Researchers from Cornell University used a specialised infrared lens to measure pupillary changes to participants watching erotic videos.
Pupils were highly telling: they widened most to videos of people who participants found attractive, thereby revealing where they were on the sexual spectrum from heterosexual to homosexual.
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.
"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," Gerulf Rieger, lead author said.
"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," Rieger said in a statement.
In the study, 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
