The eyes are the window into the soul – or at least the mind, according to a new paper published in Perspectives on Psychological Science, a journal of the Association for Psychological Science. Measuring the diameter of the pupil, the part of the eye that changes size to let in more light, can show what a person is paying attention to. Pupillometry, as it’s called, has been used in social psychology, clinical psychology, humans, animals, children, infants – and it should be used even more, the authors say.

Pupil measurement might also be useful for studying babies. Tiny infants can’t tell you what they’re paying attention to. “Developmental psychologists have used all kinds of methods to get this information without using language,” Laeng says. Seeing what babies are interested in can give clues to what they’re able to recognize – different shapes or sounds, for example.  A researcher might show a child two images side by side and see which one they look at for longer. Measuring the size of a baby’s pupils could do the same without needing a comparison.