Why would someone's pupils not dilate?

Calculate nine times 13 and your pupils will dilate slightly. Try 29 times 13 and they will widen further and remain dilated until you reach the answer or stop trying. Kahneman says in his book, Thinking Fast and Slow, that he could divine when someone gave up on a multiplication problem simply by watching for pupil contraction during the experiment. read more

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. Sometimes your pupils can dilate without any change in the light. The medical term for it is mydriasis. read more