Intelligence has traditionally been considered as more or less a single entity. English psychologist Charles Spearman first suggested this notion in 1927. He observed that people who are good at one type of thinking or cognition tend to do well in other types as well. This general ability is called the g factor.

Spearman arrived at the g theory when he found positive relationships among scores on the subtests of intelligence tests. He theorized that this positive relationship meant that the tests were measuring something in common—that general ability was expressed to some degree in all of them. According to Spearman this was evidence of the g factor.

Some correlations between subtests are much higher than others. This means some other abilities in addition to the g factor must be present. Spearman named these “s factors” for specific abilities.