It's no secret that teens take crazy risks driving too fast, say. In some cases, the paradoxical reason may be that their brains have matured too fast. Researchers at Emory University scanned the brains of risk-taking teens and found that some had white-matter development that looked more like that of adults. These kids may be intuitively aware of their greater capabilities and frustrated that they are still bound by the rules of childhood. As a result, they may be more motivated than other teens to break those rules. Tip for parents: Take away the car keys.