Editing G factor (psychometrics) (section)

===''Gf''-G''c'' theory===

{{Main|Fluid and crystallized intelligence}}

[[Raymond Cattell]], a student of Charles Spearman's, modified the unitary ''g'' factor model and divided ''g'' into two broad, relatively independent domains: fluid intelligence (G''f'') and crystallized intelligence (G''c''). G''f'' is conceptualized as a capacity to figure out novel problems, and it is best assessed with tests with little cultural or scholastic content, such as Raven's matrices. G''c'' can be thought of as consolidated knowledge, reflecting the skills and information that an individual acquires and retains throughout his or her life. G''c'' is dependent on education and other forms of acculturation, and it is best assessed with tests that emphasize scholastic and cultural knowledge.<ref name="deary2010"/><ref name="d&k"/><ref>Jensen 1998, 122–123</ref> G''f'' can be thought to primarily consist of ''current'' reasoning and problem solving capabilities, while G''c'' reflects the outcome of ''previously'' executed cognitive processes.<ref>Sternberg et al. 1981</ref>

The rationale for the separation of G''f'' and G''c'' was to explain individuals' cognitive development over time. While G''f'' and G''c'' have been found to be highly correlated, they differ in the way they change over a lifetime. G''f'' tends to peak at around age 20, slowly declining thereafter. In contrast, G''c'' is stable or increases across adulthood. A single general factor has been criticized as obscuring this bifurcated pattern of development. Cattell argued that G''f'' reflected individual differences in the efficiency of the [[central nervous system]]. G''c'' was, in Cattell's thinking, the result of a person "investing" his or her G''f'' in learning experiences throughout life.<ref name="deary2010"/><ref name="horn&mcardle">Horn & McArdle 2007</ref><ref name="d&k"/><ref>Jensen 1998, 123</ref>

Cattell, together with [[John L. Horn|John Horn]], later expanded the [[Gf-Gc model|G''f''-G''c'' model]] to include a number of other broad abilities, such as G''q'' (quantitative reasoning) and G''v'' (visual-spatial reasoning). While all the broad ability factors in the extended G''f''-G''c'' model are positively correlated and thus would enable the extraction of a higher order ''g'' factor, Cattell and Horn maintained that it would be erroneous to posit that a general factor underlies these broad abilities. They argued that ''g'' factors computed from different test batteries are not invariant and would give different values of ''g'', and that the correlations among tests arise because it is difficult to test just one ability at a time.<ref name="deary2010"/><ref name="McGrew 2005"/><ref>Jensen 1998, 124</ref>

However, several researchers have suggested that the G''f''-G''c'' model is compatible with a ''g''-centered understanding of cognitive abilities. For example, [[John B. Carroll]]'s [[Three stratum theory|three-stratum model of intelligence]] includes both G''f'' and G''c'' together with a higher-order ''g'' factor. Based on factor analyses of many data sets, some researchers have also argued that G''f'' and ''g'' are one and the same factor and that ''g'' factors from different test batteries are substantially invariant provided that the batteries are large and diverse.<ref name="d&k"/><ref>Jensen 1998, 125</ref><ref>Mackintosh 2011, 152–153</ref>