Definition
Fluid and crystallized intelligence are the two broad components of general cognitive ability proposed by Raymond Cattell and developed by John Horn. Fluid intelligence is the capacity to solve novel problems, reason abstractly, and detect patterns in unfamiliar material — the engine of figuring things out without prior practice. Crystallized intelligence is the body of knowledge, vocabulary, procedures, and learned strategies a person has accumulated through education and experience — what they already know how to do. The two are correlated but track distinctly through the lifespan and respond differently to brain ageing.
Why it matters
How it works
Cattell observed that the tests loading most heavily on general intelligence (g) split into two clusters. One cluster — Raven's matrices, novel pattern problems, working-memory tasks — measures the ability to handle material the test-taker has never seen. The other — vocabulary, general knowledge, comprehension — measures what the test-taker has acquired. The two clusters correlate with each other but show different developmental curves and different sensitivities to brain injury, fatigue, and ageing. That double dissociation is the evidence that they index different underlying capacities.
Neurally, fluid abilities depend disproportionately on the integrity of the prefrontal cortex and the speed and bandwidth of working-memory circuits — both of which decline gradually with age. Crystallized abilities depend on long-term semantic and procedural memory stores, which tend to be more robust. The practical upshot is that older adults can outperform younger ones on tasks where accumulated knowledge dominates, while younger adults retain an edge on tasks demanding fast novel reasoning. Training programmes that promise to raise general intelligence tend to show that gains transfer narrowly — fluid intelligence in particular has proved hard to durably enlarge.