The effects of base rates and individuating information on judgments about another person

Past research suggests that in judging a person's category membership, people largely ignore the population frequency of membership in the category (base rates) in favor of individuating information about the particular person. This study tested the hypothesis that base rates will be utilized to the extent that the usefulness of the individuating information for diagnosing category membership is diminished. Subjects were given problems in which both base rates of membership in each of two categories and individuating information about a target person were presented. Then, in each case, they were asked to assess the probability that the target person belonged to each category. In three sets of problems, the diagnostic usefulness of the individuating information (personality characteristics) was diminished by including individuating information that was either inconsistent or irrelevant, or by increasing the similarity of the two alternative membership categories. In a fourth set of problems, the individualistic information included consistent, relevant personal characteristics and the membership categories were dissimilar. As expected, base rates were used in each of the first three sets of problems but were ignored in the fourth set. The results were interpreted in terms of informational factors that induce a shift away from a habitual, spontaneous reliance on a source of information (e.g., personality traits) for which one has well-developed rules (e.g., intuitive personality theories), and toward a more controlled, deliberate mode of thinking wherein other sources of information (e.g., base rates) are attended to and incorporated into judgment through less frequently used rules (e.g., sampling rules).