In a classic study by Eckhard Hess in 1965, it was shown that pupil size was related to attractiveness in females. The men in the study were shown drawings of women’s faces, ones with normal sized pupils and ones with larger sized pupils. The men found that the larger pupil size was more attractive. This finding lead researchers to coin the term “pupillometry” referring to the measurement of the size of the pupil and its effect on others. Pupillometrics, on the other hand, refers to the evaluation of the pupil’s size in relation to interest and emotion.

The pupil is measured with the help of infrared cameras or sensors since visible light would throw off readings. The pupils are affected by light and open and close to allow more or less light in which assists in proper vision. In low light conditions the pupils will dilate or open to allow more light in and in bright conditions, the pupils will constrict to restrict the amount of light let it. Cameras are equipped with apertures which serve the same function. The bigger the hole, the more light comes through and the eye needs just the right amount of light to see properly.

Various studies show that our pupils also respond to positive stimuli by dilating or constricting when a person sees unpleasant or uninteresting stimuli. For example, pupils dilate more when heterosexual viewers see nude images of the opposite sex and constrict when viewing same sex images. Viewing unpleasant images such as crippled children, war scenes, or torture, leads to the constriction of the pupils. Hess and his colleagues found that an increase in pupil size was positively correlated with mental activity and problem solving and that people reached maximum dilation as they neared a solution. Further to this, images that were modified to contain female models with larger pupils tended to be rated more attractive and friendlier then images where the pupils were modified to be smaller or unmodified.

Pupil sizes can therefore be used to read people because it gives us a reliable tool for measuring interest and arousal even if in a subtle way. Subconsciously we are all aware of other’s pupil sizes as indicated by the study. Men were not able to consciously describe why they felt certain images were more attractive than others, but they did so anyway. Increased pupil size can be an indication of any positive stimuli such as food when hungry, or when seeking companionship, other potential suitors in a room. Paying at least some conscious attention to the pupil sizes of others can give us information about the overall state the people around us are in. One of the reasons pupil size is so powerful is because we are not able to consciously control the size of our pupils which means that pupils will always naturally react to stimuli we find attractive. Wearing sunglasses by professional poker players is partially explained with these findings since dilated pupils might provide a ‘tell’ to their opponents.

If you think pupils aren’t hardwired, then take the pupil test. Have someone look at the drawing in “image B” while covering “image A” then after some time have them look at “image A” while covering “image B”. You will notice that the eyes dilate in response as the brain naturally thinks it is looking at a set of eyes. Italian women in the 18th century would place eye drops called belladonna containing atropine to chemically induce their eyes to dilate in order to elicit attention from men. Today, marketing does the work for us, as images are commonly doctored by image modifying software to appeal to our innate biology. Turning the lights down or a candle lit dinner can have the same effect. Even artificially modifying pupil size by dimming the lights subconsciously produces arousal. Through their eyes they tell us that they are interested in us and so we become interested in them!