The title of this book, Why Smart People Can Be So Stupid, assumes that smart people at least sometimes do stupid things. In addition, the title implies that such stupid behavior needs explaining. The first challenge to anyone who tries to provide an explanation is that the title is phrased in terms from the common vernacular. The key words smart and stupid belong to folk psychology. As such, their meanings are vague, ambiguous, and shift with person and context. The term smart can be equated with the psychological concept of intelligence. This, in fact, is what the contributors to this volume seem to have done. Indeed, this may be the one matter upon which all these authors are in agreement. Unfortunately, the term stupid seems to have no obvious technical counterpart in psychological theory. One consequence is that the authors di√er greatly on how they treat this concept. The title makes it clear that smartness is a property of people. It is an enduring property of a person. A person who is ‘‘smart’’ today is expected to be smart tomorrow and into the foreseeable future. Of course, at least some of the authors make it clear that they do not consider intelligence to be fixed. People can, with e√ort and proper instruction, improve their intelligence. However, quick changes and major fluctuations in intelligence are rare. Stupidity, on the other hand, can be a property of an act, behavior, state, or person. We might believe that the act of smoking is stupid regardless of the intent, motivations, and construals of the people doing the smoking. Although I believe that many people apply the label of ‘‘stupid’’ to acts in