Humans have recognized the difference between having knowledge and being able to use it for at least 2,000 years. In 1963, Raymond B. Cattell was the first to propose a psychometric model for these distinct abilities (1963). Horn and Cattell (1966) further developed his theory and Horn (Horn 1968, 1982; Horn and Noll 1997) applied it to changes in cognitive abilities in old age. Horn showed that our abilities to solve problems rapidly and accurately (and so achieve high scores on intelligence tests), to respond fast to simple signals, and to quickly learn unfamiliar material such as lists of random words peak in our early twenties but decline as we grow older. These changes are slight from our 30s through our 50s but accelerate throughout our 60s, 70s, and 80s. Horn and Cattell termed these “fluid abilities” or “fluid intelligence” (gf) because they are not specific to particular problems but support performance in all mental tasks. In contrast to waning fluid abilities, Horn showed...
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