Equating Inverse Probabilities in Implicit Personality Judgments

Abstract

Meehl and Rosen (1955) observed that their colleagues tended to equate the sensitivity of a sign (its probability given a diagnostic category) with its predictive accuracy (the probability of the diagnostic category given the sign). Such equating of inverse probabilities implies ignoring base rates. Later, many researchers demonstrated such neglect in a number of experimental contexts involving hypothetical judgments; others have found some use of base rates—but only when base rates were supplied by the experimenter, in which case even irrelevant base rates influenced judgment. The present study tested for equating conditional probabilities in a context similar to the contexts involved in the initial observations—that is, subjects strove for accuracy rather than attempted to make good judgments about hypothetical problems, and the base rates were generated by the subjects themselves. Neglect of base rates was revealed by within-individual comparisons showing subjects equated inverse probabilities without equating the corresponding unconditional ones.