Article Contentsp. 1124p. 1125p. 1126p. 1127p. 1128p. 1129p. 1130p. 1131Issue Table of ContentsScience, New Series, Vol. 185, No. 4157 (Sep. 27, 1974), pp. 1089-1200Front Matter [pp. 1089-1192]LettersUranium Enrichment [p. 1109]Clean Air by 1975? [p. 1109]Sex Preselection [p. 1109]Airships [pp. 1109-1110]The Creative Process [p. 1110]World Population: World Responsibility [p. 1113]Classification: Purposes, Principles, Progress, Prospects [pp. 1115-1123]Judgment under Uncertainty: Heuristics and Biases [pp. 1124-1131]Rural Health Care in Mexico? [pp. 1131-1137]News and CommentSafeguard: Disputed Weapon Nears Readiness on Plains of North Dakota [pp. 1137-1140]Plutonium (II): Watching and Waiting for Adverse Effects [pp. 1140-1143]Briefing [p. 1142]UN Conferences: Topping Any Agenda Is the Question of Development [pp. 1143-1193]Research NewsModeling the Climate: A New Sense of Urgency [pp. 1145-1147]Speaking of Science: Skunks: On the Scent of a Myth [p. 1146]Planetary Science: First Meeting on Moons of the Solar System [pp. 1147-1148]AAAS Annual Meeting, 26-31 January 1975, New York City [pp. 1149-1157]Book ReviewsBirds Living Together: Actually and in Theory [pp. 1158-1159]How Pictures Work [pp. 1159-1160]Neuroscience [p. 1160]Honoring Dirac [pp. 1160-1161]Books Received [pp. 1161-1190]ReportsAtomic Images by Electron-Wave Holography [pp. 1163-1165]Stratospheric Ozone Destruction by Man-Made Chlorofluoromethanes [pp. 1165-1167]Methane Production in the Interstitial Waters of Sulfate-Depleted Marine Sediments [pp. 1167-1169]Melatonin: Its Inhibition of Pineal Antigonadotrophic Activity in Male Hamsters [pp. 1169-1171]Rod Origin of Prolonged Afterimages [pp. 1171-1172]Lymphocytic Choriomeningitis in a Hamster Colony Causes Infection of Hospital Personnel [pp. 1173-1174]Renal Lysosomes: Role in Biogenesis of Erythropoietin [pp. 1174-1176]Topology of the Outer Segment Membranes of Retinal Rods and Cones Revealed by a Fluorescent Probe [pp. 1176-1179]Centric Fusion, Satellite DNA, and DNA Polarity in Mouse Chromosomes [pp. 1179-1181]Pulmonary Alveolar Hypoxia: Release of Prostaglandins and Other Humoral Mediators [pp. 1181-1183]Nuclear Waste Disposal in the Oceans [pp. 1183-1184]Wilson's Disease and Copper-Binding Proteins [pp. 1184-1185]Subsidence of Venice: Predictive Difficulties [p. 1185]Back Matter [pp. 1193-1200]Judgment under Uncertainty: Heuristics and BiasesAuthor(s): Amos Tversky and Daniel KahnemanSource: Science, New Series, Vol. 185, No. 4157 (Sep. 27, 1974), pp. 1124-1131Published by: American Association for the Advancement of ScienceStable URL: http://www.jstor.org/stable/1738360Accessed: 04/01/2010 13:17Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available athttp://www.jstor.org/page/info/about/policies/terms.jsp. 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For more information about JSTOR, please contact [email protected] Association for the Advancement of Science is collaborating with JSTOR to digitize, preserve andextend access to Science.http://www.jstor.orgMany decisions are based on beliefs concerning the likelihood of uncertain events such as the outcome of an elec- tion, the guilt of a defendant, or the future value of the dollar. These beliefs are usually expressed in statements such as "I think that . .. ," "chances are . . .," "it is unlikely that . .. ," and so forth. Occasionally, beliefs concern- ing uncertain events are expressed in numerical form as odds or subjective probabilities. What determines such be- liefs? How do people assess the prob- ability of an uncertain event or the value of an uncertain quantity? This article shows that people rely on a limited number of heuristic principles which reduce the complex tasks of as- sessing probabilities and predicting val- ues to simpler judgmental operations. In general, these heuristics are quite useful, but sometimes they lead to severe and systematic errors. The subjective assessment of proba- bility resembles the subjective assess- ment of physical quantities such as distance or size. These judgments are all based on data of limited validity, which are processed according to heu- ristic rules. For example, the apparent distance of an object is determined in part by its clarity. The more sharply the object is seen, the closer it appears to be. This rule has some validity, because in any given scene the more distant objects are seen less sharply than nearer objects. However, the reliance on this rule leads to systematic errors in the estimation of distance. Specifically, dis- tances are often overestimated when visibility is poor because the contours of objects are blurred. On the other hand, distances are often underesti- mated when visibility is good because the objects are seen sharply. Thus, the reliance on clarity as an indication of distance leads to common biases. Such biases are also found in the intuitive judgment of probability. This article describes three heuristics that are em- ployed to assess probabilities and to predict values. Biases to which these heuristics lead are enumerated, and the applied and theoretical implications of these observations are discussed. Representativeness Many of the probabilistic questions with which people are concerned belong to one of the following types: What is the probability that object A belongs to class B? What is the probability that event A originates from process B? What is the probability that process B will generate event A? In answering such questions, people typically rely on the representativeness