The difference between otherwise similar people with anti-social personality disorder (APD) and psychopaths is the way the latter’s brain reacts when confronted with human emotion, a new study has found.

Researchers at the University of Michigan, US, set out to uncover why both people with APD and psychopaths both act on impulse and display a disregard for others – but psychopaths also callously manipulate others.

Luke Hyde, an assistant psychology professor told Pacific Standard magazine that because many studies had been done on criminals with very high levels of psychopathy, the team wanted to better understand adults with lower levels of anti-social and psychopathic traits.

The participants in the study were tested using a technique that experts hoped would distinctly measure the difference between people with APD and psychopaths, while confirming how the two groups also overlap in many ways.

To make their findings, published earlier this year in the 'Journal of Abnormal Psychology', researchers recruited 103 adults from a database of psychologically profiled people managed by the University of Pittsburgh’s Adult Health and Behaviour project.

First, the subjects were asked about their own personalities. Each participant was then connected to a brain scanner and shown a series of angry, distressed, and scared human faces.

What separated the psychopaths from those with APD was the way their amygdala reacted. This part of the brain detects threats and controls fear responses, and is also linked to emotions including arousal and aggression.

When psychopaths were shown a worried face, the amygdala did not respond as strongly as the average person’s. In addition, when psychopaths were questioned, they reported that they did not feel as many negative emotions.

“I was surprised to see that different measures of psychopathy and antisocial behaviour had different prediction of brain reactivity,” Hyde told the magazine.