Psychopaths lack the basic brain "hardwiring" to care for others, meaning they are neurologically stunted when it comes to empathy.
Researchers from the University of Chicago and the University of New Mexico examined the neural processes of prisoners to see how they reacted to another person being hurt.
Jean Decety, a psychology and psychiatry professor at UChicago, said: "A marked lack of empathy is a hallmark characteristic of individuals with psychopathy.
"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 team looked at 80 prisoners aged between 18 and 50 and tested them for psychopathy using standard measures.
Psychopathy is a personality disorder where the sufferer displays characteristics such as lack of empathy or remorse, antisocial behaviour, superficial charm, manipulativeness and impulsivity.
Researchers studied the prisoner's neurological function with MRI technology to establish their response to short videos showing faces of people who were being intentionally hurt.
"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," the authors wrote.
"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."
Findings showed that the participants in the psychopathy group had significantly less activity in parts of the brain involved in moral decision-making, empathic concern and valuing the wellbeing of others.
However, they also found there was more activity in the striatum and insula in the psychopathy group, which was unexpected. This region is involved in emotion and somatic resonance - which is related to awareness.
In the US psychopathy affects around one percent of the population, but around 20 to 30 percent of the prison population.
Researchers hope their results could help clinical psychologists to develop better treatment programmes to support psychopaths.