Whether it involves gambling away one's life savings or committing one murder after another, a psychopath inevitably leaves the rest of us wondering: What was going on in his head? Now researchers report that part of the answer may be hypersensitivity to rewards, which may create a pathological drive for money, sex, and status.
All psychopaths share two characteristic traits: an inability to empathize with others' emotions, such as the fear in a person's face, and impulsive, anti-social behavior, such as reckless risk taking or excessive aggression. Neuroscientists have pinpointed neural mechanisms that may cause psychopaths' lack of empathy. But very little research has looked at what leads to impulsivity-which in some ways might be more important, because it can help predict a psychopath's tendency towards violent crime.
Neuroscientist Joshua Buckholtz of Vanderbilt University in Nashville and his colleagues decided to focus on a system of interconnected brain regions called the mesolimbic system, which motivate us to hunt for rewards by releasing the neurotransmitter dopamine. Drugs like heroine-to which psychopaths are also more susceptible—can push circuits in this system into overdrive, leaving addicts compulsively seeking another hit. The researchers hypothesized that psychopaths might also overreact to other rewards.
To test their hypothesis, the scientists studied how normal personality is affected by variations in the nucleus accumbens, a part of the mesolimbic system involved in motivation. They gave 30 volunteers a small dose of amphetamine, a stimulant, and used a PET scanner to measure how much dopamine their nucleus accumbens released. They compared that measurement with tests the volunteers had taken that measure impulsivity and empathy. People scoring above a high-impulsivity cutoff released four times more dopamine than those below the cutoff. For the second experiment, subjects lay in a fMRI machine. They could score up to $5 if they pressed a button while a white square flashed on a computer screen. The fMRI detected greater levels of activity in the nucleus accumbens of impulsive high scorers as they anticipated the square.
These correlations, the researchers report online today in Nature Neuroscience, suggest that the greater the activity in a person's dopamine circuits when they anticipate rewards, the more impulsive their personality. For psychopaths, everyday rewards may trigger abnormally large mesolimbic system responses, and thus create an overwhelming drive to seek out something rewarding despite the consequences of their actions, Buchholtz says. He believes that this motivation coupled with a lack of empathy leads to crime.
While intriguing, this model "needs more data to make it fly," says behavioral neuroscientist Klaus Miczek of Tufts University in Medford, Massachusetts. The researchers especially need to account for dopamine's other roles, such as responding to stress, because its release here may not be solely due to reward. Buckholtz counters that the dopamine release data correlates with the brain activity data, suggesting that dopamine spiked as the volunteers anticipated the $5 reward.
Because of the mesolimbic system's connection with addiction, neuroscientist Kent Kiehl of the University of New Mexico, Albuquerque, believes  that the findings could ultimately have "huge implications" for substance abuse treatments that seek to dampen activity in dopamine circuits. Alcohol and drugs can further disrupt a psychopath's ability to put the brakes on aggressive, risky behavior. "So finding some way to translate this data into targeted treatments could be hugely beneficial," Kiehl says.