Is working memory capacity a causal factor in fluid intelligence?

It is well established that measures of reasoning ability and of working memory capacity (WMC) correlate positively. However, the question of what explains this relationship remains open. The purpose of this study was to investigate the capacity hypothesis, which ascribes causality to WMC. This hypothesis holds that people high in WMC are more successful in capacity-demanding cognitive tasks than people lower in WMC because they can temporarily maintain more information in the form of sub-goals, hypotheses, and partial solutions. Accordingly, this hypothesis predicts that the correlation between WMC and reasoning performance should increase as the capacity demands of the reasoning items increase. We tested this prediction using items from Raven's Progressive Matrices and two measures of WMC, complex span and the k estimate from the Visual Arrays task. Neither WMC measure showed the effect predicted by the capacity hypothesis. Furthermore, the results cannot be attributed to restriction of range in performance on the individual reasoning items. This finding adds to existing evidence calling into question the capacity hypothesis, and, more generally, the view that WMC has a causal influence on fluid intelligence.