Spending less time stuck in traffic, living in cities with little or no pollution, or leading healthy lives are outcomes that most of us would like. Achieving these outcomes, requires that people cooperate on their actions. For example, If everyone decides to use the same major expressway at the same time to get to work, they would experience congestion, the opposite of their individual goal. Similarly, if fewer people paid attention to the food that they ate and didn’t exercise, we would see more unhealthy people in the long run, increasing health costs for all.