight, gay or bisexual were recruited. All the participants were shown 30 second video clips of both sexes masturbating. Researchers tracked the gaze of the participants and used infrared lens to measure pupil dilation. The participants were also shown erotic video clips that had males and females, researchers then tracked where the participant was looking.

Women participants in the study had dilated pupils irrespective of whether they were watching a man or a woman in the video and irrespective of their self-reported sexuality. This is in line with other studies that show women are aroused by both the sexes.

According to experts, a heterosexual woman's brain is not tuned to be aroused only by men as is the case with heterosexual men.

The new study shows that this differentiation blurs in some men and they can be aroused by both sexes.

Researchers found that men who classified themselves as heterosexuals were more aroused (had dilated pupils) when they saw an erotic video of a female than a male.

Some men, who had self-reported to be bisexuals, had dilated pupils while seeing erotic videos of both sexes. The results show that these men were naturally inclined to be aroused by both men and women. Researchers, in the past have doubted the presence of ‘bisexualism’ in men and believe that bisexualism is the transition of men from thinking they are straight to accepting that they are gay.

"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 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," said Ritch C. Savin-Williams, professor in Human Development at Cornell and a co-author of the study.

Also, researchers say that testing arousal via measurements can be difficult because these arousals don't occur often during a lab-study. Eye-tracking and dilation on the other hand can be done faster and is more accurate.

Study not a threat

Dominic Davies from UK-based Pink Therapy said that it is common for people to have dilated pupils after seeing an interesting object and that this study is not alarming, GayStarNews reports.

"I find it relatively unremarkable and unsurprising. I think most people are kind of ‘so what’ about determining sexuality – especially now that it’s known to be a lot more fluid for a lot more people!," said Davies, GayStarNews reports.

The study was published in the journal PLoS ONE.