MONDAY, Aug. 6, 2012 (HealthDay News) -- Clues to a person's sexual orientation may lie in their eyes, a new study shows.
Researchers from Cornell University note that when people look at someone they find attractive, their pupils dilate, or widen. In their experiments, the team measured the size of people's pupils with a specialized infrared lens as they watched erotic videos, and these measurements predicted whether the person was homosexual, heterosexual or somewhere in between.
Measuring pupil dilation, "will give us a much better understanding how sexuality is expressed across the planet," study lead author Gerulf Rieger, a research fellow at Cornell, said in a university news release.
He explained that researchers looking into human sexual responses and orientation have long sought a less invasive tool than the genital-arousal measurements typically used in this type of research. Rieger reasoned that pupil dilation might be such a tool.
In the new study, Rieger's team found that the pupils of heterosexual men reacted strongly to sexual videos of women but did not have this response to videos depicting men. Conversely, the pupils of heterosexual women dilated while they watched videos of both men and women. That supports previous research suggesting that women's sexuality is somewhat different than that of men.
The study also revealed the pupils of bisexual men responded to sexual videos of both men and women. According to Rieger and his colleagues, this contradicts the notion that bisexual men base their sexual identity on things other than sexual arousal.
"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," the study's co-author, Ritch Savin-Williams, a professor in human development, said in the news release. "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."
The study's authors believe the findings will boost understanding of different sexual groups and better identify a broader range of sexualities.
The study was published Aug. 3 in the journal PLoS ONE.
The American Psychological Association provides more information on sexual orientation.