Muscles in the colored part of your eye, called the iris, control your pupil size. Your pupils get bigger or smaller, depending on the amount of light around you. In low light, your pupils open up, or dilate, to let in more light. When it's bright, they get smaller, or constrict, to let in less light.

What part of the eye makes your pupils dilate?

Pupil dilation occurs when the opening in the center of your iris grows bigger to let in more light. Under normal circumstances, pupils can dilate to let in more light or in response to a variety of stimuli.

When lies do pupils dilate?

In line with this assumption, previous studies have shown that lying is associated with greater pupil dilation, a behavioral cue that typically manifests itself under conditions of stress or cognitive effort. In accordance with these results, we predicted greater pupil dilation when lying than when telling the truth.