Summary of Fluid vs. Crystallized Intelligence

Fluid and crystallized intelligence are elements of general intelligence, originally identified by Raymond Cattell.[1] The concepts of fluid and crystallized intelligence were further developed by Cattell’s student John L. Horn. Here are Wikipedia’s definitions:

Fluid intelligence or fluid reasoning is the capacity to reason and solve novel problems, independent of any knowledge from the past.[2] It is the ability to analyze novel problems, identify patterns and relationships that underpin these problems and the extrapolation of these using logic. It is necessary for all logical problem solving. Fluid reasoning includes inductive reasoning and deductive reasoning.

Crystallized intelligence is the ability to use skills, knowledge, and experience. It does not equate to memory, but it does rely on accessing information from long-term memory. Crystallized intelligence is one’s lifetime of intellectual achievement, as demonstrated largely through one’s vocabulary and general knowledge. This improves somewhat with age, as experiences tend to expand one’s knowledge.

So the basic difference is that fluid intelligence involves our current ability to reason and to deal with complex information, while crystallized intelligence involves learning, knowledge, and skills acquired over a lifetime. Research has shown that these two factors of general intelligence peak at different times in life. Fluid intelligence peaks early in life,  typically in the lates teens or early twenties, while crystallized intelligence peaks much later in life, often in one’s sixties or seventies. And recent research, using large online samples, reveals that specific mental capabilities peak at different times. Here are the ages at which various capabilities peak:

  • 18-19: Information-processing speed peaks early, then begins to decline.
  • 25: Short-term memory gets better until around age 25.
  • 30: Memory for faces peaks and then starts to gradually decline.
  • 35: Short-term memory begins to weaken and decline.
  • 40s-50s: Emotional understanding peaks in middle to late adulthood.
  • 60s: Vocabulary abilities continue to increase.

(If interested you can find many graphs about age and various mental capabilities here.)

While I am not an expert on this topic the findings above match my experience. I found that teaching symbolic logic became more difficult to teach as I aged but understanding and synthesizing philosophy became easier as the storehouse of my knowledge increased.

I’m not sure how this all relates to clichés like “you can’t teach an old dog new tricks” or “old people are set in their ways.” I am a lifelong learner who believes that unlearning old falsehoods is the essence of having a childlike, inquisitive mind. To keep learning we must fight against the mental grooves that accumulate with time. Still, I admit that I was more open-minded—or impressionable if you prefer—when I was younger.

I suppose then that we need to strike a balance between being open to novel ideas and not discarding previous ones that were adopted after careful and conscientious deliberation. As the late Carl Sagan put it best in one of my favorite books, The Demon-Haunted World: Science as a Candle in the Dark, “Keeping an open mind is a virtue—but … not so open that your brains fall out.”

4 thoughts on “Summary of Fluid vs. Crystallized Intelligence

  1. I’d like to rant about a flaw in most models of cognitive abilities. These start off with a theoretical model defining a set of abilities that are hypothesized to comprise cognitive ability. A test is then devised to measure a subject’s strengths in each of these cognitive abilities. The test is applied to lots of people and various conclusions are drawn from the results.

    The huge flaw in this approach lies in the assumption that the hypothesized set of cognitive abilities properly covers the true cognitive abilities of the human mind. None of these many schemes makes any attempt to test the match. After all, how could we know what the correct set of cognitive abilities might be?

    As it happens, we have an excellent methodology for determining the correct set of cognitive abilities: evolutionary psychology. This field has been slandered by politically motivated groups who prefer to believe that the human mind is infinitely plastic and is shaped solely by cultural factors, not genetic ones. If they are correct, then all of our social problems could be solved by adjustments to our culture. They rail against what they call “genetic determinism”.

    The fact is the some of our mental traits are indeed genetically determined — after all, it’s not a cultural artifact that men and women tend to prefer coitus with the opposite sex. But there are other factors influencing the development of the mind: in utero conditions, parental behavior, peer behavior, and education. I prefer to think of it as a pyramid, with genetic factors laying a foundation on which other factors play their role.

    The fundamental idea of evolutionary psychology is that, as one scholar put it, evolution didn’t stop at the neck. The mind has adapted to environmental pressures just like every other organ in the body. This has produced the “mental modules” model of the mind, in which the mind is seen as like a Swiss Army knife, not a general-purpose computer. We have mental tools for dealing with specific environmental challenges.

    For example, our 3D binocular vision came from the need of our primate ancestors to correctly gauge distances in jumping from branch to branch in the trees. Color vision evolved from the need to determine the ripeness of the fruit upon which these primates fed. Spatial reasoning gained a lot of its power from the need of our hunter-gatherer ancestors to navigate correctly across a large landscape.

    Different scholars slice the pie of the mind into different mental modules, but four in particular seem to command broad agreement: a spatial reasoning module, a linguistic module, an encyclopedic module for cause and effect relationships in the natural world, and a social reasoning module.

    This last module is undeniably an important component of the human mind — and it is completely ignored by the various intelligence tests. I suspect that the academic creators of intelligence tests exclude it because they are unaware of its existence — how many academics do you know who are possessed of even a modicum of social intelligence? This prejudice against social reasoning results in lower scores for females, who are generally better than males at social reasoning (little girls play with dolls and little boys play with toy trucks).

    I also reject the notion of general intelligence. There is nothing in evolutionary psychology to lead us to believe in the existence of general intelligence. There are too many mental modules to enable us to measure the sum total of their performance. Worse, there’s no way to prepare a proper set of weighting factors for the different mental modules. If Joe scores 89% on his spatial reasoning test, and 22% on his social reasoning test, while Jane scores 22% on her spatial reasoning test and 89% on her social reasoning test, who is smarter?

  2. thanks for this Chris. I too believe largely in evolutionary psychology but didn’t think about it when doing this research. JGM

  3. The statistics of intelligence vs. age cited in this essay paint a fairly depressing picture. If one adds in metrics of health and athleticism, all of which generally peak in early adulthood, the picture gets worse.

    But what about “happiness”? Some studies have indicated that happiness increases with age, at least until the late sixties or early seventies. But more recent studies have cast some doubt on that. Regardless, there is a peak after which “happiness” tends to decline with increasing age. What about “wisdom” … don’t we get wiser as we get older? Unfortunately, the demographic analysis of the last Presidential election completely destroys that myth (the over sixty-five year old group went decisively for the Republican candidate).

    Bummer. It seems that the only metric that increases with age is how long you’ve lived. But “how long you’ve lived” ranks pretty low in the list of metrics that define a desirable or meaningful life.

    Based on my age, I’m definitely “past it” as the Brits would say. Yet I remain optimistic about life. How can that be? Two reasons: (1) I have a healthy dose of Stoicism that I should not be troubled or concerned about what is outside my control and (2) I keep striving. As to that second point, there are two poems by Longfellow that John has written essays about on this site: excerpts from “A Psalm of Life” and “Morituri Salutamas”. I often re-read those essays. Those poems capture my goal of trying to keep the heart of a young man while I’m plugging away at some project or activity that I think is worthwhile to me or others.

    OK, now you’re thinking: That guy is really “past it”! Well, that too is outside of my control…

  4. As always thanks for the kind words Jim. They’re really appreciated. And those are great poems by Longfellow. JGM

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