Is Intelligence One Thing, or Many?
Charles Spearman used a statistical technique called factor analysis to examine why people who score well on one type of cognitive test tend to also score well on very different types of cognitive tests. He concluded that all cognitive tests are influenced by a single underlying general intelligence factor, or "g", alongside narrower, test-specific factors ("s") unique to each particular task — his two-factor theory.
Louis Thurstone disagreed with the primacy of a single g factor, instead proposing seven Primary Mental Abilities he considered relatively independent of one another (including verbal comprehension, numerical ability, spatial visualization, and memory) — though later re-analyses found these seven abilities still correlated with each other more than chance alone would predict, suggesting SOME broad general factor persists even underneath Thurstone's multiple abilities.
Raymond Cattell (building on work with John Horn) proposed splitting general intelligence into two distinct components. Fluid intelligence (Gf) is the capacity to reason and solve novel problems, independent of any previously learned knowledge — it tends to peak in early adulthood and gradually decline with age. Crystallized intelligence (Gc) is accumulated knowledge and vocabulary built up through experience and education — it tends to remain stable, or even continue increasing, well into later life.
| Theory | Core claim |
|---|---|
| Spearman | A single general factor (g) underlies all cognitive tests |
| Thurstone | Seven relatively independent primary mental abilities |
| Cattell-Horn | Fluid intelligence (novel problem-solving) versus crystallized intelligence (accumulated knowledge) |
Modern psychometric models often synthesize these views hierarchically: a general factor (g) sits at the top, broad abilities like fluid and crystallized intelligence sit beneath it, and more specific abilities sit at the narrowest level — treating Spearman, Thurstone, and Cattell's ideas as complementary layers rather than flatly competing claims.
Common pitfall: assuming fluid and crystallized intelligence age in the same direction. They diverge sharply: fluid intelligence (novel problem-solving) tends to DECLINE with age after early adulthood, while crystallized intelligence (accumulated knowledge) tends to REMAIN STABLE or even increase — which is why older adults often outperform younger adults on vocabulary and general-knowledge tasks while underperforming on novel, abstract reasoning puzzles.