A common explanation for individual differences in the ability to draw rule-based inferences, when a putative conclusion suggests a competing belief-based inference, is that the ability to do so depends on working memory capacity. In the following studies, we examined the hypothesis that the ability to draw rule-based inferences in belief bias tasks can also be explained by individual differences in reasoning strategies and in the related attentional focus. The dual-strategy model differentiates counterexample and statistical strategies that involve different information-processing styles. In the first study (N = 139), participants completed a working memory task (operation span), a strategy diagnostic questionnaire, and a belief bias task. The results showed that individual differences in strategy use predicted performance in the belief bias problems over and above any effects of working memory capacity, with counterexample reasoners producing rule-based inferences more often than statistical reasoners. In the second study (N = 196), an eye-tracking methodology was used as a process-tracing technique to investigate attentional differences between the two strategies. On problems showing a conflict between rule-based and belief-based information, counterexample reasoners demonstrated longer fixation times on the premises than did statistical reasoners, thus providing direct evidence that individual differences in strategy use reflect different processing styles. These results clearly indicate that individual differences in strategy use are an important determinant of the way that people make inferences when rule-based and belief-based cues are both present.
