Can any ostrich fly?: some new data on belief bias in syllogistic reasoning

According to one version of the mental models theory (Oakhill, J.V., Johnso n-Laird, P.N., Garnham, A., 1989. Believability and syllogistic reasoning. Cognition 31, 117-140) beliefs exert their influence on reasoning in three ways. First they can affect the interpretation of the premises, for example by conversion. Second, they can curtail the search for alternative models of the premises, if an initial model supports a believable conclusion. Thir d, they can act as a filter on any conclusion that is eventually generated. This last influence is important in explaining the effects of belief bias in one-model syllogisms with no convertible premises, since such syllogisms , by definition, have no alternative models. However, the most natural inte rpretation of such a filter is that it filters out conclusions and leads to the response 'no valid conclusion'. The present study, which was conducted with groups of both British and Italian subjects, looked at the effect of prior knowledge on syllogistic reasoning, and showed that: (1) invalid conc lusions for such one model syllogisms, either thematic or abstract, are typ ically not of the type 'no valid conclusion', but state invalid relations b etween the end terms; (2) belief-bias is completely suppressed when previou s knowledge is incompatible with the premises, and therefore the premises t hemselves are always considered. The results an compatible with a version o f the mental models theory in which a representation of prior knowledge pre cedes modelling of the premises, which are then incorporated into the repre sentation of this knowledge. The relation between this theory and other acc ounts of belief bias in syllogistic reasoning, and the implications of thes e findings for reasoning more generally, are considered. (C) 1998 Elsevier Science B.V. All rights reserved.