The Science of Why What do our pupils say about us?
PUPILS DILATE (EXPAND) OR CONTRACT as the light dims or brightens. But pupils also change size according to what the brain behind them is doing, whether that’s recalling memories, analyzing a problem or experiencing strong emotions. We may be unaware that our eyes are giving away so much while our brains are busy, but others who are aware can use that information to gauge their responses to us.



People have been deliberately sending messages with their eyes since at least the Renaissance. Back then, Italian women used eyedrops derived from the deadly nightshade plant—which they called belladonna, or “beautiful woman”—to dilate their pupils, believing that it made them more attractive. It wasn’t until hundreds of years later that anyone could figure out why dilated pupils would be so alluring. In the 1960s, a study showed that our pupils dilate when we’re looking at something interesting or attractive. So a Renaissance man gazing into the eyes of a woman who had just used belladonna eyedrops would see dilated pupils and unconsciously assume she was looking at something she found appealing: him!



Eckhard Hess of the University of Chicago was responsible for those 1960s experiments, which were among the first to examine pupil dynamics. In Hess’s studies, volunteers were shown images on a screen, and a camera photographed their pupils as they dilated or contracted in response to the changing pictures. The light levels were constant from one image to the next, ensuring that the changes in the volunteers’ pupils were a response to mental activity rather than to light.

Hess was able to confirm the intuition of those Renaissance women: he found that men judge a woman’s face to be more attractive when her pupils are dilated. Even when men were shown the same woman twice—the only difference being the diameter of her pupils—they preferred the image with the bigger pupils. Hess also confirmed that the phenomenon was more general than that. Pupils expanded when an individual saw anything interesting or attractive. But the same person’s pupils contracted when he or she saw something unpleasant.



Did You Know . . . Another study by Eckhard Hess concluded that women who are attracted to “bad boys” (yes, they had a definition for that) responded most positively to males with dilated pupils. And another experiment, this one in the Netherlands, showed that people were more likely to give money to a virtual partner—and thus more likely to trust him or her in general—if that person’s pupils were enlarged.

Our understanding of why pupils dilate has improved since Hess’s experiments. We now know that pupils dilate in response to a range of mental activities, from recalling memories to making decisions while shopping or playing rock-paper-scissors. And it’s not just our pupils that show our brains are at work. Blinking matters, too. Blinks signal the beginning of a mental process. After we blink, our pupils remain dilated as long as we’re working on the problem. When we’re finished, we blink again as our attention switches to something else and our pupils shrink.

The best data so far suggests that our pupils dilate the most when something is emotionally engaging. It doesn’t matter whether that emotion is bad or good, just so long as it grabs our attention. In one experiment, participants filled out a survey asking if they were impulsive shoppers. They then watched a scene of people shopping. The researchers found that the people who identified as impulsive shoppers had the greatest pupil dilation—just viewing the activity of shopping was so emotionally exciting and stimulating for their brains that their pupils expanded.

That tight-knit connection between brain and pupils also happens when thinking is taking center stage. For example, while you try to solve the latest sudoku, you’re constantly juggling numbers in your working memory. As your brain is managing those digits, your pupils dilate because of the mental effort. But if you were to stop concentrating and let your mind wander away from the puzzle, your pupils would return to normal.



We sometimes hear anecdotal reports of magicians being able tell what card you’ve picked out of a deck based on the size of your pupils, or clever shopkeepers knowing what you really want to buy by reading your pupils. But there’s little research to support those stories. One experiment that came close to proving these claims used the game rock-paper-scissors to see whether people could predict their opponent’s decisions by observing his or her pupils. Instead of playing the game person-to-person, the subjects watched video replays that showed a virtual opponent. Each subject was told that the video opponent’s pupils would change when he or she chose an option (rock, paper or scissors). Once the live players had been coached about what to look for, they beat the video players more than 60 percent of the time.

There was one problem with this experiment: because the subjects were watching a replay, they were seeing the video player’s pupils dilate after his or her decision had been made. In a live game, players would have to act before that happened, so trying to use an opponent’s eyes as a crystal ball wouldn’t help most people win an ordinary game of rock-paper-scissors.

Poker, though? That’s a different story.