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Risk-taking, by definition, defies logic. Reason can't explain why people do unpredictable things — like betting on blackjack or jumping out of planes — for little or, sometimes, no reward at all. There's the thrill, of course, but those brief moments of ecstasy aren't enough to keep most risk takers coming back for more — which they do, again and again, like addicts.

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. (See the Year in Health, from A to Z.)

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."

Cohen suggests that a better understanding of novelty-seeking behavior may even help researchers find more effective treatments for addiction. If future studies validate Zald's findings and show that addicts also have fewer dopamine-inhibiting receptors than average, then medicines designed to replace the function of those receptors may help bring their dopamine levels down to normal and weaken their addiction.

On a more theoretical level, Zald's results may also help inform a long-ranging debate in the addiction field. Some experts believe that addicts suffer from a natural deficit of dopamine and self-medicate with drugs; others think addicts' brains make normal amounts of the neurotransmitter but just can't break it down and regulate it properly.

"We think a person who finds novelty and excitement more rewarding does so because he gets more dopamine release, or more of a boost," says Zald. "But it's one of the big controversies in the field of addiction research now." And it's yet another area for researchers to explore in trying to come up with a better treatment for substance abuse.

See the top 10 scientific discoveries of 2008.

See the top 10 medical breakthroughs of 2008.

• Alice Park