DEDUCTION FROM UNCERTAIN PREMISES

We investigate how the perceived uncertainty of a conditional affects a person's choice of conclusion. We use a novel procedure to introduce uncertainty by manipulating the conditional probability of the conseq uent given the antecedent. In Experiment 1, we show first that subject s reduce their choice of valid conclusions when a conditional is follo wed by an additional premise that makes the major premise uncertain. I n this we replicate Byrne (1989). These subjects choose, instead, a qu alified conclusion expressing uncertainty. If subjects are given a thi rd statement that qualifies the likelihood of the additional premise, then the, uncertainty of the conclusions they choose is systematically related to the suggested uncertainty. Experiment 2 confirms these obs ervations in problems that omit the additional premise and qualify the first premise directly Experiment 3 shows that the qualifying stateme nt also affects the perceived probability of the consequent given the antecedent of the conditional Experiment 4 investigates the effect of suggested uncertainty on the fallacies and shows that increases in unc ertainty reduce the number of certain conclusions that are chosen whil e affirming the consequent but have no effect on denying the anteceden t. We discuss our results in terms of rule theories and mental models and conclude that the latter give the most natural account of our resu lts.