Most people act differently with their friends than they do when they are alone. Teens who see their friends make risky choices are more likely to make risky decisions themselves. We wanted to learn how the brain processes information about other peoples’ actions. In our experiment, teens played a game where they chose between a safe gamble and a risky gamble. We separated teens into two groups based on whether they had ever used alcohol, tobacco or other drugs. We used functional magnetic resonance imaging (fMRI) to measure brain activity in some participants. We found that teens who had never used drugs had a stronger response to seeing their peers choose safe gambles. This result shows that positive peer influence can make a difference!