The problem of collective action is that each member of a group wants other members to make necessary sacrifices while he or she "free rides," reaping the benefits of collective action without doing the work. Inevitably the end result is that no one does the work and the common interest is not realized. This book analyzes the social pressure whereby groups solve the problem of collective action. The authors break new ground in showing that the problem of collective action requires a model of group process and cannot be deduced from simple models of individual behavior. They employ formal mathematical models to emphasize the role of small subgroups of especially motivated individuals who form the "critical mass" that sets collective action in motion.