The ability to identify emotion in other people is critical to social functioning. In a series of experiments, we explored the relationship between recognition of emotion in ambiguous facial expressions and aggressive thoughts and behavior, both in healthy adults and in adolescent youth at high risk of criminal offending and delinquency. We show that it is possible to experimentally modify biases in emotion recognition to encourage the perception of happiness over anger in ambiguous expressions. This change in perception results in a decrease in self-reported anger and aggression in healthy adults and high-risk youth, respectively, and also in independently rated aggressive behavior in high-risk youth. We obtained similar effects on mood using two different techniques to modify biases in emotion perception (feedback-based training and visual adaptation). These studies provide strong evidence that emotion processing plays a causal role in anger and the maintenance of aggressive behavior.