The present lack of integration between concepts developed in the domain of individual differences and current theoretical systems has its roots deep in the history of psychology. Bindra & Scheier (9) in discussing this lack of integration point out that "if the psychometric researcher and the experi mentalist agree on anything, and there is some doubt about this, it is that the other kind of psychologist plays in the other league (class B)." The reason for this schism goes back into the history of psychology as a science, for ex perimental psychologists of the nineteenth century were inclined to regard individual differences merely as sources of experimental error which hindered the establishment of general laws. For this reason the study of individual differences developed in its early days without reference to the expanding body of knowledge derived from laboratory experimentation. The transition of psychology from the study of states of mind to the study of behavior did not at first open a place in systematic psychology for the study of variables commonly described as individual differences, for the early type of behavior istic psychology sought laws of the general type R=f(S) which assumes that responses are a function of the stimuli to which the subject is exposed. When the limitation of such laws became apparent, and a third category of variables referred to as intervening variables was introduced, it became possible for experimental psychology and the psychology of individual differences to be come integrated; for intervening variables are those with which the student of individual differences has been primarily concerned with measuring.