The authors, to their credit, describe the evolving process of change and modification of the cognitive theory of depression in response to questions, criticisms, and new findings since the early 1960s, when Beck published his initial formulations. For example, complicating research on depression, the authors note that "pure" depressive states may be quite rare, with up to 75% of depressed patients showing comorbidity. Anxiety disorders co-occur in up to 50% of depressed individuals. Other changes that modified the Zeitgeist of cognitive behavior theory were Beck’s incorporation of Bandura’s social learning theory concepts, which rejected the strict and oversimplified stimulus-response assumptions of behaviorism; aspects of Seligman’s learned helplessness model; and Albert Ellis’s rational emotive therapy, which included the notion that irrational beliefs and negative thinking underlie psychological disturbances. As further research cast doubt on some of Ellis’s ideas, rational emotive therapy became less a part of Beck’s cognitive theory.