What is fluid intelligence?
Fluid intelligence, often written Gf, is reasoning with information you have not met before. It is what you use to find the rule in a sequence, work out how an unfamiliar device operates or plan a route through a problem with several steps. It depends heavily on working memory, the ability to hold several pieces of information in mind and manipulate them.
Tests of fluid intelligence avoid knowledge on purpose. Typical items are matrix reasoning puzzles, number and letter series, and abstract classification tasks. Because they use shapes and simple numbers, they depend less on language and schooling than other parts of an IQ test, although no test is entirely free of culture.
What is crystallized intelligence?
Crystallized intelligence, Gc, is the store of knowledge and skills you have acquired through education and experience, and your ability to use it. Vocabulary, general knowledge, reading comprehension and familiarity with how things work in your culture all count.
It is measured with vocabulary items ("What does ephemeral mean?"), general-knowledge questions and tasks that ask how two concepts are alike. Because it reflects learning, it depends on schooling, reading habits and the language of the test. A highly capable person tested in their second language can score well below their true level on crystallized subtests.
Where the distinction comes from
The psychologist Raymond Cattell proposed the split in the early 1940s and developed it with John Horn in the 1960s. Cattell's idea was that fluid ability is invested over the years in learning, and the result "crystallizes" into knowledge. People with high fluid ability tend to build up crystallized ability faster, which is why the two correlate strongly in adults.
In 1993 John Carroll reanalysed more than 460 data sets and arrived at a three-level model with general intelligence at the top. The combined Cattell-Horn-Carroll (CHC) model now underlies most modern IQ tests. Gf and Gc are two of its broad abilities, alongside visual processing, short-term memory, processing speed and others. Our CHC-format cognitive test is built on that structure.
How the two change with age
Fluid and crystallized abilities follow different paths across life:
- Fluid intelligence rises quickly through childhood, peaks in the early to mid twenties and then declines gradually. The decline becomes noticeable from the fifties and is steeper in processing speed.
- Crystallized intelligence keeps rising through adulthood and typically holds steady into the sixties or seventies, declining only late in life.
This is one reason older adults can be slower on unfamiliar puzzles yet better at tasks that draw on experience. IQ scores are age-normed, so neither trend shows up in the final number: a 70-year-old is compared with other 70-year-olds.
Can you improve fluid intelligence?
In 2008 a widely reported study suggested that training on a working-memory task, the "n-back", raised fluid intelligence. Later, larger studies and meta-analyses found that such training improves the trained task and closely similar ones but does not transfer to fluid intelligence in general.
Education is the most reliable known influence. A 2018 meta-analysis estimated that each extra year of schooling raises IQ by roughly 1 to 5 points. Practice on a specific item type also raises scores on that type, which is why retesting with the same kind of test inflates results. Crystallized intelligence can be built throughout life simply by reading, learning and staying curious.
How IQ tests measure each
The WAIS-5 measures fluid reasoning with Matrix Reasoning and Figure Weights, and crystallized ability through its Verbal Comprehension subtests, Similarities and Vocabulary. Short online tests usually lean on fluid items because they translate across languages. Mixed item types such as verbal analogies draw on both: you need to know the words, which is crystallized, and to find the relation between them, which is fluid. When a person's fluid and crystallized scores differ by a wide margin, clinicians treat the full-scale IQ with caution and report the two separately, because the single number then hides more than it shows. Our IQ test is mostly fluid reasoning; the WAIS-format test adds verbal items to give a broader picture.
Sources
Cattell, R. B. "Theory of fluid and crystallized intelligence: a critical experiment". Journal of Educational Psychology, 1963.
Carroll, J. B. Human Cognitive Abilities: A Survey of Factor-Analytic Studies. Cambridge University Press, 1993.
Melby-Lervåg, M., Redick, T. S. and Hulme, C. "Working memory training does not improve performance on measures of intelligence or other measures of far transfer". Perspectives on Psychological Science, 2016.
Ritchie, S. J. and Tucker-Drob, E. M. "How much does education improve intelligence? A meta-analysis". Psychological Science, 2018.
Frequently asked questions
What is fluid intelligence?
The ability to reason and solve new problems without relying on what you already know, for example finding the rule in a pattern. It peaks in early adulthood.
What is crystallized intelligence?
The knowledge and skills you have accumulated, such as vocabulary, facts and learned methods, and your ability to apply them. It keeps growing well into later adulthood.
What is the difference between fluid and crystallized intelligence?
Fluid intelligence is reasoning with new information; crystallized intelligence is using information you have learned. Fluid ability declines gradually after the twenties, while crystallized ability holds up into the sixties or seventies.
Which declines with age, fluid or crystallized intelligence?
Fluid intelligence. It falls gradually from the late twenties, with a clearer drop from the fifties. Crystallized intelligence stays stable or rises until late in life.
Can you improve fluid intelligence?
Not much through brain-training games; meta-analyses find no general transfer. Education does raise IQ, by roughly 1 to 5 points per extra year of schooling according to a 2018 meta-analysis.
Is IQ fluid or crystallized intelligence?
Both. A full-scale IQ combines fluid reasoning, verbal knowledge, working memory and processing speed. Short online tests usually weight fluid reasoning more heavily.