Economists are quick to assume opportunistic behavior in almost every walk of life other than our own. Our empirical methods are based on assumptions of human behavior that would not pass muster in any of our models. The solution to this problem is not to expect a mass renunciation of data...

In many cases, aggregate data is used to make inferences about individual level behavior. If there are social interactions in which one person's actions influence his neighbor's incentives or information, then these inferences are inappropriate. The presence of positive social interactions, or...