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. Myelination, the scientific name for this process, strengthens and accelerates the communication between brain regions and underlies a person’s basic learning abilities.
The myelination process starts from the back of the brain and works its way to the front. That means the prefrontal cortex, the area of the brain involved in decision-making, planning and self-control, is the last part to mature. It’s not that teens don’t have frontal- lobe capabilities but rather that their signals are not getting to the back of the brain fast enough to regulate their emotions. It’s why risk-taking and impulsive behavior are more common among teens and young adults. “This is why peer pressure rules at this time of life,” says Jensen. “It’s why my teenage boys would come home without their textbook and realize at 8 p.m. that they have a test the next day. They don’t have the fully developed capacity to think ahead at this time.”
Although the development of the prefrontal cortex is the last step on the development checklist, teenagers undergo major changes in their limbic system—the area of the brain that controls emotions—at the onset of puberty, which is typically around the ages of 10 to 12. Doctors now believe that this mismatch in development of the impulse-control part of the brain and the hormone- and emotion-fueled part of the brain is what causes the risk-taking behaviors that are so common among teenagers. “The prefrontal cortex
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