Understanding conjunction effects in probability judgments: the role of implicit mental models

Subjective probability judgments often violate a normative principle in tha t the conjunction of two events is judged to be more likely than the probab ility of either of the two events occurring separately,. Most previous expl anations of these conjunction effects have assumed that probability judgmen ts depend on some psychological relation (e.g. representativeness) between the constituents mentioned explicitly in the stimulus information. In contr ast, the present approach highlights the fundamental role of implicitly inf erred information. Participants are assumed to transform the explicit stimu lus information into implicit mental models in their attempt to make sense of the experimental task. Probability judgments should then reflect the deg ree of activation of such a mental model in memory given a set of propositi ons, rather than the quantitative Jit ol likelihood of the propositions the mselves. Two studies are reported which provide converging evidence for the proposed mental model approach. In the first study, using graded conjuncti ons of one to Jive propositions, probability judgments are shown to vary as a function of the activation of a mental model rather than the likelihood of the component events. In a second study, a priming procedure is employed to activate mental models that either fit an event conjunction or do not, leading to an increase or decrease of conjunction effects in probability ju dgment. Copyright (C) 1999 John Wiley & Sons, Ltd.