The impact of necessity and sufficiency in the Wason four-card selection task

Performance in the Wason card selection task is often improved given thematic content. Such content effects have been considered evidence against human rationality. We propose that the role of content lies in specifying premises underlying "if P, then Q" rules. Unlike thematic rules, abstract conditional rules do not explicitly provide material interpretation (P is a sufficient but not necessarily a necessary condition for Q), resulting in nonnormative responses. When necessity-sufficiency relations were explicated, normative responses were elicited and effects of other logically irrelevant components disappeared. The results suggest that content effects are compatible with human rationality.