Debiasing by instruction: The case of belief bias

Abstract The study is concerned with the question of whether robust biases in reasoning can be reduced or eliminated by verbal instruction in principles of reasoning. Three experiments are reported in which the effect of instructions upon the belief bias effect in syllogistic reasoning is investigated. Belief bias is most clearly marked by a tendency for subjects to accept invalid conclusions which are a priori believable. Experiment 1 attempted to replicate and extend an experiment reported by Newstead, Pollard, Evans and Allen (1992). In contrast with their experiment, it was found that belief bias was maintained despite the use of augmented instructions which emphasised the principle of logical necessity. Experiment 2 provided an exact replication of the augmented instructions condition of Newstead et al., including the presence of problems with belief-neutral conclusions. Once again, significant effects of conclusion believability were found. A third experiment examined the use of elaborated instructions which lacked specific reference to the notion of logical necessity. The use of these instructions significantly reduced the effects of belief on the reasoning observed. Taking the current findings together with the experiment of Newstead et al., the overall conclusion is that elaborated instructions can reduce the belief bias effect in syllogistic reasoning, but not eliminate it. This conclusion is discussed with reference to (1) the practical implications for improving thinking and reasoning via verbal instruction and (2) the nature of the belief bias phenomenon.