Both the pattern of brain use and the structure of brain regions change through the teen years. The good news is that, around puberty, the brain blossoms with new brain cells and neural connections, something that was thought to happen only in the first 18 months of life. Then, between puberty and young adulthood the frontal lobes—responsible for such “executive” functions as self-control, judgment, emotional regulation, organization and planning—undergo wholesale renovation. They shrink. The reason seems to be that extraneous neuronal branchings get pruned back. Pruning also occurs in infancy, creating efficient, well-organized circuitry. The teen years are, then, a second chance to consolidate circuits that are used and prune back those that are not—to hard-wire an ability to hit a curve ball, juggle numbers mentally or turn musical notation into finger movements almost unconsciously. “Teens have the power to determine their own brain development, to determine which connections survive and which don’t, [by] whether they do art, or music, or sports, or videogames,” says Giedd.

The immaturity of the frontal lobes during the teen years may explain why the brain regions that teens use for several tasks differ from the regions adults use. McLean’s Yurgelun-Todd and colleagues showed pictures of people wearing fearful expressions to teenagers between 11 and 17. Compared to adults, she finds, the teens’ rational, thoughtful frontal lobes light up less and their amygdala, which registers emotions such as fear, light up more. Yet the teens often misread facial expressions, seeing sadness or anger or confusion where there was fear. The results, says Yurgelun-Todd, suggest that “in teens, the judgment, insight and reasoning power of the frontal cortex is not being brought to bear on the task as it is in adults. Teens just process information differently from adults.”

Developing the ability to plan, to organize, to manage emotions, to understand others, to exhibit judgment and even to master logic and algebra requires more than slipping software into existing hardware. That’s what learning is. Instead, it requires changing the very hardware of the brain. The brain you have when you enter your teen years is not the one you have when you grow out of them. Thank goodness.