THE EFFECT OF PREMISE ORDER IN CONDITIONAL REASONING - A TEST OF THE MENTAL MODEL-THEORY

The difference in difficulty between modus ponens (if p then q; p; the refore q) and modus tollens (if p then q; not-q; therefore not-p) argu ments has been traditionally explained by assuming that the mind conta ins a rule for modus ponens, but not for modus tollens. According to t he mental model theory, modus tollens is a more difficult deduction th an modus ponens because people do not represent the case not-q in thei r initial model of the conditional. On the basis of this theory, we pr edicted that conditions in which reasoners are forced to represent the not-q case should improve correct performance on modus tollens. In pa rticular, we predicted that the presentation of the minor premise (not -q) as the initial premise should produce facilitation. Experiment 1 s howed that this is the case: whereas the inversion of the premise orde r did not affect modus ponens, it produced a significant increase of v alid conclusions for modus tollens. Experiment 2 showed that this faci litation does not depend on the negative form (contrary vs. contradict ory) of the minor premise. Experiments 3 and 4 (and/or some of their r eplications) demonstrated that facilitation also occurs when participa nts are asked to find the cases compatible with not-q or to evaluate a p conclusion. No premise order effect was found for sentences which ma ke explicit the not-q case right from the start, i.e. p only if q cond itionals and biconditionals (Experiments 5 and 6). Finally, Experiment s 7 and 8 showed that the conditional fallacies are not significantly affected by the premise order.