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It is a common notion that a person's sexual orientation could be told by the pupil dilation at the sight of people one is attracted to, but there has been no scientific study to support the theory. 

Now, the researchers at Cornell University have made the first attempt ever to record the pupillary changes in people with the help of infrared lenses to determine their sexual orientation.

The participants were shown erotic videos of people, and it was found that the pupils revealed a lot. The pupils widened the most when a participant saw an attractive person in the video, thus revealing their sexual orientation.  

Earlier, a person's sexuality could be determined by either by asking the person or by using physiological measures such as assessing their genital arousal. These invasive methods, however, are not agreeable to many. 

"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 research fellow at Cornell in the news 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 new study also reveals much more than just the sexual orientation of people. The study results showed that as expected, heterosexual men showed strong pupillary responses to sexual videos of women, and little to men. However, it was found that heterosexual women displayed dilated pupils for men as well as women. This finding adds evidence to previous research that suggests women have a very different kind of sexuality than what men do. 

Also, another finding of the study reveals that unlike what was previously believed, most bisexual men do base their sexual identity on physiological sexual arousal, and not on romantic and identity issues.

Bisexual men during the study displayed dilated pupils for 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 Ritch C. Savin-Williams, co-author and professor in Human Development at Cornell. "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," Savin-Williams notes.

According to the researchers, this new method of detecting sexual orientation will be helpful in understanding these groups better.

The findings were published August 3 in the scientific journal PLoS ONE.