Pupil dilation has long held interest of many a psychologist. No one knows for sure what the changes in pupil size mean in different circumstances. It is certainly an involuntary result of our nervous system processing important information obtained from the various stimuli it comes in contact with. There are number of things that case pupils to dilate and it is impossible to figure out what it means each time. However, in context of this question, research has found links between liking something and dilation of pupils.
Pioneering study in this regard was done by a psychologist named Eckhard Hess at the University of Chicago who also coined the term pupillometrics in 1975 for the study of pupil size as an indicator of emotions. He was looking at photos of animals one night and his wife noticed that his pupils were expanded. She asked him if the light was not enough, and he said there was good enough light. This instance struck him as interesting enough to embark on research about what caused pupils to dilate.
Research shows that pupils dilate when we see or hear something in which we are interested. In order words, when something we see or hear interests us the pupils get bigger. In one study the subjects were given to listen to three excerpts from books. The first one was erotic, the second one involved mutilation and the third one was neutral. It was observed that the subjects’ pupils got bigger for all three passages but only in case of the first two – the erotic and the one involving mutilation – remained wide. It means that the pupils dilate when we hear or see something interesting but remain dilated only when it continues to be interesting.
The opposite is also true. Pupils are constricted when we see something which we find disgusting. This is also shown by a study in which the subjects were shown pictures of injured children. The results of the study were actually in line with what we learned from the above paragraph. In this study when the subjects were shown the pictures of injured children, their pupils got bigger at first (as their interest was piqued) but very soon they constricted because what they saw was disgusting. Unlike in the neutral case wherein the pupils become normal again due to the object not continuing to hold one’s interest, the opposite happens when the object holds negative interest.