The pupil is a hole located in the center of the iris that allows light to strike the retina. It appears black because light rays entering the pupil are absorbed. (The iris is the colored part.) You know that its size varies due to the amount of light, your emotional state, cognitive effort, drugs and, well, death.
A dark room means it dilates to allow more light. The same is true for an orgasm, a difficult test question and when you're lying. "Lying Eyes" isn't just an Eagles song, because pupil dilation is considered a reliable indicator of lying. Enlarged pupils are a sign that your brain is working hard, and that happens when you lie.
The study of all this, pupillometry, is often used in psychological research.
So, saying someone has big pupils probably isn't a compliment. Or is it? Scientists have known for more than 50 years that the size of the pupil is related to more than just the amount of light entering the eyes.
A standard experiment in memory shows that the pupil dilates each time a new item is held in memory (more difficult), and constricts as each item is subsequently recalled (easier).
A recent study found that:
Pupil dilations of the eye are known to correspond to central cognitive processes. However, the relationship between pupil size and individual differences in cognitive ability is not as well studied. A peculiar finding that has cropped up in this research is that those high on cognitive ability have a larger pupil size, even during a passive baseline condition.
It seems that there is relationship between pupil size and cognitive ability across individuals. The researchers looked at the correlation between baseline (resting) pupil diameter and fluid intelligence. Fluid intelligence is thought to be a major component of IQ.
Yes, the paper also admits that "these findings were incidental and lacked a clear explanation" but maybe this means that saying you have big pupils is really a compliment.
I have also read that pupil size diminishes with age. What's with that?