Fluid intelligence shares closer to 60% of its variance with working memory capacity and is a better indicator of general intelligence

• WMC and gf share 60% of their true score variance.
• WMC and gf have equally strong unique factors.
• gf is a stronger indicator of g than WMC.

A number of empirical studies have examined the association between working memory capacity (WMC) and fluid intelligence (gf), with conclusions varying from constructs that are substantially distinct to constructs that are isomorphic. A review of the empirical literature suggests that these disparate conclusions are likely due to a number of factors, including the use of tests of varying quality, tests which share method variance, small sample sizes, and samples of varying representativeness of the population. Consequently, in this investigation, the association between WMC and gf was estimated based on well validated adult Wechsler scale subtests and a large, normative sample (N = 2200). Based on a correlated two-factor model, the correlation between WMC and gf was estimated at r = .77 (95% CI: .74/.80), suggesting a substantial level of shared variance, but a meaningful level of uniqueness, as well. Furthermore, based on a bifactor model and omega specific (ωs), both WMC and gf were found to be associated with approximately equal standardized levels of unique common variance: WMC ωs = .18 (95% CI: .12/.23) and gf ωs = .19 (95% CI: .13/.25). Finally, further evidence of divergent validity was obtained, as the gf subtests were observed to be more substantial indicators of general intelligence (g) than the WMC subtests. It is concluded that WMC and gf share approximately 60% of their true score variance, rather than the commonly cited 50%. Additionally, gf should be considered a better indicator of g than WMC on both empirical and theoretical grounds.