A new study of teenagers and their moms reveals how adolescent brains negotiate risk -- and the factors that modulate their risk-taking behind the wheel. Researchers observed that teens driving alone found risky decisions rewarding. Blood flow to the ventral striatum, a "reward center" in the brain, increased significantly when teen drivers chose to ignore a yellow stoplight and drove through the intersection anyway. A mother's presence, however, blunted the thrill of running the yellow light.