Pupil dilation can reveal sexual desires and arousal, according to a new study.
Researchers at Cornell University found that pupil dilation is not only a way to understand what arouses someone, but can also be used to determine sexual orientation.
Live Science reported that the study is the first large-scale experiment that proves that dilation of the pupils is an accurate indicator of sexual desire.
To get their results, the study authors showed subjects short videos of men and women masturbating in neutral landscape scenes, reported the Daily Mail, while their pupils were watched by researchers.
Scientists found the volunteers' pupils dilated depending on what video they were turned on by with some obvious and some not so obvious results.
Interestingly, heterosexual females responded to both men and women masturbating, suggesting a fluidity in their sexual orientation.
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Bisexual men did not respond to either men or women oddly.
They also found that sexual orientation could easily be determined by watching the eyes.
"So if a man says he's straight, his eyes are dilating towards women," study author Ritch Savin-Williams, a developmental psychologist at Cornell University told Live Science.
"And the opposite with gay men, their eyes are dilating to men."
According to ABC News, the dilation occurs as the autonomic nervous sytem begins to speed up and as the heart rate and breathing begin to increase.
The study authors say that proving the connection between pupil dilation and sexual arousal will also be helpful for further studies, which may no longer need to use invasive techniques to study desire.
The study was published in the journal PLoS One.