| When estimating likelihood, people seldom use the prescriptions of probability theory. Instead, decision heuristics which simplify the decision-making process and reduce complexity and cognitive load are used (Tversky & Kahneman, 1974). According to the Representativeness Heuristic, judgments of group membership are based on the similarity between a target and the larger reference class (Kahneman & Tversky, 1973). While there is some evidence to support the use of this heuristic in judgments of AIDS likelihood (Raghubir & Menon, 1998; Triplet, 1992) little research on the heuristic in this context exists.; In the current study, a regression analysis technique (Kern & Thijis, 1996) was used to examine whether AIDS likelihood judgments were consistent with Bayes' Theorem or whether they reflected the use of the Representativeness Heuristic. The moderating roles of cognitive ability, as measured by the Scholastic Aptitude Test (SAT), and cognitive motivation, as measured by Need for Cognition (NFC) were also examined. Participants were given information concerning the prevalence of AIDS in the U.S. population (prior likelihood) and individual-specific descriptions. The descriptions implied high-risk-group, low-risk-group, or they were neutral with regard to risk-group membership. No actual risk factors were specified.; If people were using the Representativeness Heuristic, the regression weight for the description's likelihood ratio (its perceived diagnostic value) would be greater than the regression weight of the prior likelihood. If judgments conformed to Bayes' Theorem, the regression weight for the prior likelihood and the regression weight for the description's likelihood ratio would both equal 1.; The analysis indicated that while the ideal Bayesian weighting was not used, the possibility that the two variables received equal weight could not be ruled out. The description likelihood ratios were typically underweighted. However, comments referring to implied risk factors accounted for a significant amount of the variance in the AIDS likelihood judgments, suggesting that the diagnostic value of the descriptions were not adequately measured by the description likelihood ratios. People who scored higher on the SAT were more likely to rely on the prior likelihood stated in the instructions than participants who scored lower on the SAT. NFC was not associated with judgment approach. |
