Proceedings of the Annual Meeting of the Cognitive Science Society

People tend to give subadditive probability judgments when asked to assess each in a set of three or more exclusive hypotheses. The degree of subadditivity in such judgments is determined in large part by the evidence upon which the judgments are based, but the characteristics of the evidence that influence subadditivity have yet to be fully specified. In the present experiments, this issue was addressed using a classification learning task, in which the relationship between the evidence and the hypotheses under consideration can be controlled experimentally. Two potential evidential influences on subadditivity--cue conflict and cue frequency--are distinguished and tested in three experiments. The results indicate that (a) people's probability judgments are systematically subadditive--in violation of standard probability theory--even when the judgments are based on cues learned within the experimental context, contrary to the predictions of "ecological" theories of human judgment which attribute such biases to nonrepresentative item selection; and (b) cue conflict has a reliable influence on the degree of subadditivity exhibited in probability judgments.