ABSTRACT
The measurement of individual differences in intellectual functioning is well founded in mental test theory and has gained social significance through an impressive array of instruments that predict socially important criterion achievements, such as job performance and learning outcomes. Yet despite its high level of statistical and methodological sophistication, the differential approach to human intelligence never assumed a central role in the development of psychology as a science, even though intelligence tests are often hailed as one of psychology's greatest practical triumphs. The increasing isolation of differential psychology from the mainstream of psychology during the first half century of scientific study of individual differences was in large measure due to the fact that differential psychology was unable to achieve one of its central goals: the identification of the mental processes that underlie intelligent functioning. Instead, differential psychology produced theories that described the organization of individual differences in traits thought to comprise human intelligence. Furthermore, despite the fact that some differential psychologists have long recognized the need to identify the mental processes that produce intelligent behavior (Cronbach, 1957; Freeman, 1926; McNemar, 1964; Spearman, 1927; Thurstone, 1947), it was only recently that many in the field acknowledged that a research program dominated by factor analyses of test intercorrelations was incapable of producing an explanatory theory of human intelligence.
