In this new edition of his critically acclaimed book, Jon Elster examines the nature of social behavior, proposing choice as the central concept of the social sciences. Extensively revised throughout, the book offers an overview of key explanatory mechanisms, drawing on many case studies and experiments to explore the nature of explanation in the social sciences; an analysis of the mental states - beliefs, desires, and emotions - that are precursors to action; a systematic comparison of rational-choice models of behavior with alternative accounts, and a review of mechanisms of social interaction ranging from strategic behavior to collective decision making. A wholly new chapter includes an exploration of classical moralists and Proust in charting mental mechanisms operating 'behind the back' of the agent, and a new conclusion points to the pitfalls and fallacies in current ways of doing social science, proposing guidelines for more modest and more robust procedures.