This book invites readers to discover the simple heuristics that people use to navigate the complexities and surprises of environments populated with others. The social world is a terrain where humans and other animals compete with conspecifics for myriad resources, including food, mates, and status, and where rivals grant the decision maker little time for deep thought, protracted information search, or complex calculations. The social world also encompasses domains, however, where social animals such as humans learn from one another how to deal with the vagaries of a natural world that both inflicts unforeseeable hazards and presents useful opportunities and dare to trust and forge alliances with one another to boost their chances of success. According to the book's thesis, the undeniable complexity of the social world does not dictate cognitive complexity as many scholars of rationality argue. Rather, it entails circumstances that render optimization impossible or computationally arduous: intractability, the existence of incommensurable considerations, and competing goals. With optimization beyond reach, less can be more. That is, heuristics-simple strategies for making decisions when time is pressing and careful deliberation an unaffordable luxury-become indispensible mental tools. As accurate or even more accurate than complex methods when used in the appropriate environments, these heuristics are good descriptive models of how people make many decisions and inferences, but their impressive performance also poses a normative challenge for optimization models. In short, the homo socialis may prove to be a homo heuristicus whose intelligence reflects ecological rather than logical rationality.