When Jay Giedd tells people that he studies the brains of adolescents, they often respond with a wisecrack such as "that must be a short study" or "what, they have brains?" In fact, as he and a handful of researchers showed at the American Psychological Association's annual conference last week, teens have quite active brains that are undergoing myriad changes.

Giedd, a psychologist at the National Institute of Mental Health, pointed out that once children become teenagers there is a sharp increase in death and injuries, often caused by risk-taking behavior—drinking too much, driving too fast, or simply making dumb decisions.

Now, his and others' research is showing that changes in the brain as it matures from childhood to adulthood could drive teens toward some of these behaviors.

Adolescence, said Giedd, is a time when various brain processes are being refined. One is myelination, the streamlining of the connections between neurons. Children's neurons form many diverse connections with one another, though they are not very efficient. As people mature, their connections decrease, though each connection becomes quicker, enabling faster processing. Giedd, who grew up in North Dakota, compares it to the difference between a country road—gravely, curvy, and slow—and a four-lane highway, which has fewer turn-offs but allows for faster driving.

A second study of the brains of teens showed that when they are asked to inhibit a natural response, in this case looking at a flashing light in a dark room, they invoke different areas of their brains than do adults.

Beatriz Luna, a professor at the University of Pittsburgh, put subjects in a dark room, then flashed a light in their peripheral vision. She told them, when they saw the light, to look the other way. While many people initially looked at the light and then, realizing they had made a mistake, looked the other way, most learned how to look immediately the other way after several trials. Teens and adults were both quicker to learn than kids, with teens and adults making about the same number of mistakes before learning it correctly.

However, teens and adults use different parts of their brains to accomplish that task, as Luna learned by using an MRI machine during the task. The adolescents put more strain on the prefrontal cortex, the part of the brain generally thought to be responsible for executive decisions, than did adults, who integrated several different brain areas.

Luna says the experiment suggests that teens may not be as capable as adults of inhibiting tempting behaviors, especially if they are under stress. If they are leaning so heavily on the prefrontal cortex to accomplish simple inhibition, such as not looking at a light, they may be unable to resist other temptations—like staying out past curfew.