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.