Pupils: Windows to the Soul

People often call eyes the windows to the soul. But what exactly do we see when we gaze into the eyes of another person? In fact, the eyes do provide lots of information about another person’s emotional state.

We can tell a true (or Duchenne) smile from a fake by looking at a person's eyes. The mouth shape of a smile is easy to fake—we do it all the time out of politeness. But the eyes are the giveaway: When we’re truly happy, we not only smile but also crinkle the corners of our eyes in a “crow’s feet” pattern. But when people fake a smile, they usually forget about their eyes.

If the eye is the window, the pupil is the opening to the eye. Psychologists Sebastiaan Mathôt and Stefan Van der Stigchel argue that there’s a lot more to the pupillary light response than, well, meets the eye. They claim that the size of the pupils tells us a lot about the emotions and intentions of their owners. According to the researchers, the pupillary light response isn’t just a mechanical reaction to ambient light. Rather, as we shift our gaze from one spot to another, our pupils adjust their size in advance to the amount of light we expect to encounter at the new location. Because the pupillary light response is relatively slow—about a quarter of a second—anticipating the amount of light at the new location improves vision once our gaze gets there.

The pupillary light response is only one reason why the pupils change size. They also dilate when we’re aroused. This occurs whenever we detect a threat or an opportunity in our environment. Meet an attractive person at a party, and what happens to your body? Your heart and breathing rates increase, you begin to sweat—and your pupils dilate. Psychologists consider pupil dilation to be an honest cue to sexual or social interest. That’s because pupil size isn’t under your voluntary control. Let’s say you’re trying to fake interest as your coworker recounts every play in his weekend golf game. You can force a smile. You might even remember to crinkle the corners of your eyes, to make that smile look real. But your tiny pupils will reveal your lack of interest. Thus, the eyes may be the windows to the soul, but the pupils tell a lot about what’s going on in the mind of another person.