Charles Spearman (1863–1945) believed we have one general intelligence (often shortened to g) that is at the heart of all our intelligent behavior, from navigating the sea to excelling in school. He granted that people often have special, outstanding abilities. But he noted that those who score high in one area, such as verbal intelligence, typically score higher than average in other areas, such as spatial or reasoning ability. Spearman’s belief stemmed in part from his work with factor analysis, a statistical procedure that identifies clusters of related items.
This idea of a general mental capacity expressed by a single intelligence score was controversial in Spearman’s day, and so it remains. One of Spearman’s early opponents was L. L. Thurstone (1887–1955). Thurstone gave 56 different tests to people and mathematically identified seven clusters of primary mental abilities (word fluency, verbal comprehension, spatial ability, perceptual speed, numerical ability, inductive reasoning, and memory). Thurstone did not rank people on a single scale of general aptitude. But when other investigators studied these profiles, they detected a persistent tendency: Those who excelled in one of the seven clusters generally scored well on the others. So, the investigators concluded, there was still some evidence of a g factor.
“ g is one of the most reliable and valid measures in the behavioral domain . . . and it predicts important social outcomes such as educational and occupational levels far better than any other trait.”
Behavior geneticist Robert Plomin (1999)
We might, then, liken mental abilities to physical abilities: Athleticism is not one thing, but many. The ability to run fast is distinct from the eye-hand coordination required to throw a ball on target. Yet there remains some tendency for good things to come packaged together—for running speed and throwing accuracy to correlate. So, too, with intelligence. Several distinct abilities tend to cluster together and to correlate enough to define a general intelligence factor. Distinct brain networks enable distinct abilities, with g explained by their coordinated activity (Cole et al., 2015; Hampshire et al., 2012). The result is a chorus of actions orchestrated through the best possible distribution of mental resources (Carroll & Bright, 2016; Lee et al., 2015).