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Spearman's General Intelligence (g): Spearman's two-factor theory

The conceptual foundation of modern psychometrics. Charles Spearman's 1904 paper proposed that all cognitive abilities share a common factor he called g (general intelligence). Spearman invented factor analysis to demonstrate g empirically. The 1904 paper is the most-cited single document in the history of intelligence research.

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About the Spearman's General Intelligence (g)

By 1903, Charles Spearman had a hypothesis that he could not quite prove: he believed all cognitive abilities shared a common underlying factor, and that observed differences between tests reflected this common factor plus test-specific factors. The problem was statistical - the methods available at the time could not test his hypothesis on real data.

Spearman invented the missing methods. His 1904 paper introduced tetrad differences (a precursor to factor analysis) to demonstrate that intercorrelations between cognitive tests followed a specific pattern: they could all be explained by a single general factor (g) plus test-specific factors (s). This was the two-factor theory of intelligence.

Spearman's two-factor theory dominated British psychometrics for fifty years. American psychologists (Thorndike, then Thurstone) initially rejected it in favor of multi-factor models, but by the 1990s the Carroll-Cattell-Horn synthesis had brought g back: modern hierarchical models accept g at the top of the cognitive ability hierarchy with broad and narrow factors below. The 1904 paper remains the most-cited document in intelligence research.

Copyright note: Spearman's 1904 paper proposes a theory and demonstrates it with factor analysis on existing data. There is no 'Spearman test' per se - his framework was applied to other tests (notably Binet items). The page below describes the theoretical framework and shows a sample tetrad-difference calculation. There are no interactive items because Spearman did not propose specific test items.

The 1 subtests

#1
Spearman's Theoretical Framework (no test items) Spearman's 1904 paper is a theoretical and statistical contribution, not a test instrument. The 'method' is factor analysis applied to existing test batteries.
Theory / Statistical Method

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Spearman, C. (1904). General Intelligence, Objectively Determined and Measured. American Journal of Psychology, 15(2), 201-292.

Public domain. Charles Spearman (1863-1945) was a British military officer turned psychologist who studied under Wilhelm Wundt at Leipzig. His 1904 paper is the founding document of modern psychometric theory. Read it on Internet Archive: https://archive.org/details/jstor-1412107.

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Historical test materials are obsolete and are not valid modern IQ assessments. This page is preserved for educational, research, and historiographic purposes.

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