Early Theories

Theories of intelligence start emerging in the early twentieth century. Early theorists disagreed on the question of intelligence as a single factor or multiple components.

Intelligence as One General Ability

Charles Spearman believed that intelligence is general. People who are bright in one area are bright in other areas as well. Spearman came to this conclusion based on research that consistently showed that specific dimensions or factors of intelligence were all measuring pretty much the same thing. Thus, Spearman's "g" factor theory describes intelligence as a single general factor made up of specific dimensions. The three dimensions that are correlated are verbal, spatial, and quantitative intelligence. Verbal intelligence is the ability to solve problems and analyze information using language-based reasoning. Quantitative intelligence is the ability to reason and solve problems by carrying our mathematical operations and using logic. Spatial intelligence describes the ability or mental skill to solve spatial problems such as navigating, and visualizing objects from different angles. His theory was criticized early on because intelligence tests do not measure all aspects of what it means to be intelligent.

Intelligence as Multiple Abilities

Theorists in the Intelligence-as-Multiple-Abilities camp believe that IQ test scores should not be used as the only measurement of intelligence. They believe that other important aspects of intelligence are not measured by IQ tests. Therefore, the multiple-factor theory of intelligence states that different aspects of intelligence are distinct enough that multiple abilities must be considered. L.L. Thurstone, Director of the Psychometric Laboratory and President of the American Psychological Association (1933) believed that intelligence encompasses seven mental abilities that are relatively independent of one another: verbal comprehension, word fluency, number facility, spatial visualization, associative memory, perceptual speed, and reasoning. For an overview of Thurstone's work, go to http://www.intelltheory.com/lthurstone.shtml.

In contrast, American psychologist Raymond Cattell divided mental abilities into two clusters. The first is crystallized intelligence, or abilities such as reasoning and the verbal and numerical skills that are accumulated from education and experience. The second is fluid intelligence, or skills such as spatial and visual imagery, the ability to notice visual details, and rote memory. Fluid intelligence or raw mental ability is considered culture free.

John Carroll extended Cattell's model into a hierarchy of three levels of intelligence: general intelligence which is similar to Spearman's g factor, broad intelligence, and narrow intelligence. Both crystallized intelligence and fluid intelligence as well as memory, learning, and processing speed are considered broad intelligence. Narrow intelligence, on the other hand, consists of approximately 70 distinct abilities such as speed of reasoning, general sequential reasoning for fluid intelligence and reading, spelling, and language comprehension for crystallized intelligence.

Becasue the model includes Cattell and Horn's crystalized and fluid intelligences, it has become known as the Cattell-Horn-Carroll model of intelligence.

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