History

Charles Spearman, an early psychometrician, found that schoolchildren's grades across seemingly unrelated subjects were positively correlated, and discovered that these correlations reflected the influence of a dominant factor, which he termed g for "general" intelligence. He developed a model where all variation in intelligence test scores can be explained by two factors. The first is the factor specific to an individual mental task: the individual abilities that would make a person more skilled at one cognitive task than another. The second is g, a general factor that governs performance on all cognitive tasks. Spearman's theory proved too simple, however, as it ignored group factors in test scores (corresponding to broad abilities such as spatial visualization, memory and verbal ability) that may also be found through factor analysis.

The accumulation of cognitive testing data and improvements in analytical techniques have preserved g's central role and led to the modern conception of g. A hierarchy of factors with g at its apex and group factors at successively lower levels, is presently the most widely accepted model of cognitive ability. Other models have also been proposed, and significant controversy attends g and its alternatives.