ILLUSORY INFERENCES ABOUT PROBABILITIES

The mental model theory postulates that reasoners build models of the situations described in premises. A conclusion is possible if it holds in at least one model of the premises; it is probable if it holds in most of the models; and it is necessary if it holds in all of the mode ls. The theory also postulates that reasoners represent as little info rmation as possible in explicit models and, in particular, that they r epresent only information about what is true. One unexpected consequen ce of this assumption is that there should be a category of illusory i nferences: they will have conclusions that seem obvious, but that are wholly erroneous. Experiment 1 established the existence of such illus ory inferences about probabilities. Overall, 88% of the intelligent ad ult subjects chose as more probable an outcome that was impossible for at least one of the illusory problems. Experiment 2 corroborated the phenomenon and showed that illusory inferences include a wide variety of problems. Finally, the paper argues that current theories based on formal rules of inference are unlikely to be able to explain the illus ions.