During the 20th century, theories of the structure of mental abilities have evolved from the two-factor theory of Spearman, which hypothesized only a single factor common to all tests of ability, to the more differentiated structure of the Horn-Cattell and Carroll models. In these models, the two most widely studied of the second-stratum factors are Gc and Gf. Gc, or crystallized intelligence, reflects stored cultural knowledge and corresponds closely with the verbal factor often reported in factor analyses of the Wechsler batteries. Gf, or fluid intelligence, is a dimension representing reasoning or thinking skills; the performance factor identified in factor analyses of the Wechsler batteries appears to be an amalgamation of Gf and Gv (or visualization skills).

Some movement has already taken place in structuring intelligence tests to acknowledge the utility of the Horn-Cattell and Carroll models. For example, the Stanford-Binet IV yields a composite IQ, but it was based on a theoretical model that included subareas for crystallized abilities (verbal reasoning and quantitative reasoning), fluid-analytic abilities (abstract/visual reasoning), and short-term memory. Furthermore, one battery—the Woodcock-Johnson—was explicitly designed to assess all second-stratum dimensions in the Horn-Cattell model. During the next decade, even greater alignment of intelligence tests and the IQ scores derived from them and the Horn-Cattell and Carroll models is likely. As a result, the future will almost certainly see greater reliance on part scores, such as IQ scores for Gc and Gf, in addition to the traditional composite IQ. That is, the traditional composite IQ may not be dropped, but greater emphasis will be placed on part scores than has been the case in the past. As this movement to part scores develops, it will most likely occur first for Gc and Gf, the most central of the second-stratum factors, and then extend to other second-stratum dimensions as they are determined to be useful for differential prediction.