In an article on contemporary personality psychology in a recent edition of the Annual Review of Psychology, Ravenna Helson and Valory Mitchell note:

As psychologists since Freud have known, jokes and delusions all contain some grain of truth, and as psychologists for over a decade have known, the above quotation is neither entirely a joke nor wholly a delusion.

Personality psychology is a splintered field, for some time in the throes of a move away from global theorizing, emphasizing instead the development of second- and third-order theories and empirical research. In a rather ironic sense, psychology has always had multiple personalities: the first major theorist, Freud, had little influence on academic psychology (especially in America) until around the time of his death; and shortly thereafter the universities produced their first really comprehensive theory of personality, Skinnerian behaviorism, which split the discipline into two mutually hostile camps. From the behaviorist matrix have emerged cognitive social learning approaches that focus on cognitive processes, the interaction between personality variables and specific situations, and socially mediated learning. In the 1950s a “third force” in personality theory arose, which includes existential and humanistic psychology. Research in personality psychology also includes a number of other empirical approaches, collectively known as trait psychology, which attempt the delineation and measurement of various dimensions of personality (e.g., intro- version/extroversion, perceived locus of control, Machiavellianism).