They say that the eyes are a window into the soul. And it’s not only romantics who agree, but also scientists. In fact, eyes may reveal the way you feel about someone before you even know.
Pupils respond to more than changes in light. They dilate and contract in response to various cognitive and emotional events. In one of the earlier studies, researchers looked at changes in pupil size as heterosexuals looked at half-naked pictures of the opposite sex and pictures of babies (Hess & Polt, 1960). They found that pupils dilated more when looking at semi-nudes of the opposite sex, offering evidence for the link between sexual interest and pupil size. And similar results were found when homosexual individuals were shown photos of semi-nudes of the same sex. As a matter of fact, such findings aided the Canadian government to identify gays and lesbians during the Cold War. They were deemed a risk for national security, thus the government aimed to remove them from the civil service to reduce susceptibility to Soviet blackmail.
On a more positive note, it is worth pointing out that pupil size not only indicates sexual arousal, but also serves to induce it. The interest for pupil size dates back to the 16th century, when Italian women used eye drops made out of toxic herb Belladona to keep their pupils large, which was the secret to a seductive look at the time. These efforts were not futile, as a more recent study revealed (Tombs & Silverman, 2002). Participants had to rate pictures of normatively attractive individuals of the preferred gender. Overall, they had a strong preference for those with dilated pupils.
However, things are quite messy. This research is not to show that pupil enlargement is a sole indicator of love, though it could be. Pupils can expand when you are being sexually aroused, when you are olving a difficult math problem, or when you’re scared. More generally, pupils enlarge when you’re being intellectually or emotionally stimulated. And they do so reliably enough to be regarded as a psychological method in itself. The measurement of pupil diameter (aka pupillometry) is now used for studying other psychological processes, including racial bias, moral judgment, and depression. Since pupils dilate in a host of other situations, they cannot be taken as reliable indicator of sexual interest
It seems that being attracted to someone could, in fact, make you more attractive to them. Next time you and your loved one exchange gazes, look straight into their pupils. If they are dilated, he or she may find you either attractive or mindfucking. If anything, you did not go unnoticed.