Working memory capacity and its relation to general intelligence.

In the past decade, cognitive scientists have entertained the notion that working memory capacity (WMC) is the ‘Factor X’ that underlies individual differences in general intelligence (or Spearman's g). Much of the speculation on this topic has been motivated by a series of studies published in 1990 by Kyllonen and Christal, in which they demonstrated strong correlations between WMC and reasoning ability. As impressive as these correlations were, Kyllonen and Christal themselves had serious reservations about the battery of WMC tasks used in their 1990 studies: ‘We concede to a certain degree of arbitrariness in creating tasks according to such a broad and vague definition of their requirements, and readers may find fault with the way we operationalized the working memory factor. But without well-developed models of information-processing requirements of the tasks, we can only proceed with what is available.’

More recently, differential, experimental, and neuroimaging research projects have uncovered important details about the information-processing requirements of WMC tasks. The purpose of this paper is to review this recent progress, and in so doing, re-examine the relation between WMC and g in light of these new findings. The main points to emerge from the review are: (1) the creation of WMC tasks is a much more principled and much less arbitrary endeavor than it was in 1990; (2) WMC and g are indeed highly related, but are not the same construct; and (3) the basis of the WMC–g relation is most likely to be an executive-attention control mechanism, which is mediated by portions of the prefrontal cortex.

The research to be reviewed draws upon methodology from both the differential and experimental traditions of psychology. Because the Kyllonen and Christal data provide an excellent point of departure for this review, we begin with a discussion of the differential approach. We then review experimental investigations into the basic processes contributing to the performance of WMC tasks and consider how those investigations inform the differential data. Finally, we review studies of the neural bases of WMC and g and consider how this work might further constrain theories of working memory, intelligence, and their relation. We conclude with problems that still exist with respect to the measurement of WMC and provide suggestions for future research.

Early investigations of working memory capacity (WMC) and reasoning ability suggested that WMC might be the basis of Spearman's g. However, recent work has uncovered details about the basic processes involved in working memory tasks, which has resulted in a more principled approach to task development. As a result, claims now being made about the relation between WMC and g are more cautious. A review of the recent research reveals that WMC and g are indeed highly related, but not identical. Furthermore, WM span tasks involve an executive-control mechanism that is recruited to combat interference and this ability is mediated by portions of the prefrontal cortex. More combined experimental–differential research is needed to understand better the basis of the WMC–g relation.

In summary, several recent latent variable analyses suggest that WMC accounts for at least one-third and perhaps as much as one-half of the variance in g. What seems to be important about WM span tasks is that they require the active maintenance of information in the face of concurrent processing and interference and therefore recruit an executive attention-control mechanism to combat interference. Furthermore, this ability seems to be mediated by portions of the prefrontal cortex.

Although progress has been substantial in recent years, much more work remains to be done, particularly with respect to the measurement of WMC. Specifically, experimental task analyses need to be conducted on all the different types of WMC tasks that are currently being used in the field. And, latent variable analyses of different WMC tasks need to be conducted to determine the relationship among different WMC tasks and between these tasks and g.