British psychologist Raymond Cattell introduced the concepts of fluid and crystallized intelligence in the early 1940s. He defined fluid intelligence as "the ability to reason, analyze, and solve novel problems—what we commonly think of as raw intellectual horsepower." In contrast, crystallized intelligence "is the ability to use knowledge gained in the past, [like] possessing a vast library and understanding how to use it. It is the essence of wisdom."
"Innovators typically have an abundance of fluid intelligence. It is highest relatively early in adulthood and diminishes starting in one’s 30s and 40s." Older people can find innovation more challenging. Crystallized intelligence is enhanced by "accumulating a stock of knowledge, and it "tends to increase through one’s 40s, and does not diminish until very late in life."
This contrast is illustrated by two example careers. "Dean Keith Simonton has found that poets—highly fluid in their creativity—tend to have produced half their lifetime creative output by age 40 or so. Historians—who rely on a crystallized stock of knowledge—don’t reach this milestone until about 60."
The good news is that "no matter what mix of intelligence your field requires, you can always endeavor to weight your career away from innovation and toward the strengths that persist, or even increase, later in life."
For example, "teaching is an ability that decays very late in life, a principal exception to the general pattern of professional decline over time. A study in The Journal of Higher Education showed that the oldest college professors in disciplines requiring a large store of fixed knowledge, specifically the humanities, tended to get evaluated most positively by students. This probably explains the professional longevity of college professors, three-quarters of whom plan to retire after age 65—more than half of them after 70, and some 15 percent of them after 80. (The average American retires at 61.) One day, during my first year as a professor, I asked a colleague in his late 60s whether he’d ever considered retiring. He laughed, and told me he was more likely to leave his office horizontally than vertically."
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