Crystallized intelligence is the ability to retrieve and apply the knowledge, vocabulary, and procedural skill built up through learning and experience. It forms a pair with fluid intelligence and does the central work in situations such as recalling a date from history, explaining what a technical term means, or carrying out a familiar procedure. Its trajectory with age differs markedly from that of fluid intelligence: as long as suitable learning continues, it can keep growing across a lifetime.
Contrast with Fluid Intelligence
Where fluid intelligence finds a rule in a problem met for the first time, crystallized intelligence depends on how well stocked your store of knowledge is and how readily you can reach into it. The two are not independent: one view holds that understanding gained through fluid work accumulates and becomes part of the crystallized store. Their relationship with age is the opposite of each other - fluid intelligence peaks relatively early in adulthood and then declines gradually, while crystallized intelligence can keep growing for a long time as experience accumulates. That overall intellectual functioning is often maintained at a high level well into middle age and beyond is explained partly by these two trajectories offsetting one another.
Its Place in Measurement
In standardized intelligence tests, subtests probing vocabulary and general knowledge capture the crystallized side. Because these items are affected by schooling and native language, comparisons between populations that differ in culture or language also pick up those differences. Two people with the same composite score can have different profiles depending on whether their strength lies on the fluid or the crystallized side, so the composite alone does not show the character of someone's ability.