Fluid and crystallized intelligence are elements of general intelligence, originally identified by Raymond Cattell. 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. 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 late 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 to being more open-minded—or impressionable if you prefer—when I was younger. As I’ve gotten older some ideas have simply been discarded and I don’t have the time or inclination to revisit them.
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 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.”