In this concluding chapter, we examine a theoretical issue that has been the subject of increasing attention in recent years: the question of human rationality. Most people think they know what they mean when they say that someone’s thoughts or actions are rational or irrational, and official versions of this intuition can be found, for instance, in the law or in medical practice. People have been debating rationality ever since Aristotle, who first claimed the capacity for reason as a distinguishing mark of humanity: the human species thus became the “rational animal” (see Wetherick, 1993). The rationality debate was a philosophical matter until quite recently; only in the last couple of decades or so have the findings of psychological research, and the theoretical ideas they have led to, had much of a bearing on the case. I shall concentrate on the psychological aspect of the debate, since this is a psychology book, and leave it up to you to go further into the philosophical (and political and economic) areas, should you choose to.