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Teenagers are a famously reckless species. They floor the gas and experiment with drugs and play with guns; according to Centers for Disease Control and Prevention figures, more than 16,000 young people die each year from unintentional injuries. The most common-sense explanation for teens' carelessness is that their brains just aren't developed enough to know better. But new research suggests that in the case of some teens, the culprit is just the opposite: the brain matures not too slowly but, perhaps, too quickly.

In a paper just published in PLoS ONE — a journal of the Public Library of Science — a team led by psychiatrist Gregory Berns of Emory University in Atlanta shows that adolescents who engage in more dangerous activities have white-matter pathways that appear more mature than those of risk-averse youths. White matter is essentially the brain's wiring — the neural strands that connect the various gray-matter regions, where the actual nerve cells reside, that are otherwise independent of one another. Maturation of white matter is important because it increases the brain's processing speed; nerve impulses travel faster in mature white matter.

Berns and his colleagues recruited 91 kids ages 12 to 18 and asked them to fill out a questionnaire about their tendency to engage in behaviors such as driving without a license, having unprotected sex and using drugs. Then they had the kids undergo a relatively new kind of brain scan called diffusion tensor imaging, a type of magnetic resonance imaging that is used to look at dense tissues like white matter. After analyzing the scans, the authors found a strong correlation between how risky the students described their behavior to be and how sophisticated their white matter was. The more mature the look of the brain, the more risk-taking the teenager tended to report.

The results challenge the accepted notion that teens make dumb decisions because their brains are immature. Although previous research has shown that most teens' gray-matter structures — including those involved in decision-making — are less advanced than those of adults, as you would expect, until now no one had studied teens' white matter, which works along with gray matter to produce decisions. The key part of white matter is called myelin, a fatty substance that coats the individual neural strands, or axons, that make up white matter. Myelination of axons begins during childhood and is completed at the end of adolescence, around the mid-20s. Myelination in the frontal lobe — the brain region responsible for decision-making — happens last, and it was in this region that the brains of risk-seeking teens in the study showed greater development compared with the frontal lobes of their more restrained peers.

Why would kids who take more risks turn out to have more adultlike white matter than other kids? The authors suggest that some risk-taking among adolescents is evidence that they are trying out more adultlike roles. Having unsafe sex and driving too fast may be mistakes, but kids often have to experiment with limits in order to learn how to live within them. Which, in turn, is a sign of maturity. "Adolescents who engage in [risky] behaviors obtain more experience in a variety of domains," the authors write. "Their more conservative peers, in contrast, do not have as much 'life experience' and therefore might be expected to have more immature brains."

Another possible explanation is that some teenagers whose brains develop more quickly than others become uncomfortable with the gap between their biological capabilities and the social rules they must follow as kids. "Precocious development of these [white-matter] tracts may predispose some adolescents to engage in behaviors that society considers too adult in nature for their chronological age," the authors write. In other words, having a more mature brain may actually spur some kids to seek out new and potentially harmful experiences.

For now, these theories are just speculation, and the researchers concede that the interaction of white and gray matter is so complex that hard conclusions remain elusive. "We have a new piece to the puzzle here," says Emory's Monica Capra, one of the study's authors. "But we don't have it all together."

One further x-factor in the study: What if teens weren't being honest in their self-reports about risky behavior? Berns' team addressed this question by drug-testing the 91 research subjects. Only nine had actually done drugs — in each case, marijuana — but eight of those nine admitted their drug use in the survey. No students who tested negative falsely claimed to have tried drugs. The teen brain, it appears, can be often an honest thing — even if it's not always a wise one.

• John Cloud