National Geographic : 2011 Oct
rough the ages, most answers have cited dark forces that uniquely a ect the teen. Aris- totle concluded more than 2,300 years ago that "the young are heated by Nature as drunken men by wine." A shepherd in William Shake- speare's e Winter's Tale wishes "there were no age between sixteen and three-and-twenty, or that youth would sleep out the rest; for there is nothing in the between but getting wenches with child, wronging the ancientry, stealing, ghting." His lament colors most modern scienti c in- quiries as well. G. Stanley Hall, who formalized adolescent studies with his 1904 Adolescence: Its Psychology and Its Relations to Physiology, An- thropology, Sociology, Sex, Crime, Religion and Education, believed this period of "storm and stress" replicated earlier, less civilized stages of human development. Freud saw adolescence as an expression of torturous psychosexual con- ict; Erik Erikson, as the most tumultuous of life's several identity crises. Adolescence: always a problem. Such thinking carried into the late 20th century, when researchers developed brain- imaging technology that enabled them to see the teen brain in enough detail to track both its physical development and its patterns of activity. ese imaging tools o ered a new way to ask the same question---What's wrong with these kids?---and revealed an answer that surprised almost everyone. Our brains, it turned out, take much longer to develop than we had thought. This revelation suggested both a simplistic, un attering explanation for teens' maddening behavior---and a more complex, a rmative ex- planation as well. THE FIRST FULL SERIES of scans of the devel- oping adolescent brain---a National Institutes of Health (NIH) project that studied over a hundred young people as they grew up during the 1990s---showed that our brains undergo a massive reorganization between our 12th and 25th years. The brain doesn't actually grow very much during this period. It has already reached 90 percent of its full size by the time a person is six, and a thickening skull accounts for most head growth a erward. But as we move through adolescence, the brain undergoes ex- tensive remodeling, resembling a network and wiring upgrade. For starters, the brain's axons---the long nerve bers that neurons use to send signals to other neurons---become gradually more insulated with a fatty substance called myelin (the brain's white matter), eventually boosting the axons' trans- mission speed up to a hundred times. Mean- while, dendrites, the branchlike extensions that neurons use to receive signals from nearby axons, grow twiggier, and the most heavily used syn- apses---the little chemical junctures across which axons and dendrites pass notes---grow richer and stronger. At the same time, synapses that see little use begin to wither. is synaptic pruning, as it is called, causes the brain's cortex---the outer layer of gray matter where we do much of our conscious and complicated thinking---to become thinner but more e cient. Taken together, these changes make the entire brain a much faster and more sophisticated organ. is process of maturation, once thought to be largely nished by elementary school, con- tinues throughout adolescence. Imaging work done since the 1990s shows that these physical changes move in a slow wave from the brain's rear to its front, from areas close to the brain stem that look a er older and more behaviorally basic functions, such as vision, movement, and fundamental processing, to the evolutionarily newer and more complicated thinking areas up front. e corpus callosum, which connects the brain's le and right hemispheres and car- ries traffic essential to many advanced brain functions, steadily thickens. Stronger links also develop between the hippocampus, a sort of memory directory, and frontal areas that set goals and weigh di erent agendas; as a result, we get better at integrating memory and experi- ence into our decisions. At the same time, the frontal areas develop greater speed and richer David Dobbs is the author of Reef Madness, on Darwin's controversial theory of coral reef origins. is is Kitra Cahana's rst story for the magazine.