The correlation between individual differences in working memory capacity and performance on the Raven's Advanced Progressive Matrices (RAPM) is well documented yet poorly understood. The present work proposes a new explanation: that the need to use a new combination of rules on RAPM problems drives the relation between performance and working memory capacity scores. Evidence for this account is supported by an item-based analysis of performance during standard administration of the RAPM and an experiment that manipulates the need to use new rule combinations across 2 subsets of RAPM items. (Contains 3 figures and 5 tables.)