Proceedings of the Annual Meeting of the Cognitive Science Society

Two experiments investigate the conjunction fallacy (judging that conjunctive probabilities are higher than the probabilities of the constituents). The conjunction fallacy was much less for P(E|C) tasks than for P(C|E) tasks. The results are explained in terms of the way people interpret the conditional probabilities. We argue that people prefer to reason from cause to effect (cause-to-effect reasoning heuristic), and for that reason, the instructions given for P(C|E) tasks were misinterpreted, resulting in apparent fallacy. In addition, we provide evidence showing that likelihood judgments are higher with more evidence (more-is-better heuristic).