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Spearman's two-factor theory proposes that intelligence has two components: general intelligence ("g") and specific ability ("s"). [7] To explain the differences in performance on different tasks, Spearman hypothesized that the "s" component was specific to a certain aspect of intelligence.
When a more appropriate method of analysis, multigroup confirmatory factor analysis, is applied, it has been found that Spearman's hypothesis (i.e., that the difference is due to differences in general intelligence) is only one of several models that could give rise to the observed distributions in test scores (Dolan, 2000).
Thurstone ultimately agreed with Spearman that there was a general factor among ability measures. Subsequently, Raymond Cattell supported a version of the general ability concept theorized by Spearman but highlighted two forms of ability, distinguished by their noegenetic properties: fluid and crystallized intelligence. [13]
This is known as Spearman's two-factor theory. Later research based on more diverse test batteries than those used by Spearman demonstrated that g alone could not account for all correlations between tests. Specifically, it was found that even after controlling for g, some tests were still correlated with each other.
Spearman found that a single common factor explained the positive correlations among tests. Spearman named it g for " general intelligence factor ". He interpreted it as the core of human intelligence that, to a larger or smaller degree, influences success in all cognitive tasks and thereby creates the positive manifold.
The book traces the origins of the idea of individual differences in general mental ability to 19th century researchers Herbert Spencer and Francis Galton. Charles Spearman is credited for inventing factor analysis in the early 20th century, which enabled statistical testing of the hypothesis that general mental ability is required in all mental efforts.
Charles Spearman, bearing in mind the influential theory that he originated—that intelligence comprises both a "general factor" and "special factors" more specific to particular mental tasks—wrote in 1927, "Every normal man, woman, and child is, then, a genius at something, as well as an idiot at something." [104]
Vernon preferred factor analysis for research and applied this approach to intelligence. At the top of his hierarchical model was Spearman 's g and then there were two major group factors; verbal-educational ability ( v:ed ) and practical-spatial-mechanical abilities ( k:m ) which could always be decomposed into smaller factors.