Using a specialized infrared lens to measure pupillary changes to participants watching erotic videos, they noticed that pupils widened most to videos of people who participants found attractive, thereby revealing where they were on the sexual spectrum from heterosexual to homosexual.

The findings were published August 3 in the scientific journal PLoS ONE.

"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, the study's lead author and a research fellow at Cornell, said in a statement. "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."

The paper says that heterosexual men showed strong pupillary responses to sexual videos of women and little to men "but 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."

The researchers say the 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, the team says.

"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," says Ritch C. Savin-Williams, the study's co-author. "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."