Rj. Stevenson et De. Over, DEDUCTION FROM UNCERTAIN PREMISES, The Quarterly journal of experimental psychology. A, Human experimental psychology, 48(3), 1995, pp. 613-643
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.