Everyone knows that your pupils will change size according to the amount of light you are experiencing. With more light, the pupil constricts and becomes smaller. Less light and your pupil dilates, letting more light into the back of the eye. It is the muscles of the iris working with your autonomic nervous system (ANS) to adjust the iris so the right amount of light enters the eye – like the aperture of a camera.

The iris is made up of two types of muscle:

  • Sphincter muscles that are like concentric rings that constrict the pupil to as small as two millimeters across
  • Dilator muscles that are laid out like the spokes of a bicycle wheel and can expand the pupil up to eight millimeters across

dilated pupils respond
But the ANS is not only concerned with light reflex, it also reveals emotional and mental responses. The sympathetic branch of the ANS responds to a person being under stress, triggering the “fight or flight” response, which will cause the pupil to dilate. On the other hand, the parasympathetic branch known for “rest and digest” will cause pupil constriction. At any given time, your pupil is balancing between both the light and emotional reactions.

Here are some of examples of mental responses:

Princeton University psychologist Daniel Kahneman demonstrated that pupil size increases in proportion to the difficulty of the task being performed. Calculating 8 x 21 will cause your pupil to dilate slightly, however calculating 8 x 47 will cause them to dilate even more. Whatever the problem, they will remain dilated until you come up with the answer or give up.

Even memory recall creates a pupil response. When subjects were instructed to remember and recite a series of seven digits, their pupils would grow steadily as they learned each number, but reduce as steadily when they recited back each of the numbers.

Wolfgang Einhauser-Treyer, a neurophysicist at Philipps University Marburg in Germany, found that “pupil dilation can betray an individual’s decision before it is openly revealed.” He asked people to push a button at any point during a span of 10 seconds. Dilation began about one second before they pressed the button and continued to peak one to two seconds after the push.

This study of pupil size is known as pupillometry and is used to investigate a wide range of psychological phenomena including sleepiness, introversion, sexual interest, racial bias, schizophrenia, moral judgment, autism and depression. Kahneman said he has “never done any work in which the measurement is so precise.” And while “nobody really knows for sure what these changes do,” according Stuart Steinhauer, director of Biometric Research Lab at the University of Pittsburgh, pupillometry is a valuable tool for psychological research.

So the next time you look into someone’s eyes, know that you have the potential to see more than just their eye color. You might have a clue as to what is going on in their mind.


Susan DeRemerSusan DeRemer, CFRE
Vice President of Development
Discovery Eye Foundation

Pupils Respond To More Than Light
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