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. We 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.

Evans, Barston and Pollard, (1983) found that on the syllogistic evaluation task participants tended to endorse believable conclusions as being valid but reject unbelievable conclusions as invalid. A phenomenon known as "Belief Bias". Additionally, they collected verbal protocols from participants and established that this influence of belief was primarily associated with initial reference to the conclusions of these syllogistic arguments. In contrast, better logical reasoning was associated with initial reference to the premises. This experiment was designed to try to direct participants' anention to either the conclusion or the premises of a syllogistic argument with the intention of manipulating participants' logical reasoning ability and susceptibility to belief. The results reflected an inability to alter the influence of beliefs, but in one condition where the conclusion was presented prior to the premises, there was a successful reduction in participants' reasoning ability. The results are discussed with respect to the current theories of belief bias.