Psychopaths lack basic hardwiring in the brain that enables most people to be compassionate and caring, new research has shown.

American scientists studied 80 male prisoners aged 18 to 50 who were assessed for psychopathic traits.

Around 20% to 30% of the US prison population are believed to be affected by psychopathy compared with 1% of the general population

Participants underwent brain scans while being shown videos of people being intentionally hurt and others of faces reacting to pain.

The results of the functional magnetic resonance imaging scans showed distinct differences in the brain responses of highly psychopathic and non-psychopathic individuals.

Psychopaths displayed significantly less activity in the ventromedial prefrontal cortex, lateral orbitofrontal cortex, amygdala and periaqueductal grey parts of the brain. Conversely, more activity was seen in the striatum and insula regions.

"A marked lack of empathy is a hallmark characteristic of individuals with psychopathy," said lead researcher Professor Jean Decety, from the University of Chicago. "This is the first time that neural processes associated with empathic processing have been directly examined in individuals with psychopathy, especially in response to the perception of other people in pain or distress."

The findings are published online in the journal JAMA Psychiatry. They may help explain why criminal psychopaths appear so lacking in remorse or compassion.