We first uncovered this surprising relationship while studying differences in the amount of mental effort people used to complete memory tasks. We used pupil dilations as an indicator of effort, a technique psychologist Daniel Kahneman popularized in the 1960s and 1970s. When we discovered a relationship between baseline pupil size and intelligence, we weren’t sure if it was real or what it meant. Intrigued, we conducted several large-scale studies in which we recruited more than 500 people aged 18 to 35 from the Atlanta community. We 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. We 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, we then calculated each participant’s average pupil size. To be clear, pupil size refers to the diameter of the black circular aperture in the center of the eye. It can range from around two to eight millimeters. The pupil is surrounded by the colorful area known as the iris, which is responsible for controlling the size of the pupil. Pupils constrict in response to bright light, among other things, so we kept the laboratory dim for all participants.