The Influence of Logic on Conditional Reasoning Performance

Two experiments are reported, in each of which subjects were asked to decide whether or not a number of statements, including the inverse, converse and contra-positive, followed logically from a given conditional rule of the form “if P then Q”. Rules referred to letter, number relationships, the linguistic form being manipulated by systematic negation of the antecedent and consequent components. The influence of logical validity on responses was investigated by examining differential frequencies with which inferences were drawn and by testing for consistent behaviour across inferences which depend upon the same logical principle. These analyses revealed little evidence for an influence of logic. Responses were found to be substantially influenced by a response bias, such that subjects showed a preference for agreeing with statements having affirmative antecedents and negative consequents. This finding was in part a replication of parallel findings on other inference tasks. In addition, correlational evidence suggested that subjects’ susceptibility to these biases was consistent across problems. A possible explanation of the non-logical biases, in terms of a “caution” effect, was proposed and it was argued that these, and other, findings indicate that logical validity plays little role in mediating behaviour regarding the consequent of a conditional rule.