There's more to the working memory capacity-fluid intelligence relationship than just secondary memory.

The present study examined the claim that secondary memory processes account for the correlation between working memory capacity and fluid intelligence via a latent variable analysis. In the present study, participants performed multiple measures of secondary memory, working memory capacity, and fluid intelligence. Structural equation modeling suggested that both secondary memory and working memory capacity account for unique variance in fluid intelligence. These results are inconsistent with recent claims that working memory capacity does not account for variance in fluid intelligence over and above what is accounted for by secondary memory. Rather, the results are consistent with models of working memory capacity that suggest that both maintenance and retrieval processes are needed to account for the substantial relation between working memory capacity and fluid intelligence.