BIAS IN CONDITIONAL INFERENCE - IMPLICATIONS FOR MENTAL MODELS AND MENTAL LOGIC

Three experiments are reported in which subjects are given the opportu nity to make any of the four inferences associated with conditional st atements: modus ponens (MP), denial of the antecedent (DA), affirmatio n of the consequent (AC), and modus tollens (MT). The primary purpose of the research was to establish the generality and robustness of pola rity biases that may be occasioned by systematic rotation of negative components in the conditional rules. In Experiments 1 and 2, three for ms of conditionals were used: ''if (not) p then (not) q'', ''(not) p o nly if (not) q'' and ''(not) q if (not) p''. Experiment 1 used a concl usion evaluation task, whereas Experiment 2 used a conclusion producti on task. In Experiment 3, thematic conditionals were presented with an d without a preceding scenario. The biases investigated were (a) affir mative premise bias-the tendency to draw more inferences from affirmat ive premises and (b) negative conclusion bias-the tendency to draw mor e inferences with negative conclusions. The suggestive evidence for af firmative premise bias in the literature was not supported: very littl e evidence was found for it in the current experiments. Robust finding s of negative conclusion bias were, however, found across the three ex periments, although the bias was mostly restricted to DA and MT infere nces. This suggests that the bias is best regarded as a difficulty wit h double negation. The results are discussed with respect to both the mental logic and mental model accounts of propositional reasoning. Nei ther theory as currently formulated can explain all of our findings, a lthough a plausible revision of each is considered.