If you’re interested in making sure kids learn a lot in school, yes, intervening in early childhood is the time to do it … But if you’re interested in how people become who they are, so much is going on in the adolescent years.

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It turns out that just before adolescence, the prefrontal cortex—the part of the brain that governs our ability to reason, grasp abstractions, control impulses, and self-­reflect—undergoes a huge flurry of activity, giving young adults the intellectual capacity to form an identity, to develop the notion of a self. Any cultural stimuli we are exposed to during puberty can, therefore, make more of an impression, because we’re now perceiving them discerningly and metacognitively as things to sweep into our self-concepts or reject (I am the kind of person who likes the Allman Brothers). “During times when your identity is in transition,” says Steinberg, “it’s possible you store memories better than you do in times of stability.”

At the same time, the prefrontal cortex has not yet finished developing in adolescents. It’s still adding myelin, the fatty white substance that speeds up and improves neural connections, and until those connections are consolidated—which most researchers now believe is sometime in our mid-­twenties—the more primitive, emotional parts of the brain (known collectively as the limbic system) have a more significant influence. This explains why adolescents are such notoriously poor models of self-­regulation, and why they’re so much more dramatic—“more Kirk than Spock,” in the words of B. J. Casey, a neuroscientist at Weill Medical College of Cornell University. In adolescence, the brain is also buzzing with more dopamine activity than at any other time in the human life cycle, so everything an adolescent does—everything an adolescent feels—is just a little bit more intense. “And you never get back to that intensity,” says Casey. (The British psychoanalyst Adam Phillips has a slightly different way of saying this: “Puberty,” he writes, “is everyone’s first experience of a sentient madness.”)