They say the eyes are window to the soul, and now scientists say the pupils can also reveal a person's sexual orientation.
For the first time, researchers at Cornell University have used a specialized infrared lens to measure pupillary changes while subjects watched erotic videos to determine which gender they found attractive.
The results of the study were published Aug. 3 in the scientific journal PLoS One.
In most cases, a person's stated sexual orientation matched the dilation of their pupils, which show signs of arousal. Previously, scientists used instruments to measure genital arousal, methods that were "too invasive."
Arousal or "interest" is associated with pupil dilation.
"The idea was to find an unconscious measure," said lead researcher and research fellow Gerulf Rieger. "We tried to find measures that were not so invasive, but reliable. The eye tracker infrared camera focuses on the eye while the person watches videos or pictures and measures the changes in pupil size."
The Cornell scientists had 325 subjects -- 165 men and 160 women, aged 20 to 35, all of whom were open about their sexual orientation.
As a control, subjects were shown naked photos and videos of the gender they were not attracted to. Subjects were also shown "boring" videos of people talking about the weather.
Predictably, heterosexual men showed more pupil dilation after being shown sexual videos of women, and little to men.
Heterosexual women, however, responded to both sexes, confirming previous research that women have more fluidity in their sexuality.
"They respond to anything that is in some way sexual," said Rieger. "That doesn't mean they are bisexual. Their body is not connected to their mind, which is very different from guys."
The most plausible explanation for the female response is evolutionary. Primates show a lot of forced copulation that can be "brutal and painful" to females. The physical response seems to be "protective," according to Rieger.
The study did have some surprising results on men who said they were bisexual. The majority of men who identified with that orientation did respond to the 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," said co-researcher Ritch C. Savin-Williams, who is a professor of human development at Cornell.
"In fact, not even a division into 'straight,' 'bi,' and 'gay' tells the full story," he said. "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."
Measuring sexual arousal has always been a challenge for researchers.
"What if someone comes in and says one thing and responds a different way?" said Rieger. "We wanted to really find a measure of sexuality that goes beyond what people tell us."
The test might have some practical applications, when arousal and stated sexual orientation don't match. Therapists or doctors could guide a person through their own results.
In the past, genital arousal was measured differently for men and women. Men had a rubber band placed around their penis to measure expansion; women inserted a photo receptor instrument into the vagina that measured changes in blood flow.
As experts in human development, the Cornell team was trying to understand sexuality from the beginning. Because they now have a tool that is not so invasive, they can recruit younger subjects and learn more about human sexuality.
"We would not expose children to pornography, but there are different studies that we can now do that are not ethically problematic," said Rieger.
"Even if you say it's an unconscious response, doesn't mean it's all biological," he said. "In an ideal research world, you would study a child and see how early it starts and what it says about social influences."