Grown-ups have noted since Aristotle, and probably before that as well, that teens tend to take more risks and to be less responsible than adults. It’s easy to write that off as immaturity and to tell ourselves that it is one of those proverbial “stages.” More recently, a rather provocative idea has been discussed in cognitive and neuro- psychology: risk taking may actually serve some adaptive purposes in helping teens to grow up. Underlying this idea is a growing understanding of some of the neurotransmitters . . .