Charles Spearman investigated correlations among cognitive performances and developed an account of a shared general intelligence factor with task-specific contributions. This helped make intelligence a subject of explicit measurement and statistical modeling. A fitted common factor describes covariation under a model; it is not automatically a uniquely identified biological mechanism. His name is also attached to Spearman's rank correlation coefficient. [ref]
General Intelligence, Objectively Determined and Measured by Charles Spearman. 1904, original correlation analysis and general/specific factor account
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Charles Spearman (1863–1945) was a British psychologist known for his work in statistics and psychology, particularly in the field of intelligence testing. He is best known for developing the concept of the "g factor," or general intelligence, which posits that a common factor underlies various cognitive abilities. Spearman introduced a statistical method called factor analysis, which allows researchers to identify underlying relationships between different variables, including cognitive tasks.