The concepts of fluid intelligence (gf) and crystallized intelligence (gc) were introduced in 1963 by the psychologist Raymond Cattell. According to Cattell's psychometrically-based theory, general intelligence (g) is subdivided into gf and gc. Fluid intelligence is the ability to solve novel reasoning problems and is correlated with a number of important skills such as comprehension, problem-solving, and learning. Crystallized intelligence, on the other hand, involves the ability to deduce secondary relational abstractions by applying previously learned primary relational abstractions. History Fluid and crystallized intelligence are constructs originally conceptualized by Raymond Cattell. The concepts of fluid and crystallized intelligence were further developed by Cattell and his former student John L. Horn. Fluid versus crystallized intelligence Fluid intelligence (gf) refers to basic processes of reasoning and other mental activities that depend only minimally on prior learning (such as formal and informal education) and acculturation. Horn notes that it is formless and can "flow into" a wide variety of cognitive activities. Tasks measuring fluid reasoning require the ability to solve abstract reasoning problems. Examples of tasks that measure fluid intelligence include figure classifications, figural analyses, number and letter series, matrices, and paired associates. Crystallized intelligence (gc) refers to learned procedures and knowledge. It reflects the effects of experience and acculturation. Horn notes that crystallized ability is a "precipitate out of experience," resulting from the prior application of fluid ability that has been combined with the intelligence of culture. Examples of tasks that measure crystallized intelligence are vocabulary, general information, abstract word analogies, and the mechanics of language. Example application of fluid and crystallized abilities to problem-solving Horn provided the following example of crystallized and fluid approaches to solving a problem. Here is the problem he described: "There are 100 patients in a hospital. Some (an even number) are one-legged but wearing shoes. One-half of the remainder are barefooted. How many shoes are being worn?" The crystallized approach to solving the problem would involve the application of high school-level algebra. Algebra is an acculturational product. is the number of shoes worn, where equals the number of one-legged patients. equals the number of two-legged patients. The solution boils down to 100 shoes. In contrast to the crystallized approach to solving the problem, Horn provided a made-up example of a fluid approach to solving the problem, an approach that does not depend on the learning of high school-level algebra. In his made-up example, Horn described a