Suppression of valid inferences and knowledge structures: The curious effect of producing alternative antecedents on reasoning with causal conditionals

These studies looked at the difficulty that reasoners have in accepting con ditional ("If P then Q") major premises that are not necessarily true empir ically, as a basis for deductive reasoning. Preliminary results have shown that when reasoners are asked to produce possible alternate antecedents to the major premise ("If A then Q"), they paradoxically tend to deny the modu s ponens (MP) inference ("If P is true, then Q is true"). Three studies fur ther explored these results. The first study gave university students paper -and-pencil tests in which instructions to "suppose that the major premise is true" was followed by a request to determine the next number in a sequen ce, to retrieve information unrelated to the premises, or to retrieve a pos sible case of "If A then Q." Relative to a control group, reasoners asked t o produce an alternative antecedent showed a significant tendency to deny t he MP inference, whereas no such tendency was observed for the two other ta sks used. A second study compared performance on a condition in which reaso ners were asked to produce an alternative antecedent with that when they we re given an explicit alternative. Premises used in this study were such tha t the latter alternative antecedent was also spontaneously produced by over 7096 of reasoners. Results showed that the tendency to refuse the MP premi se could not be accounted for by the specific nature of the alternative pro duced. A third study found that the tendency to refuse the MP inference aft er producing an alternative antecedent was affected by the number of "disab ling conditions" (i.e., conditions that allow "P to be true" and "Q to be f alse") available for the major premise. These results are interpreted as be ing consistent with a model that supposes that logical reasoning requires s elective inhibition of real-world knowledge.