A new theory of the development of expertise in medicine is outlined. Contrary to existing views, this theory assumes that expertise is not so much a matter of superior reasoning skills or in-depth knowledge of pathophysiological states as it is based on cognitive structures that describe the features of prototypical or even actual patients. These cognitive structures, referred to as "illness scripts," contain relatively little knowledge about pathophysiological causes of symptoms and complaints but a wealth of clinically relevant information about disease, its consequences, and the context under which illness develops. By contrast, intermediate-level students without clinical experience typically use pathophysiological, causal models of disease when solving problems. The authors review evidence supporting the theory and discuss its implications for the understanding of five phenomena extensively documented in the clinical-reasoning literature: (1) content specificity in diagnostic performance; (2) typical differences in data-gathering techniques between medical students and physicians; (3) difficulties involved in setting standards; (4) a decline in performance on certain measures of clinical reasoning with increasing expertise; and (5) a paradoxical association between errors and longer response times in visual diagnosis.

Medical diagnosis is a categorization task that allows physicians to make predictions about features of clinical situations and to determine appropriate course of action. The script concept, which first arose in cognitive psychology, provides a theoretical framework to explain how medical diagnostic knowledge can be structured for diagnostic problem solving. The main characteristics of the script concept are pre-stored knowledge, values acceptable or not acceptable for each illness attribute, and default values. Scripts are networks of knowledge adapted to goals of clinical tasks. The authors describe how scripts are used in diagnostic tasks, how the script concept fits within the clinical reasoning literature, how it contrasts with competing theories of clinical reasoning, how educators can help students build and refine scripts, and how scripts can be used to assess clinical competence.