A new study by Ritch Savin-Williams, a developmental psychologist at Cornell University, claims pupil dilation is an accurate indicator of sexual orientation.
The study found that when people watch porn featuring their gender preference, their pupils dilate in response to arousal.
In several previous studies, subjects were asked to look at pornographic material with instruments hooked up to their genitals to measure sexual arousal. However, Savin-Williams says some people can suppress genital arousal or just aren't keen to get turned on in a laboratory setting.
"Some people just don't want to be involved in research that involves their genitals," Savin-Williams told Live Science.
The connection between pupils and sex dates back centuries. In the 1600s, Italian women would administer eyedrops containing poisonous belladonna to dilate their pupils, a feature that was considered seductive at the time.
Read the full study on livescience.com.