A new study by researchers at Vanderbilt University in Nashville and Albert Einstein College of Medicine in New York City suggests a biological explanation for why certain people tend to live life on the edge — it involves the neurotransmitter dopamine, the brain's feel-good chemical...

Dopamine is responsible for making us feel satisfied after a filling meal, happy when our favorite football team wins, or really happy when we use stimulating drugs like amphetamines or cocaine, which can artificially squeeze more dopamine out of the nerve cells in our brain. It's also responsible for the high we feel when we do something daring, like skiing down a double black diamond slope or skydiving out of a plane. In the risk taker's brain, researchers report in the Journal of Neuroscience, there appear to be fewer dopamine-inhibiting receptors — meaning that daredevils' brains are more saturated with the chemical, predisposing them to keep taking risks and chasing the next high: driving too fast, drinking too much, overspending or even taking drugs.

David Zald, a professor of psychology and psychiatry at Vanderbilt, studied whether the brains of those thrill seekers differed in any way from those of the less adventuresome when it comes to dopamine. He gave 34 men and women a questionnaire to assess their novelty-seeking tendencies, then scanned their brains using a technique called positron emission tomography to figure out how many dopamine receptors the participants had. Zald and his team were on the lookout for a particular dopamine-regulating receptor, which monitors levels of the neurotransmitter and signals brain cells to stop churning it out when there's enough.

Earlier studies in rats had shown that animals that tend to explore and take more risks in new environments also tend to have fewer of these inhibitory receptors, and Zald wanted to find out if the same was true in people.

"This is one of those situations where the data came out essentially perfectly," he says. "The results were exactly as we predicted they would be, based on the animal data." That is, like the rats, humans who were more spontaneous and eager to take risks had fewer dopamine-regulating receptors than those who were more cautious.

The findings support Zald's theory that people who take risks get an unusually big hit of dopamine each time they have a novel experience, because their brains are not able to inhibit the neurotransmitter adequately. That blast makes them feel good, so they keep returning for the rush from similarly risky or new behaviors, just like the addict seeking the next high.

"This finding is really interesting," says Dr. Bruce Cohen, director of the Frazier Research Institute at McLean Hospital in Boston and a professor of psychiatry at Harvard Medical School. "It's a piece of the puzzle to understanding why we like novelty, and why we get addicted to substances ... Dopamine is an important piece of reward."

Abnormalities in how the nucleus accumbens, highlighted here, processes dopamine have been found in individuals with psychopathic traits and may be linked to violent, criminal behavior. Credit: Gregory R.Samanez-Larkin and Joshua W. Buckholtz

The brains of psychopaths appear to be wired to keep seeking a reward at any cost, new research from Vanderbilt University finds. The research uncovers the role of the brain's reward system in psychopathy and opens a new area of study for understanding what drives these individuals.

"This study underscores the importance of neurological research as it relates to behavior," Dr. Francis S. Collins, director of the National Institutes of Health, said. "The findings may help us find new ways to intervene before a personality trait becomes antisocial behavior."

The results were published March 14, 2010, in 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 the Department of 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."

Previous research on psychopathy has focused on what these individuals lack—fear, empathy and interpersonal skills. The new research, however, examines what they have in abundance—impulsivity, heightened attraction to rewards and risk taking. Importantly, it is these latter traits that are most closely linked with the violent and criminal aspects of psychopathy.

"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," David Zald, associate professor of psychology and of psychiatry and co-author of the study, said. "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."

To examine the relationship between dopamine and psychopathy, the researchers used positron emission tomography, or PET, imaging of the brain to measure dopamine release, in concert with a functional magnetic imaging, or fMRI, probe of the brain's reward system.

"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.

Study volunteers were given a personality test to determine their level of psychopathic traits. These traits exist on a spectrum, with violent criminals falling at the extreme end of the spectrum. However, a normally functioning person can also have the traits, which include manipulativeness, egocentricity, aggression and risk taking.

In the first portion of the experiment, 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."

In the second portion of the experiment, the research subjects were told they would receive a monetary reward for completing a simple task. Their brains were scanned with fMRI while they were performing the task. The researchers found in those individuals with elevated psychopathic traits the dopamine reward area of the brain, the nucleus accumbens, was much more active while they were anticipating the monetary reward than in the other volunteers.

"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."