Understanding the nature of brain development in adolescence helps explain why adolescents can vacillate so often between mature and immature behavior. Early and middle adolescence, in particular, are times of heightened vulnerability to risky and reckless behavior because the brain's reward center is easily aroused, but the systems that control impulsive behavior are still relatively immature. Assignments that require teenagers to think ahead, make a plan, and carry it out may stimulate the maturation of brain systems that enable more mature self-regulation. (Contains 1 endnote.)