His sexual orientation can be seen from his eyes

A new study finds that pupil dilation is an accurate indicator of sexual orientation. When people look at pornographic images and become aroused, their pupils dilate unconsciously, making it possible to determine sexual orientation without requiring a genital diagnosis.

Ritch Savin-Williams, a developmental psychologist at Cornell University and the lead researcher on the study, said this is the first large-scale experiment to show that pupil dilation coincides with sexual arousal in people. "So if a man says he's straight, then his pupils dilate when he sees a beautiful woman," Savin-Williams told LiveScience. "In contrast, a homosexual will have his pupils dilate when he sees a handsome man."

Eyes know the truth

The link between pupil size and sexual arousal is long-standing. In 16th century Italy, women used eye drops made from the poisonous herb hyssop, which dilated their pupils and made them look sexy.

In fact, many exciting or interesting things, such as a loved one's face or a beautiful work of art, can cause the pupils to dilate slightly, Savin-Williams says. This expansion is a sign of excitement in the autonomic nervous system, which controls involuntary actions like pulse and breathing.

Traditionally, when researchers examine sexual arousal and orientation, volunteers are hooked up to devices that measure genital blood flow while they watch pornographic films or images. In men, the circumference of the penis is measured, while in women, a probe measures changes in pressure in the blood vessels of the vaginal wall. This measurement is flawed, Savin-Williams said. Some people can suppress their sexual arousal or even have no arousal response at all in a laboratory setting, so genital invasive diagnosis is necessary to find out the true situation. "Some people just don't want to be involved in research involving their genitals," Savin-Williams said.

Savin-Williams said that simply asking people whether they are aroused by a particular stimulus is not objective, and some people may be ashamed to admit their orientation or even deny it. And asking direct questions about sexual orientation is difficult in many cultures.

Detecting sexual arousal

To address these questions, Savin-Williams and his Cornell University colleague Gerulf Rieger turned their attention to the pupil. They recruited 165 gay, straight, and bisexual men and 160 bisexual women. The volunteers watched a one-minute video of a man masturbating, a woman masturbating, and a landscape video. This video has been brightness matched, and different lighting will not affect the results.

A pupil-tracking camera measures tiny changes in the pupil during these videos. The volunteers also reported how they felt after watching each video. The results showed that pupil dilation matched the characteristics of sexual arousal. In men, this trait is usually straightforward: heterosexuals respond to female images, homosexuals respond to male images, and bisexuals respond to both men and women.

For women, it's not so simple, Savin-Williams said. Homosexual women, similar to heterosexual men, react to images of other women with pupil dilation. Despite reporting feelings for men, heterosexual women responded to erotic images of both men and women.

This doesn't mean all straight women are secretly bisexual, says Savin-Williams

That being said, their subjective sexual excitement does not necessarily correspond to their bodies. Sex researchers aren't sure why. One theory is that because women have historically been at risk of rape, they have evolved to respond to any sexual stimulation, no matter how boring. This, the thinking goes, will allow women to be less vulnerable to sexual assault and make their genes more likely to be passed on.

The researchers' detailed findings were published today (August 3) in the journal PLoS ONE. Savin-Williams said the next step will be to do both pupillometry and genitaliametry to see how consistent their results are.

Finally, he said, the technique could be applied to sexuality research in a variety of cultures because pupil dilation is independent of sexual orientation and is universal across populations of different languages. Savin-Williams also said the method could even be used to help people who are unsure of their sexual orientation.