The Discovery of g
Spearman invented 1 a method, factor analysis, that permitted a rigorous statistical test of Spencer's and Galton's hypothesis that a general mental ability enters into every kind of activity requiring mental effort. A well-established empirical finding--positive correlations among measures of various mental abilities--is putative evidence of a common factor in all of the measured abilities. The method of factor analysis makes it possible to determine the degree to which each of the variables is correlated (or loaded) with the factor that is common to all the variables in the analysis. Spearman gave the label g to this common factor, which is manifested in individual differences on all mental tests, however diverse.
Spearman's two-factor theory held that every mental test, however diverse in the contents or skills called for, measures only two factors: g and s, a factor specific to each test. But later research based on larger numbers of tests than were available in Spearman's early studies showed that g alone could not account for all of the correlations between tests. So Spearman had to acknowledge that there are other factors besides g, called group factors, that different groups of tests, each with similar task demands (such as being either verbal, spatial, numerical, or mechanical), have in common.
By comparing tests with high and low g factor loadings, Spearman concluded that g is most strongly reflected in tests that call for the "eduction of relations and correlates," for example, reasoning to solve novel problems, as contrasted with recalling previously acquired knowledge or using already well-learned skills.
Spearman thought of g metaphorically as "mental energy" that could be applied to any and every kind of mental task, and likened group factors and specificity to specialized "engines" for the per-