According to a new study, the idea that your pupils dilate when aroused is no popular myth.
Researchers from Cornell University used a specialized infrared lens to measure the changes to individuals' eyes while they watched erotic videos.
The study published in the scientific journal PLoS ONE, noted that a person's pupils widened most to videos of people who participants found attractive.
Pupil dilation could thereby reveal where participants were on the sexual spectrum from heterosexual to homosexual.
In a statement, researchers said that until now there was no scientific evidence that sexual orientation could be revealed by pupil dilation to attractive people.
Previous research explored these mechanisms either by simply asking people about their sexuality, or by using physiological measures such as assessing their genital arousal. These methods, however, come with substantial problems.
Find out what else your eyes reveal about you... BELOW
"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," says Gerulf Rieger, lead author and research fellow at Cornell, in a statement.
"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 Cornell study adds considerably more to the field of sexuality research than merely a novel measure.
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 very 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 co-author Ritch 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.
Here are more tips on how to find out if someone's straight or gay...