Discussion Task Demands and Revising Probabilities in the Selection Task: A Comment on Green,Over, and Pyne

Green, Over, and Pyne's (1997) paper (hereafter referred to as “GOP”) seems to provide a novel approach to examining probabilistic effects in Wason's selection task. However, in this comment, it is argued that their chosen experimental paradigm confounds most of their results. The task demands of the externalisation procedure (Green, 1995) enforce a correlation between card selections and the probability of finding a counterexample, which was the main finding of GOP's experiments. Consequently GOP cannot argue that their data support Kirby's (1994) proposal that people's normal strategy in the selection task is to seek falsifying evidence. Despite this methodological problem, effects of the probability of the antecedent (p) of a conditional rule, if p then q, predicted by Kirby (1994) and by Oaksford and Chater (1994) were observed, although they were inconsistent between Experiments 1 and 2. Moreover, the probability estimates that GOP collected, which are not vulnerable to that methodological criticism, do support the idea that when P (p)> P (q), participants revise P (p) down as suggested by Oaksford and Chater (1994).