The American psychologist L.L. Thurstone disagreed with Spearman’s theory, arguing instead that there were seven factors, which he identified as the “primary mental abilities.” These seven abilities, according to Thurstone, were verbal comprehension (as involved in the knowledge of vocabulary and in reading), verbal fluency (as involved in writing and in producing words), number (as involved in solving fairly simple numerical computation and arithmetical reasoning problems), spatial visualization (as involved in visualizing and manipulating objects, such as fitting a set of suitcases into an automobile trunk), inductive reasoning (as involved in completing a number series or in predicting the future on the basis of past experience), memory (as involved in recalling people’s names or faces, and perceptual speed (as involved in rapid proofreading to discover typographical errors in a text).

Although the debate between Spearman and Thurstone has remained unresolved, other psychologists—such as Canadian Philip E. Vernon and American Raymond B. Cattell—have suggested that both were right in some respects. Vernon and Cattell viewed intellectual abilities as hierarchical, with g, or general ability, located at the top of the hierarchy. But below g are levels of gradually narrowing abilities, ending with the specific abilities identified by Spearman. Cattell, for example, suggested in Abilities: Their Structure, Growth, and Action (1971) that general ability can be subdivided into two further kinds, “fluid” and “crystallized.” Fluid abilities are the reasoning and problem-solving abilities measured by tests such as analogies, classifications, and series completions. Crystallized abilities, which are thought to derive from fluid abilities, include vocabulary, general information, and knowledge about specific fields. The American psychologist John L. Horn suggested that crystallized abilities more or less increase over a person’s life span, whereas fluid abilities increase in earlier years and decrease in later ones.

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