Fluid intelligence is the ability to use information, work with it, apply it, and so on. Solving a Rubik’s cube requires fluid intelligence, as does working out why your partner isn’t talking to you when you have no memory of doing anything wrong. In each case, the information you have is new and you have to work out what to do with it in order to arrive at an outcome that benefits you.
Crystallized intelligence is the information you have stored in memory and can utilize to help you get the better of situations. Knowing the lead actor in an obscure 1950s film for a trivia game requires crystallized intelligence. Knowing all the capital cities of the northern hemisphere is crystallized intelligence. Learning a second (or third or fourth) language utilizes crystallized intelligence. Crystallized intelligence is the knowledge you have accumulated, where fluid intelligence is how well you can use it or deal with unfamiliar things that need working out.
It’s fair to say that fluid intelligence is another variation of g and working memory; the manipulation and processing of information. But crystallized intelligence is increasingly viewed as a separate system, and the workings of the brain back this up. One quite telling fact is that fluid intelligence declines as we age; someone aged eighty will perform worse on a fluid intelligence test than he or she did aged thirty, or fifty. Neuroanatomical studies (and numerous autopsies) revealed the prefrontal cortex, believed responsible for fluid intelligence, atrophies more with age than most other brain regions.
Contrastingly, crystallized intelligence remains stable over a lifetime. Someone who learns French at eighteen will still be able to speak it at eighty-five, unless they stopped using it and forgot it at nineteen. Crystallized intelligence is supported by long-term memories, which are distributed widely throughout the brain and tend to be resilient enough to withstand the ravages of time. The prefrontal cortex is a demanding energetic region that needs to engage in constant active processing to support fluid intelligence, actions that are quite dynamic and thus more likely to result in gradual wear and tear (intense neuronal activity tends to give off a lot of waste products such as free radicals, energetic particles that are harmful to cells).
Both types of intelligence are interdependent; there’s no point in being able to manipulate information if you can’t access any of it, and vice versa. It’s tricky to separate them clearly for study. Luckily, intelligence tests can be designed to focus mostly on either fluid or crystallized intelligence. Tests that require individuals to analyze unfamiliar patterns and identify odd ones out or work out how they are interconnected are thought to assess fluid intelligence; all the information is novel and needs to be processed, so crystallized-intelligence use is minimal. Similarly, tests of recall and knowledge such as word-list memory, or the aforementioned trivia games, focus on crystallized intelligence.
It’s never quite that simple of course. Tasks where you have to sort unfamiliar patterns still rely on an awareness of images, colors, even the means by which you complete the test ( if it’s rearranging a series of cards, you’ll be using your knowledge of what cards are and how to arrange them). This is another thing that makes brain-scanning studies tricky; even doing a simple task involves multiple brain regions. But, in general, tasks for fluid intelligence tend to show greater activity in the prefrontal cortex and associated regions, and crystallized-intelligence tasks suggest a role of the wider cortex, often the parietal-lobe (the upper-middle bit of the brain) regions, such as the supramarginal gyrus and Broca’s area. The former is often thought of as being necessary for storage and processing of information concerning emotion and some sensory data, while the latter is a key part of our language-processing system. Both are interconnected, and suggest functions requiring access to long-term memory data. While it’s still not clear-cut, there’s mounting evidence to support this fluid/crystallized distinction of general intelligence.
Miles Kingston captures the theory brilliantly: “Knowledge is knowing that a tomato is a fruit; wisdom is not putting it in a fruit salad.” It requires crystallized intelligence to know how a tomato is classed, and fluid intelligence to apply this information when making a fruit salad. You might now think that fluid intelligence sounds a lot like common sense. Yes, that would be another example. But, for some scientists, two distinct types of intelligence are still not enough. They want more.
The logic is that a single general intelligence is insufficient for explaining the wide variety of intellectual abilities humans can demonstrate. Consider soccer players—they often didn’t thrive academically, but being able to play a complicated sport like soccer at a professional level requires a great deal of intellectual ability such as precise control, calculating force and angles, spatial awareness of a wide area, and so on. Concentrating on your job while filtering out the rantings of the obsessive fans takes considerable mental fortitude. The common concept of “intelligence” is clearly a bit restrictive.
Perhaps the starkest examples are “savants,” individuals with some form of neurological disorder, who show an extreme affinity or ability for complex tasks involving math, music, memory, etc. In the film Rain Man, Dustin Hoffman plays Raymond Babbit, an autistic but mathematically gifted psychiatric patient. The character was inspired by a real individual called Kim Peek who was dubbed a “mega-savant” for his ability to memorize, to the word, up to twelve thousand books.
These examples and more led to the development of multiple intelligence theories, because how can someone be both unintelligent in one sphere and a gifted in another if there’s only one type of intelligence? The earliest theory of this nature is probably that put forward by Louis Leon Thurstone in 1938, who proposed that human intelligence was made up of seven Primary Mental Abilities:
Verbal comprehension (understanding words: “Hey, I know what that means!”)
Verbal fluency (using language: “Come here and say that, you acephalous buffoon!”)
Memory (“Wait, I remember you, you’re the cage-fighting world champion!”)
Arithmetic ability (“The odds of me winning this fight are about 82523 to 1.”)
Perceptual speed (spotting and linking details: “Is he wearing a necklace made of human teeth?”)
Inductive reasoning (deriving ideas and rules from situations: “Any attempt to placate this beast is only going to anger him further.”)