Proving a Disjunctive Rule

This experiment was designed to determine whether individuals reason correctly about disjunctive rules. The task consisted in the selection of appropriate instances either to prove, or to disprove, a given disjunctive rule. When the first component of the rule was negated, i.e. when the rule was logically equivalent to implication (p v q), the selection of appropriate instances was significantly more difficult than when the first component was not negated. The majority of subjects, however, revealed patterns of reasoning which were unstable and labile. The results are discussed in relation to those of previous experiments in which subjects had to reason about conditional sentences.