Your pupils tend to dilate, or widen, when eyeing somebody that you find particularly attractive. Now, newly published research suggests that measuring pupil responses may be an effective way of assessing a person's sexual orientation.
Cornell Researchers Gerulf Fieger and Ritch C. Savin Williams summarize their findings in the latest issue of PLoS ONE:
In general, self-reported sexual orientation corresponded with pupil dilation to men and women. Among men, substantial dilation to both sexes was most common in bisexual-identified men. In contrast, among women, substantial dilation to both sexes was most common in heterosexual-identified women.
"The pupil reacts very quickly, and it is unconscious," explains Rieger, "so it's a method that gives us a subconscious indicator of sexuality."
As L.A. Times' Thomas H. Maugh points out, the findings aren't exactly revelatory, but they do confirm a belief among sexual researchers "that has apparently not been studied in any depth before." But what is perhaps most interesting about the study is the insight it provides into the evolutionary development of sexual responses, and the evolutionary explanation for the differences in arousal that we see between genders. Maugh explains:
The findings support the idea that sexual response has different biological functions in men and women. For men, an important function is to facilitate erection and penetration. For women, the function is to stimulate lubrication and prevent genital injury in case of penetration — a response that may have developed early in evolutionary history in response to episodes of rape by males.