Gc and Gf are both valid intelligence factors: Commentary on Robinson (2005)

We examine objections raised by Robinson (2005) to arguments in Ashton, Lee, and Vernon (2001), with reference to three methods we suggested for comparing the extent to which fluid (Gf) and crystallized (Gc) abilities represent “intelligence”. We suggest that results of analyses using these methods do not support the arguments that “Gc is the true intelligence or IQ factor” (Robinson, 1999, p. 727) or that traditional measures of Gf, such as Raven's Matrices or Block Design, “are certainly not good measures of intelligence” (Robinson, 1999, p. 732). Instead, these results show that measures of crystallized and fluid abilities show similar relations with various intelligence criteria. In addition, we note that the factor analyses by Robinson (1999) tended to separate IQ scores from fluid ability subtests, by using age-adjusted IQ scores in a sample in which fluid ability was strongly negatively correlated with age, and by using an IQ score based on all ability subtests except one fluid ability subtest. We discuss the issue of crystallized bias in the Wechsler test batteries, and the relations of fluid and crystallized abilities with age and with biological markers of intelligence.