Book excerpt: This important new book explores the psychological motives that shape the extent and nature of people's cooperative behavior in the groups, organizations and societies to which they belong. Individuals may choose to expend a great deal of effort on promoting the goals and functioning of the group, they may take a passive role, or they may engage in behaviors targeted towards harming the group and its goals. Such decisions have important implications for the group's functioning and viability, and the goal of this book is to understand the factors that influence these choices.