Spearman vs. Thurstone
Is intelligence one single capacity, or a collection of many? Charles Spearman used factor analysis to show that scores on diverse cognitive tests correlate, leading to his two-factor theory: a single general intelligence factor (g) drives all test performance, alongside narrow test-specific factors (s).
Louis Thurstone challenged this, proposing seven Primary Mental Abilities—such as verbal comprehension, spatial visualization, and memory—as independent faculties.
| Theorist | Core Structure | Primary Claim |
|---|
| Spearman | Hierarchical | One core g factor plus task-specific s |
| Thurstone | Pluralistic | Seven distinct, independent mental abilities |
Pitfall: Later re-analyses proved Thurstone's seven abilities still correlate with each other above chance, meaning a broad general factor lurks even beneath his multiple abilities.
Cattell's Fluid and Crystallized Intelligence
Raymond Cattell split general intelligence into two distinct components that age in opposite directions.
Fluid intelligence (Gf) is the capacity to reason and solve novel problems independently of learned knowledge. It peaks in early adulthood and declines with age.
Crystallized intelligence (Gc) is accumulated knowledge and vocabulary built through experience. It remains stable or increases well into later life.
| Intelligence Type | Core Definition | Trajectory with Age |
|---|---|---|
| Fluid (Gf) | Novel problem-solving | Declines after early adulthood |
| Crystallized (Gc) | Accumulated knowledge | Stable or increases with age |
Modern models synthesize these views hierarchically: g sits at the top, broad abilities like Gf and Gc sit beneath it, and narrow skills form the base.