The theoretical role of individual differences in psychological theory has been a divisive issue among psychologists for a long time (Cronbach, 1957). It has been argued that the divergent positions taken regarding individual diffferences may have their roots in a philosophical disagreement about the proper focus of psychological theory, i.e., whether psychology seeks to conceptualize persons or to predict behaviors (Fiske, 1979). Naturally, the choice of research tools reflects this fundamental schism, resulting in mutually defacing strategies with respect to the role of individual differences in explaining behavior, as exemplified by methods favored by the ``experimentalists'' in one camp and by the ``correlationists'' in the other.
Along with many psychologists today, we conclude that neither approach, by itself, is able to give as complete an account of psychological functioning as researchers in each discipline have hoped. The emphasis on treatment variation in the experimental study of behavior has no place for differential (individual difference) variables. Underwood (1975) pointed out that this is clearly a mistake because establishing theoretical links to differential variables is necessary for theory construction. In focusing on the products of behavioral processes, differential psychology assumes that traits are sufficient for describing behavior. This too becomes questionable when critics point out that behavioral predictions from traits have been dismal (e.g., Mischel, 1968), just as others start to question the very status of trait concepts themselves (e.g., Schweder, 1975). As these difficulties become better recognized, researchers from both disciplines realized that they must not only pool their research talents but that they must, to paraphrase Cronbach (1957, p. 682), bring their collective effort to bear on the same targets. Both behavioral variability and individual differences should be considered together for a fuller description of psychological behavior (Brower, 1949; Cattell, 1952; Cronbach, 1957; Gulliksen, 1974; Hull, 1945; Underwood, 1975).