Probability judgment in three-category classification learning.

People give subadditive probability judgments--in violation of probability theory--when asked to assess each in a set of 3 or more mutually exclusive hypotheses, as indicated by their sum exceeding 1. Three potential evidential influences on subadditivity--cue conflict, cue frequency, and cue redundancy--are distinguished and tested in 5 experiments using a classification-learning task. Results indicate that (a) judgments of probability and of frequency are systematically subadditive even when the judgments are based on cues learned within the experimental context, (b) cue conflict has a reliable influence on the degree of subadditivity, and (c) judgments in this context are well described by a linear-discounting model within the framework of support theory.