Lay theories - the informal, common-sense explanations people give for particular social behaviours - are often very different from formal 'scientific' explanations of what actually happens. While they have been studied in the past, this is the first attempt to review, in detail, the nature of these beliefs. More specifically, it is the first study to consider such fundamental questions as the structure, aetiology, stability and consequence of lay theories about a range of topics. Each chapter covers a different area, such as psychology, psychiatry, medicine, economics, statistics, law and education.