The Suppression of card selections in Wason's selection task: evidence that inference plays a role

We report the results of two experiments designed to investigate the role of inference in Wason's selection task. In Experiment 1 participants received either a standard one rule problem or a task which contained an additional rule. This additional rule specified an alternative antecedent. Both groups of participants were asked to select those cards they considered necessary to test whether the rule common to both problems was true or false. The results showed a significant suppression of "q" card selections. In addition there was weak evidence for increased "not-q" selection. In Experiment 2 we manipulated number of rules, as before, and the presence or absence of explicit negation on the cards. Once again "q" card selections were suppressed, but there was no evidence of an increase in "not-q" selection. There was also no effect of type of negation. Our results suggest that inferences about the unseen side of the cards underlie participants' selections. W e argue firstly that these findings are inconsistent with current views of selection task performance (Oaksford and Chater, 1994, Evans and Over, 1996) and secondly, that they support accounts which emphasise the role of inference in the task.