Abstract

Two experiments were conducted with variants of Wason's (1966) selection task. The common focus was the effect of differences in the sizes of the sets represented by P and not-Q in assertions of the form _If P then Q_ (conditional) or _All P are Q_ (categorical). Results support the conclusion that such set size differences affect the strategies people adopt when asked to determine, efficiently, the truth or falsity of such assertions, but they do not entirely negate the tendency to select the term represented by P (the antecedent of the conditional or the subject of the categorical) in preference to the term represented by not-Q (the complement of the consequent of the conditional or the complement of the predicate of the categorical). Several possible explanations of this perseverance are considered. Results also support the conclusion that people find it easier to assess categorical assertions than logically equivalent conditional assertions