“One of the responsibilities of the prefrontal cortex is to assess risk versus reward and to decide whether to inhibit behavior that may cause harm. With addicts, it’s not so much that they make bad choices as that they fail to inhibit behavior that has become reflexive. We know from studies in animals and humans that cocaine, for one, damages nerve cells in the prefrontal cortex and even reduces gray matter. And in recent years, imaging studies have shown that the prefrontal cortex doesn’t fully develop until we are well into our twenties, which could explain why most people who experiment with drugs and get hooked do so as teenagers or during early adulthood, when their inhibition hasn’t fully developed. “They end up with a hypersensitive system that wants drugs, and they make very bad decisions,” says Robinson. “It’s the worst of both possible worlds.”