By Cory Turner, NPR

This story was written for the series, “Being 12: The Year Everything Changes,” from member station WNYC.

If adolescence has a poster child, it’s a teenager. In a car. Smoking, drinking, and driving badly while also, somehow, having sex in the back seat.

But changes in the brain that lead to the famously bad choices of adolescence don’t start at 16 or 17 years old. They start around 11 or 12 and the beginning of puberty.

This is the dirty little secret of adolescence: The cloudy judgment and risky behavior may not last a year or two. Try a decade.

To understand why, let’s start with an experiment. At Temple University, psychology professor Laurence Steinberg and his team put a bunch of adolescents into an FMRI machine — a brain scanner — and asked them to play a driving game.

“Your perspective is that of a person behind the wheel,” Steinberg says, describing the set-up. “And you come to a series of intersections, and the lights turn yellow. And you have to decide whether to put the brakes on or not.”

Now, what do you think the adolescents did in this situation?

Wrong.

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