Thrill seeking and poor judgment go hand in hand when it comes to teenagers—an inevitable part of human development determined by properties of a growing but immature brain. Right? Not so fast. A study being published tomorrow turns that thinking upside down: The brains of teens who behave dangerously are more like adult brains than are those of their more cautious peers.

Psychologists have long believed that the brain's judgment-control systems develop more slowly than emotion-governing systems, not maturing until people are in their mid-20s. Hence, teens end up taking far more risks than adults do. Evidence supporting this idea comes from studies looking at functional and structural properties of gray matter, the important part of the brain that contains the neurons that relay brain signals.

At least two observations undermine this theory, however. First, American-style teen turmoil is absent in more than 100 cultures around the world, suggesting that such mayhem is not biologically inevitable. Second, the brain itself changes in response to experiences, raising the question of whether adolescent brain characteristics are the cause of teen tumult or rather the result of lifestyle and experiences. Because brain research is virtually always correlational in design, determining whether brain properties are causes or effects is impossible.

Now neuroscientists Gregory S. Berns, Sara Moore and Monica Capra of Emory University suggest that teen risk-taking is associated not with an immature brain but with a mature, adultlike brain—exactly the opposite of conventional wisdom.

Risk and white matter

The researchers assessed 91 teens from ages 12 to 18 in two ways: First, teens completed the Adolescent Risk-Taking Questionnaire (ARQ), designed to measure their engagement in dangerous behaviors such as drinking, smoking, taking drugs, and having unprotected sex. Subjects were also screened for actual drug use, which was predicted well by test scores. Second, a technology known as diffusion tensor imaging (DTI) was used to assess the development of white matter in the frontal cortex of teens' brains. White matter consists of myelin, a fatty substance that coats the long axons, which carry brain signals; its main function is to increase the efficiency of neural signaling. Between childhood and adolescence, it grows in volume and becomes better organized, improving our ability to think and function.

According to Berns, DTI technology takes advantage of the fact that water molecules tend to move along myelin pathways. In an immature brain, water throughout the brain diffuses in a roughly spherical, cloudlike pattern, but as the brain matures and myelin "tracts" form and grow around axons, water starts to move along those tracts in patterns that the imaging technique picks up as lines radiating throughout the brain.

If the existing theory about the teen brain is correct, then the higher the ARQ score, the less developed the white matter should be—but that is not what the Berns team discovered. "It was surprising," Berns says. "I assumed we'd find that risk-taking would be associated with an immature brain." In fact, he found the opposite—a strong positive correlation between engagement in dangerous behaviors and the increased myelination typical of mature brains. In other words, young people who engage in dangerous behaviors generally have a more adultlike brain than their conservative peers.

Against conventional wisdom

As for the conventional thinking about the teen brain, according to Berns, "after reviewing all of the neurodevelopment stuff, I couldn't really find any link between brain development and adolescent risk-taking. Nobody denies that the brain develops or that teens take risks, but how the two got intertwined is beyond me." Nevertheless, the accepted view of the teen brain is so powerful, Berns says, that his paper faced a lengthy and tumultuous review process. It appears in Wednesday's PLoS ONE.

If valid, the study has important implications for interpreting risk-taking in teens. It suggests that the brains of many teens who behave dangerously are maturing early: Reckless behavior might in fact be a sign of adultness. Some adults do risky things (speeding, drinking, having unprotected sex) quite commonly without causing great alarm. Automatically considering such behaviors to be more objectionable just because someone is young runs into what the researchers call in their paper "a conundrum of defining risk (or dangerousness) based not on the objective attributes of the activity but on the person engaging in them."

Room for skepticism

Berns also acknowledges that the new study says nothing about causation, just like the gray matter studies. "Could someone whose brain develops earlier start to engage in adult activities earlier?" That is one possibility, he says, but it is also possible that "engaging in adult activities makes the brain mature faster," he says.

Not everyone thinks the new study will overturn thinking about the teenage brain. Developmental psychologist Laurence Steinberg of Temple University says he has been aware of the Berns research for several years and that it is flawed. "There are findings from other studies that in some respects contradict these findings," Steinberg says. "For instance, it's been shown that individuals with more developed white matter tracts are less oriented toward immediate rewards and less susceptible to peer pressure”—meaning they are probably less prone to risk-taking.

Michael S. Gazzaniga of the University of California, Santa Barbara, one of the pioneers in modern neuroscience, sees the Berns study more positively. The large number of subjects makes the results hard to ignore, he says, and they need to be taken seriously. Says Gazzaniga: "So much for the much touted model that the tumultuous teenage brain is that way because it is not fully developed. Back to the drawing board again."