Abstract

It was hypothesized that the perceived irrelevance of the proposition “Some X are not Y” is a factor contributing to the difficulty of nearly all the determinate syllogisms classed as multiple model by Johnson-Laird and Byrne, according to mental models theory. Experiment 1 supported this hypothesis by showing that subjects frequently correctly evaluate valid “Some … not” conclusions but rarely produce them, even when they have evaluated them elsewhere. Explanations of these findings based on the use of superficial strategies were ruled out. Experiment 2 further supported the hypothesis by showing that performance increased across the no-conclusion, multiple-choice, and evaluation task formats, and that this effect generalized to problems containing the quantifier “only”. However, the initial hypothesis was rejected in light of Experiment 3, which found no difference between multiple-choice and no-conclusion formats when the number of allowable conclusions was controlled for. Nevertheless, superior performance remained in the evaluation format, and it is suggested that offered conclusions may be used as a goal for the reasoning process. This interpretation is supported by the finding that subjects appear to search only for alternative conclusions that maintain the subject-predicate structure of the offered conclusion