The inverse fallacy: an account of deviations from Bayes's theorem and the additivity principle.

In judging posterior probabilities, people often answer with the inverse conditional probability--a tendency named the inverse fallacy. Participants (N = 45) were given a series of probability problems that entailed estimating both p(H / D) and p(approximately H / D). The findings revealed that deviations of participants' estimates from Bayesian calculations and from the additivity principle could be predicted by the corresponding deviations of the inverse probabilities from these relevant normative benchmarks. Methodological and theoretical implications of the distinction between inverse fallacy and base-rate neglect and the generalization of the study of additivity to conditional probabilities are discussed.