By Alexandra Sifferlin September 8, 2017
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The following story is excerpted from TIME’s special edition, The Science of Childhood, which is available in stores, at the TIME Shop and at Amazon.
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.