This. Pupil size correlates with stimulation level, which in turn correlates with attention and performance. And it's not a simple one-way street - drugs of co

This. Pupil size correlates with stimulation level, which in turn correlates with attention and performance. And it's not a simple one-way street - drugs of course have an effect (caffeine, for a start) - but psychological factors also come into play - someone who is more able to activate their attention in response to a task will likely show a physiological response. Your pupils will "pop" in response to both pleasant (sexually attractive person) and unpleasant (unexpected large tax bill) tasks or stimuli, but the amount by which they do so is going to vary from one pair of (background conditions, situation) to the next.