How Pupil Size Correlates to Intelligence

It has been said that the eyes are the window to the soul, but new research suggests that they may be a window to the brain as well. The larger the pupils, the higher the intelligence, as measured by tests of reasoning, attention and memory. In fact, across three studies, scientists found that the difference in baseline pupil size between people who scored the highest on the cognitive tests and those who scored the lowest was large enough to be detected by the unaided eye.

Scientists first uncovered this surprising relationship while studying differences in the amount of mental effort people used to complete memory tasks. Scientists used pupil dilations as an indicator of effort in the 1960s and 1970s. When scientists discovered a relationship between baseline pupil size and intelligence, they weren’t sure if it was real or what it meant. Intrigued, scientists conducted several large-scale studies in which they recruited more than 500 people aged 18 to 35 from the Atlanta community. Scientists measured participants’ pupil size using an eye tracker, a device that captures the reflection of light off the pupil and cornea using a high-powered camera and computer. They measured participants’ pupils at rest while they stared at a blank computer screen for up to four minutes. All the while, the eye tracker was recording. Using the tracker, scientists then calculated each participant’s average pupil size.

In the next part of the experiment, participants completed a series of cognitive tests designed to measure fluid intelligence, the capacity to reason through new problems, working memory capacity, the ability to remember information over a period of time, and “attention control,” the ability to focus attention amid distractions and interference. Scientists found that a larger baseline pupil size was correlated with greater fluid intelligence, attention control and, to a lesser degree, working memory capacity—indicating a fascinating relationship between the brain and eye. Interestingly, pupil size was negatively correlated with age: older participants tended to have smaller, more constricted, pupils. Once standardized for age, however, the relationship between pupil size and cognitive ability remained.