Don't blame Hannibal Lecter, he cannot help being a callous murdering monster.
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 (fMRI) 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 today in the journal JAMA Psychiatry. They may help explain why criminal psychopaths such as Lecter, portrayed chillingly on screen by Sir Anthony Hopkins, appear so lacking in remorse or compassion.
Psychopaths are known to be responsible for a disproportionate amount of repetitive crime and violence.
The scientists were surprised by the high insula activity seen in psychopaths, since this brain region is central to emotion.
However, the stunted response observed in the ventromedial prefrontal cortex and amygdala was consistent with previous studies of psychopathy.
The amygdala, an almond-shaped bundle of neurons deep within the brain, plays an important role in processing emotions such as fear, anger and pleasure.
Most people flinch from seeing others in pain or distress - which serves a useful social purpose, the scientists pointed out.
They wrote in their paper: "The neural response to distress of others such as pain is thought to reflect an aversive response in the observer that may act as a trigger to inhibit aggression or prompt motivation to help.
"Hence, examining the neural response of individuals with psychopathy as they view others being harmed or expressing pain is an effective probe into the neural processes underlying affective and empathy deficits in psychopathy."