Despite the variety of concepts of intelligence, the approach to understanding intelligence with the most supporters and published research over the longest period of time is based on psychometrics testing. Such intelligence quotient (IQ) tests include the Stanford-Binet, Raven's Progressive Matrices, the Wechsler Adult Intelligence Scale and the Kaufman Assessment Battery for Children.
Charles Spearman is generally credited with discovering general intelligence, which he reported in his 1904 American Journal of Psychology article titled "General Intelligence," Objectively Determined and Measured. Based on the results of a series of studies collected in Hampshire, England, Spearman concluded that there was a common function (or group of functions) across intellectual activities including what he called intelligence (i.e., school rank, which Spearman thought of as “present efficiency” in school courses; the difference between school rank and age, which was conceptualized as “native capacity;” teacher ratings; and peer ratings provided by the two oldest students, which was termed “common sense”) and sensory discriminations (i.e., discrimination of pitch, brightness, and weight). This common function became known as “g” or general intelligence.
To objectively determine and measure general intelligence, Spearman invented the first technique of factor analysis (the method of Tetrad Differences) as a mathematical proof of the Two-Factor Theory. The factor analytic results indicated that every variable measured a common function to varying degrees, which led Spearman to develop the somewhat misleadingly named Two-Factor Theory of Intelligence. The Two-Factor Theory of Intelligence holds that every test can be divided into a “g” factor and an “s” factor. The g-factor measures the “general” factor or common function among ability tests. The s-factor measures the “specific” factor unique to a particular ability test. Based on a more modern interpretation of his work, Spearman’s g factor represents the fact that any set of cognitive ability tests, no matter how different, tend to all correlate positively.
L. L. Thurstone extended and generalized Spearman’s method of factor analysis into what is called the Centroid method and which became the basis for modern factor analysis. Thustone demonstrated that Spearman’s one common factor method (Spearman’s method yielded only a single factor) was a special case of his multiple factor analysis. Thurstone’s research lead him to propose a model of intelligence that included seven orthogonal (unrelated) factors (i.e., verbal comprehension, word fluency, number facility, spatial visualization, associative memory, perceptual speed and reasoning) referred to as the Primary Mental Abilities.
In a critical review of the adult testing literature, Raymond B. Cattell found that a considerable percentage of intelligence tests that purported to measure adult intellectual functioning had all of the trappings of using college students in their development. To account for differences between children/adolescents and adults, which past theory did not address, Cattell proposed two types of cognitive abilities in a revision of Spearman’s concept of general intelligence. Fluid intelligence (Gf) was hypothesized as the ability to discriminate and perceive relations (e.g., analogical and syllogistic reasoning), and crystallized intelligence (Gc) was hypothesized as the ability to discriminate relations that had been established originally through Gf, but no longer required the identification of the relation (commonly assessed using information or vocabulary tests). In addition, fluid intelligence was hypothesized to increase until adolescence and then to slowly decline, and crystallized intelligence increases gradually and stays relatively stable across most of adulthood until it declines in late adulthood.
With his student John L. Horn, Cattell indicated that Gf and Gc were only two among several factors manifest in intelligence tests scores under the umbrella of what became known as Gf/Gc Theory. General visualization (Gv; visual acuity, depth perception), general fluency (F, facility in recalling words), general speediness (Gs; performance on speeded, simple tasks) were among several cognitive ability factors added to Gf/Gc Theory.
J. P. Guilford sought to more fully explore the scope of the adult intellect by providing the concept of intelligence with a strong, comprehensive theoretical backing.The Structure-of-Intellect model (SI model) was designed as a cross classification system with intersections in the model providing the basis for abilities similar to Mendeleev’s periodic table in chemistry. The three-dimensional cube—shaped model includes five content categories (the way in which information is presented on a test; visual, auditory, symbolic, semantic, and behavioral), six operation categories (what is done on a test; evaluation, convergent production, divergent production, memory retention, memory recording, and cognition), and six product categories (the form in which information is processed on a test; units, classes, relations, systems, transformations, and implications). The intersection of three categories provides a frame of reference for generating one or more new hypothetical factors of intelligence.
John B. Carroll re-analyzed 461 datasets in the single most comprehensive study of cognitive abilities. This analysis led him to propose the Three-Stratum Theory, which is a hierarchical model of intellectual functioning. The stratums represent three different levels of generality over the domain of cognitive abilities. At the bottom is the first stratum, which is represented by narrow abilities that are highly specialized (e.g., induction, spelling ability). The second stratum is represented by broad abilities that include moderate specializations in various domains. Carroll identified eight second-stratum factors: fluid intelligence, crystallized intelligence, general memory and learning, broad visual perception, broad auditory perception, broad retrieval ability, broad cognitive speediness, and processing speed (reaction time decision speed). Carroll has noted the similarity of his second stratum abilities and the Gf/Gc factors, although the Three-Stratum Theory does not incorporate the developmental trajectories associated with Gf/Gc Theory. Carroll accepted Spearman’s concept of general intelligence, for the most part, as a representation of the uppermost third stratum.
More recently, an amalgamation the Gf-Gc theory of Cattell and Horn with Carroll's Three-Stratum theory has led to the Cattell-Horn-Carroll (CHC) theory of cognitive abilities. CHC researchers have produced numerous studies that have influenced diagnostic issues and test development.
Intelligence, as measured by IQ and other aptitude tests, is widely used in educational, business, and military settings due to its efficacy in predicting behavior. g is highly correlated with many important social outcomes - individuals with low IQs are more likely to be divorced, have a child out of marriage, be incarcerated, and need long term welfare support, while individuals with high IQs are associated with more years of education, higher status jobs and higher income. Intelligence is significantly correlated with successful training and performance outcomes, and g is the single best predictor of successful job performance.