Pupils never lie.

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. When people are sad or worried, they furrow their brow, which makes the eyes look smaller. Yet when people are cheerful, we correctly call them “bright eyed.” That’s because people raise their eyebrows when they’re happy, making the eyes look bigger and brighter. 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.

     David Ludden Ph.D.    David Ludden Ph.D. is a professor of psychology at Georgia Gwinnett College. He received his Ph.D. in cognitive psychology from the University of Iowa, and is the author of The Psychology of Language: An Integrated Approach. He will be focusing on the role that language plays in human psychology–from perception to persuasion, from attention to attitudes, from motor skills to mental states.

Editor:  Saad Shaheed

If the eye is the window into the soul, the pupil is—quite literally—an opening into the eye. The pupil acts like the aperture on a camera, dilating or contracting to regulate the amount of light coming into the eye. We all know that our pupils get smaller in the light and bigger in the dark. This is the pupillary light response.

In an article published in a recent issue of Current Directions in Psychological Science, 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.

Consider working at a computer: Most of the time, our gaze is fixed on the bright screen, so our pupils are contracted. But every now and then, we glance down at the keyboard, as when we need to reposition our fingers. The authors of the article claim that the pupils begin to dilate even before the downward eye movement begins. 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. (All of this, of course, operates below the level of consciousness.)

The pupillary light response is only one reason why the pupils change size. They also dilate when we’re aroused. The body has an alarm network called the autonomic nervous system that prepares us to take action whenever we detect a threat—or an opportunity—in our environment.

Encounter a bear while walking through the woods, and your autonomic nervous system goes on alert. Your heart and breath rates increase, you begin to sweat as your muscles tense up, and, among other bodily reactions, your pupils dilate. The autonomic nervous system prepares your body to take action against the threat—perhaps scampering up the nearest tree.

We also need to take action when we encounter opportunities. 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.

Arousal increases pupil size irrespective of the amount of ambient light present because optimal pupil size involves a trade-off between two factors. The first is visual acuity, or how well you can see the details of whatever you’re looking at. In this case, you need “Goldilocks” pupils—not too bright and not too dark, with just the right amount of light coming in. Thus, the pupillary light response is important for visual acuity.

The second factor is visual sensitivity, or how well you can detect something that’s in the environment. If you want to really see what’s out there, you need to have your eyes—especially your pupils—wide open. This is where the connection between arousal and pupil dilation comes in.

Psychologists consider pupil size in terms of the two functions of vision—exploration and exploitation. When we’re exploring our environments, we’re on the lookout for threats and opportunities, so we’re in a heightened state of arousal. Visual sensitivity is most important in exploration, so our pupils are wide open, taking in as much visual information as possible.

Once we’ve identified an object of interest and have it under our control, we shift to exploitation mode: We need to examine the item carefully to find all the ways we can use it, to understand it as fully as possible. Now, visual acuity is most important, and our pupils dilate or contract so that just the right amount of light comes in.

The so-called pupillary light response isn’t just a mechanical reaction to the amount of ambient light, as is the aperture on a camera. Instead, the pupils also adjust according to our emotions and expectations. 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.

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