Usually, our pupils dilate when we see something that upsets or offends us, due to an adrenaline rush caused by the “fight, flight, or freeze” response. They also get bigger if we see something that excites us, which is why sophisticated card players often look at their opponents’ eyes to see if they have a good hand.
As psychopaths didn’t seem to have such a strong reaction to horrible imagery, it suggests they aren’t so susceptible to feeling threatened or scared.
“The pupil has long been known to be an indicator of a person’s arousal,” said Dan Burley from Cardiff University’s School of Psychology. “The pupil usually dilates when an image shocks or scares us. The fact that this normal physiological response to threat is reduced in psychopathic offenders provides us with an obvious physical marker for this condition.”