People differ in important and measurable ways along a number of dimensions of human functioning. The study of such individual differences has been undertaken traditionally by psychometricians, and their work has shaped current ideas concerning intelligence and personality. Other branches of psychology have been concerned also with the question of individual differences, but from a different point of view: namely, assuring through methods of control that these differences do not obscure or bias the results of experimental studies. Thus, the psychometrician finds error variance the focus of interest, whereas the experimental psychologist seeks to minimize it relative to treatment variance. The distinction between the two enterprises has been the nature of their respective goals: Psychometrics has concentrated on issues of measurement and structure of proposed dimensions of human difference, whereas experimental psychology has concentrated on general laws of behavior and processes that transcend the boundaries of the individual.

As more traditional views of experimental psychology have been replaced with the newer interests of cognitive psychology, however, there has been an increased interest in the interaction between basic cognitive characteristics of the individual and more complex aspects of cognitive performance. The work of Hunt and his colleagues on components of verbal ability (Hunt, 1978; Hunt, Lunneborg, & Lewis, 1975), for example, has generated a great deal of interest. The appearance of edited volumes devoted to the topic of individual differences in cognitive performance (e.g., Dillon & Schmeck, 1983) further attests to the growing importance of individual-difference concepts in current investigations of human cognition.