There are biological brain differences that mark out psychopaths from other people, according to scientists.
The scans showed brain activity
Psychopaths showed less activity in brain areas involved in assessing the emotion of facial expressions, the British Journal of Psychiatry reports.
In particular, they were far less responsive to fearful faces than healthy volunteers.
The Institute of Psychiatry, Kings College London team say this might partly explain psychopathic behaviour.
Criminal psychopaths are people with aggressive and anti-social personalities who lack emotional empathy.
They can commit hideous crimes, such as rape or murder, yet show no signs of remorse or guilt.
It has been suggested that people with psychopathic disorders lack empathy because they have defects in processing facial and vocal expressions of distress, such as fear and sadness, in others.
Professor Declan Murphy and colleagues set out to test this using a scan that shows up brain activity.
They showed six psychopaths and nine healthy volunteers pictures of faces showing different emotions.
Both groups had increased activity in brain areas involved in processing facial expressions in response to happy faces compared with neutral faces, but this increase was smaller among the psychopaths.
By contrast, when processing fearful faces compared with neutral faces, the healthy volunteers showed increased activation and the psychopaths decreased activation in these brain regions.
The researchers said: "These results suggest that the neural pathways for processing facial expressions of happiness are functionally intact in people with psychopathic disorder, although less responsive.
"In contrast, fear is processed in a very different way."
This failure to recognise and emotionally respond to facial and other signals of distress may underlie psychopaths' failure to block behaviour that causes distress in others and their lack of emotional empathy, the scientists suggest.
Dr Nicola Gray, from Cardiff University's School of Psychology, has also been studying what underpins psychopathy.
"What we are trying to understand are the cognitive deficits underpinning the behaviour of psychopaths.
"If people with psychopathy can't process the emotion of fear and that is mirrored in terms of their brain activity, as this study suggests, that will help us understand the cognitive deficits.
"But it is still a long way to finding out what to do about that. We are a long way from knowing how to treat psychopathy."