Psychological Review VOLUME 90 NUMBER 4 OCTOBER 1983 Extensional Versus Intuitive Reasoning: The Conjunction Fallacy in Probability Judgment Amos Tversky Daniel Kahneman Stanford University University of British Columbia, Vancouver, British Columbia, Canada Perhaps the simplest and the most basic qualitative law of probability is the con- junction rule: The probability of a conjunction, P(A&B), cannot exceed the prob- abilities of its constituents, P(A) and .P(B), because the extension (or the possibility set) of the conjunction is included in the extension of its constituents. Judgments under uncertainty, however, are often mediated by intuitive heuristics that are not bound by the conjunction rule. A conjunction can be more representative than one of its constituents, and instances of a specific category can be easier to imagine or to retrieve than instances of a more inclusive category. The representativeness and availability heuristics therefore can make a conjunction appear more probable than one of its constituents. This phenomenon is demonstrated in a variety of contexts including estimation of word frequency, personality judgment, medical prognosis, decision under risk, suspicion of criminal acts, and political forecasting. Systematic violations of the conjunction rule are observed in judgments of lay people and of experts in both between-subjects and within-subjects comparisons. Alternative interpretations of the conjunction fallacy are discussed and attempts to combat it are explored. Uncertainty is an unavoidable aspect of the the last decade (see, e.g., Einhorn & Hogarth, human condition. Many significant choices 1981; Kahneman, Slovic, & Tversky, 1982; must be based on beliefs about the likelihood Nisbett & Ross, 1980). Much of this research of such uncertain events as the guilt of a de- has compared intuitive inferences and prob- fendant, the result of an election, the future ability judgments to the rules of statistics and value of the dollar, the outcome of a medical the laws of-probability. The student of judg- operation, or the response of a friend. Because ment uses the probability calculus as a stan- we normally do not have adequate formal dard of comparison much as a student of per- models for computing the probabilities of such ception might compare the perceived sizes of events, intuitive judgment is often the only objects to their physical sizes. Unlike the cor- practical method for assessing uncertainty. rect size of objects, however, the "correct" The question of how lay people and experts probability of events is not easily defined. Be- evaluate the probabilities of uncertain events cause individuals who have different knowl- has attracted considerable research interest in edge or who hold different beliefs must be al- lowed to assign different probabilities to the This research was supported by Grant NR197-058 from same event > no sin g! e value can be correct for the U.S. Office of Naval Research. We are grateful to friends all people. Furthermore, a correct probability and colleagues, too numerous to list by name, for their cannot always be determined even for a single useful comments and suggestions on an earlier draft of person Outside the domain of random sam- 'Xuets for reprints should be sent to Amos Tversky, P lin S> Probability theory does not determine Department of Psychology, Jordan Hall, Building 420, the probabilities of uncertain events—it merely Stanford University, Stanford, California 94305. imposes constraints on the relations among Copyright 1983 by the American Psychological Association, Inc. 293