Why teenage brains are so hard to understand

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When Frances Jensen’s eldest son, Andrew, reached high school, he underwent a transformation. Frances’s calm, predictable child changed his hair color from brown to black and started wearing bolder clothing. It felt as if he turned into an angst-filled teenager overnight. Jensen, now the chair of the neurology department at the Perelman School of Medicine at the University of Pennsylvania, wondered what happened and whether Andrew’s younger brother would undergo the same metamorphosis. So she decided to use her skills as a neuroscientist to explore what was happening under the hood. “I realized I had an experiment going on in my own home,” says Jensen, author of The Teenage Brain.
That was about 10 years ago, when society at large was only beginning to catch up to the idea that the teen brain was not a fully developed adult brain, just with less mileage. For generations, the overarching thinking was that the brain had reached its full growth by the time a child reached puberty. But thanks to the research of people like Jensen and many others, beginning in the 1990s, it’s become clear that the teenage brain is some- thing much more complex—and special.
Doctors, parents and teachers have long held preconceived notions about why teenagers act so reckless and emotional, and many of these explanations have turned out to be incorrect. It was once believed that teens were impulsive due to raging hormones and that they were difficult because they hated authority. But advances in brain imaging, which gathered force in the 2000s, told a much more complicated story. It turns out the teenage brain is nowhere near fully baked and that the brain’s structure and its effects on development continue into a person’s 20s.
Advanced brain imaging has revealed that the teenage brain has lots of plasticity, which means it can change, adapt and respond to its environment. The brain does not grow by getting substantially larger during the teenage years but rather through increased connectivity between brain regions. This growth in connectivity presents itself as white matter in the brain, which comes from a fatty substance called myelin. As the brain develops, myelin wraps itself around nerve cells’ axons—long, thin tendrils that extend from the cell and transmit information—like insulation on an electrical wire.