Typicality Can Create, Eliminate, and Reverse the Dilution Effect

Nisbett, Zukier, and Lemley presented evidence for a "dilution effect," in which information nonpredictive of a stimulus per- son's behavior "watered down" or diluted the predictive value of categorical information diagnostic of that behavior. Two experi- ments suggest, however, that nondiagnostic information influ- ences prediction by altering the perceived goodness of fit between the stimulus person and the diagnostic category. The authors conclude that (a) Nisbett et al. found evidence for dilution because their nondiagnostic items decreased the goodness of fit between the stimulus person and the diagnostic category and (b) depending on the typicality of the nondiagnostic items, it is possi- ble to dilute, enhance, or leave unchanged the predictability between category and behavior. The structure of social categories, and the importance of typicality in the stereotyping process, is discussed. In 1981, Nisbett, Zukier, and Lemley presented an interesting idea about people's use of diagnostic and nondiagnostic information in prediction. They showed that the predictive power of diagnostic information can be reduced by the addition of nondiagnostic informa- tion, which they called "the dilution effect." For a given criterion behavior (e.g., "engages in child abuse"), diagnostic items predictive of the behavior (e.g., "has a drinking problem") and nondiagnostic items irrelevant to the behavior (e.g., "he manages a hardware store") were developed with pretest samples. Participants who were presented only with diagnostic information gave more extreme predictions than did participants pre- sented with the same diagnostic information plus addi- tional nondiagnostic information. The addition of non- diagnostic information, although independently judged to be of no value in the prediction, diluted the effects of otherwise diagnostic information. An individual with a drinking problem who managed a department store was judged less likely to be a child abuser than an individual identified only as having a drinking problem. These findings are nonintuitive, nonnormative, and carry important implications for real-life judgments. Information judged to be of little value in predicting an outcome should not alter a prediction. A medical doc- tor's diagnosis of her patient's disease, for example, should not be influenced by patient information unre- lated to the disease. A psychologist's evaluation of the dangerousness of his client should not be reduced by exposure to the unrelated details of his client's life. Nisbett et al. (1981) explained the dilution effect in terms of Tversky's (1977) features of similarity model, in which the presence of nondiagnostic information reduces the similarity between the stimulus person and the target behavior by increasing the number of distinc- tive elements. According to Tversky's model, similarity is some function of the difference between common and distinctive features. In the Nisbett et al. paradigm, the overlap between the stimulus person and the target behavior represents the common features and thus the nondiagnostic items add to the number of distinctive features (present for the stimulus person and absent for the target behavior). As the number of nondiagnostic features increase, similarity between stimulus person and target decreases and the stimulus person is viewed as less likely to engage in the target behavior.