The study from from Vanderbilt University in Nashville, Tenn. is published in the March 14 issue of the research journal Nature Neuroscience.
“Psychopaths are often thought of as cold-blooded criminals who take what they want without thinking about consequences,” Joshua Buckholtz, a graduate student in psychology and lead author of the new study, said. “We found that a hyper-reactive dopamine reward system may be the foundation for some of the most problematic behaviors associated with psychopathy, such as violent crime, recidivism and substance abuse.”
Dopamine is the brain chemical most closely associated with pleasure and excitement.
Previous research on psychopathy has focused on what these people lack—fear, empathy and interpersonal skills. The new research, however, examines what they have in abundance—impulsivity, heightened attraction to rewards and risk taking, said Buckholtz and his co-authors. Importantly, the latter traits are those most closely linked with the violent and criminal aspects of psychopathy, researchers said.
“There has been a long tradition of research on psychopathy that has focused on the lack of sensitivity to punishment and a lack of fear, but those traits are not particularly good predictors of violence or criminal behavior,” said Vanderbilt psychologist David Zald, co-author of the study.
“Our data is suggesting that something might be happening on the other side of things. These individuals appear to have such a strong draw to reward—to the carrot—that it overwhelms the sense of risk or concern about the stick.”
The researchers used a brain imaging technique called positron emission tomography, or PET, to measure dopamine release, in concert with a probe of the brain’s reward system using functional magnetic imaging, or fMRI. “The really striking thing is with these two very different techniques we saw a very similar pattern—both were heightened in individuals with psychopathic traits,” Zald said.
Volunteers for the study took a personality test to gauge their level of psychopathic traits. These traits lie on a spectrum: violent criminals fall at its extreme end, but a normally functioning person can also have psychopathic traits to some degree. These traits include manipulativeness, egocentricity, aggression and risk taking.
The researchers gave the volunteers a dose of amphetamine, or speed, and then scanned their brains using PET to view dopamine release in response to the stimulant. Substance abuse has been shown in the past to be associated with alterations in dopamine responses. Psychopathy is strongly associated with substance abuse.
“Our hypothesis was that psychopathic traits are also linked to dysfunction in dopamine reward circuitry,” Buckholtz said. “Consistent with what we thought, we found people with high levels of psychopathic traits had almost four times the amount of dopamine released in response to amphetamine.”
The research subjects were later told they would receive some money for completing a simple task. Their brains were scanned with fMRI while they were performing the task. The researchers found in those participants with more psychopathic traits the dopamine reward area of the brain, the nucleus accumbens, was much more active while they were anticipating the reward.
“It may be that because of these exaggerated dopamine responses, once they focus on the chance to get a reward, psychopaths are unable to alter their attention until they get what they’re after,” Buckholtz said. Added Zald, “It’s not just that they don’t appreciate the potential threat, but that the anticipation or motivation for reward overwhelms those concerns.”
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