- Abstract Informal reasoning fallacies are arguments that are psychologically persuasive but not valid. In the context of informal reasoning fallacies, the truth-value of the reason in support of a given claim is irrelevant to judging the validity of the argument as a whole. This property stands in sharp contrast with the general role of the reason's truth-value in judging arguments in general. The aim of this study was to test the hypothesis that in the context of informal reasoning fallacies the truth-value of a reason influences pupils' judgment of the extent to which the reason supports the claim. The results of the study confirm the research hypothesis.