A lack of emotion isn’t the only thing driving psychopaths. It now seems that their brains may overvalue the pleasure associated with getting what they want. In extreme psychopaths, this may result in callous and manipulative acts.

Joshua Buckholtz, a neuroscientist at Vanderbilt University in Nashville, Tennessee, discovered that the brains of people with psychopathic tendencies are rich with dopamine, a “reward” chemical that makes us seek out pleasure.

This might drive psychopathic behaviour by encouraging people to pursue activities that give them this kind of reward at the expense of considering the costs of their actions – such as hurting others.

“This might cause them to pay more attention to obtaining rewards like money, sex or status, as opposed to the costs,” he says.


Drugs and alcohol

Psychopathy is a spectrum of personality traits, including fearlessness, callousness, narcissism and impulsivity, and everyone has a place somewhere on this spectrum. People who score above a certain threshold on enough of these traits are known as psychopaths, but there are still varying degrees of psychopathy among the rest.

What drives these traits is still unknown. Previous studies have found that brain centres involved in emotion tend to be less active in psychopathic individuals. “Psychopaths seem to experience little to no fear and have very little empathy. They appear to be unable to put themselves in other people’s shoes,” Buckhotlz says.

But that alone can’t explain some of the other behaviours often found in psychopaths. For example, psychopathic criminals are far more likely than other criminals to abuse drugs and alcohol. And people who get high scores in tests for the psychopathic trait of antisocial impulsivity – a person’s willingness to manipulate people to achieve their own goals – are more likely to break the law than those who score highly for fearlessness.

Putting these findings together, Buckholtz’s team wondered whether psychopathic tendencies might stem from abnormal production of dopamine in a region of the brain called the nucleus accumbens that is critical to processing rewards and is also implicated in drug addiction.

Cash rewards

To investigate, Buckholtz’s team gave 30 volunteers a kind of amphetamine that attaches itself to dopamine-producing neurons. The drug had been radioactively labelled, so the researchers could see where and how much dopamine was released in response to it.

The team couldn’t work with true psychopaths but had to rely on volunteers who had only some psychopathic traits. That’s because people who score highly enough on a personality test to be rated as full-blown psychopaths often have a history of substance abuse, which might have confounded any innate differences in dopamine production.

The researchers found that those volunteers who scored highest on antisocial impulsivity tended to produce much more dopamine in their nucleus accumbens than those who scored lower on this trait. This correlation did not exist for volunteers scoring highly on fearlessness, indicating that higher dopamine production may drive only some aspects of psychopathy.

The team also investigated another way of triggering dopamine production: they asked 24 of the volunteers to play a guessing game with cash rewards while in a functional MRI scanner. In nearly all of them, the nucleus accumbens lit up when they expected a pay-off, but this change was greatest in those who scored high on antisocial impulsivity.

Rewarding violence?

Buckholtz concludes that over-anticipating rewards – which could even include violence – and disregarding the costs of obtaining them, combined with an insensitivity to fear and the emotion of others, could produce psychopathic behaviour.

“This is an intuitive, sensible finding,” says Joseph Newman a psychologist at the University of Wisconsin-Madison who studies psychopathy. However, he questions how relevant these results are to people with extremely psychopathic personalities. Researchers are increasingly studying psychopathic traits in people with more mainstream psychopathic profiles, but Newman says that care must be taken when extrapolating from this.

Antisocial impulsivity, for instance, may have a different cause in homicidal psychopaths than in people who are a bit manipulative but don’t go as far as breaking the law.

Journal reference: Nature Neuroscience, DOI: 10.1038/nn.2510