[[ClinPsy}} Belief bias is a bias of deductive reasoning. In a series of experiments by Evans, et al., subjects were presented with deductive arguments (in each of which a series of premises and a conclusion are given) and asked to indicate if each conclusion necessarily follows from the premises given. In other words, the subjects are asked to make an evaluation of logical validity. The subjects, however, exhibit belief bias when they reject valid arguments with unbelievable conclusions, and endorse invalid arguments with believable conclusions. It seems that instead of following directions and assessing logical validity, the subjects base their assessments on personal beliefs.

It has been argued that like in the case of the matching bias, using more realistic content in syllogisms can facilitate more normative performance, and the use of more abstract, artificial content has a biasing effect on performance.


  • Cohen, L.J. (1981). Can human irrationality be experimentally demonstrated? The Behavioral and Brain Sciences, 4, 317-370.
  • Evans, J. St. B. T., Barston, J.L., & Pollard, P. (1983). On the conflict between logic and belief in syllogistic reasoning. Memory and Cognition, 11, 295-306.

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