Two studies were conducted to examine possible cases of bias and distortion in the self‐evaluation of an ability, particularly as a function of social comparison information. Despite considerable differences in procedure, both studies found that subjects used strictly logical deductions as the primary method of evaluation. There were two instances of a non‐logical evaluational tendency, but these were not irrational in the sense of a cost–benefit analysis, nor were they easily avoidable given the experimental context. It was proposed that the results were due to the practical importance of having accurate perceptions of one's abilities. The discussion considered the proposition that people are in general highly rational, and will typically display bias and distortion in their judgements only when these tendencies are less costly than the use of