Dilated pupils are considered more feminine.

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How Pupils Advertise Attraction

Italian women in the Middle Ages were a step ahead of science. Recognizing the beauty endowed by wide-open pupils, they would dilate their own eyes with belladonna [source: Gowin]. (Incidentally, the word "belladonna" means "beautiful woman" in Italian.) Unfortunately, the plant secreted not only the chemical atropine, which draws back the irises, but also a toxin that would deteriorate these women's eyesight and possibly poison them [source: Swaminathan].

Although belladonna wasn't an optimal dilating agent, the come-hither effect of large pupils was a scientifically valid assumption. In 1965, pupillometry pioneer and psychologist Eckhardt Hess asked men to compare the attractiveness of images of women with average-sized pupils to drawings in which the women's pupil sizes were enhanced. Consistently, men ranked the doe-eyed gazes as prettier, since the subtle ocular opening unconsciously signals sexual attraction on the woman's part [source: Tombs and Silverman]. In response to attraction, the brain secretes norepinephrine, which then flexes the eyeball's dilator muscles [source: Murphy]. Therefore, men may unwittingly read pupil dilation as an advertisement of interest.

Studying women's preferences for male pupil sizes wasn't so predictable, however. A 2002 study at York University identified medium-sized pupils as most appealing to many female participants [source: Tombs and Silverman]. Whereas men seek out the maximum sexual attraction, as hinted at by women's fully dilated pupils, the same isn't true of women seeking men, probably because unbridled sexual attraction in males has greater chance of fostering overly aggressive or violent behavior [source: Tombs and Silverman]. Medium pupils, on the other hand, indicate interest but not blinding lust.

But, as implied earlier, that evolutionary principle doesn't hold true for all women all the time. Those looking for short-term mating options who tend to date "bad boys" may be more drawn to fully expanded pupils, research has affirmed [source: Tombs and Silverman]. Monthly menstrual cycles also appear to hold sway over female pupil preferences. During the follicular phase, which culminates with ovulation, desire for doe-eyes peaks [source: Caryl et al].

In other words, the more physiologically primed for sex they are, the more males and females both innately keep an eye out for dilated pupils. Given that evolutionary phenomenon, the eyes are less the windows to one's soul than the windows to his or her bedroom.