Bayes Rule as a Descriptive Model: The Representativeness Heuristic

Results of experiments designed to test the claim of psychologists that expected utility theory does not provide a good descriptive model are reported. The deviation from tested theory is that, in revising beliefs, individuals ignore prior or base-rate information contrary to Bayes rule. Flaws in the evidence in the psychological literature are noted, an experiment avoiding these difficulties is designed and carried out, and the psychologists' predictions are stated in terms of a more general model. The psychologists' predictions are confirmed for inexperienced or financially unmotivated subjects, but for others the evidence is less clear.