Evaluating a rule against possible instances.

This investigation is concerned with a reasoning task in which subjects are given a sentence expressing a material implication and are asked to select those instances which would have to be examined in order to determine the truth or falsity of the sentence. A model is developed to describe the process of evaluating the sentence against possible instances. Two sources of differences among subjects are incorporated into the model: differences in interpretation of the sentence, and differences in interpretation of the reversibility of the stimulus cards which are the instances. This evaluation model is then combined with a model previously developed by P. C. Wason and P. N. Johnson-Laird to explain the process which subjects employ to select the instances which they believe must be examined. Predictions derived from the combined model were tested in an experiment which used an expanded version of a set of ‘therapeutic’ procedures developed by Wason and Johnson-Laird. Subjects were found to make initial selections and to modify their choices in ways consistent with the model's predictions. The results are briefly discussed in terms of the ambiguity of aspects of the task when the stimulus materials are arbitrary symbols without reference to a realistic context.