The next major event in the history of hypnotism came as a result of the progress of behavioural psychology in American university research. Clark L. Hull, an eminent American psychologist, published the first major compilation of laboratory studies on hypnosis, Hypnosis & Suggestibility (1933), in which he conclusively proved that the state of hypnosis and the state of sleep had nothing in common. Hull published many quantitative empirical findings derived from experiments using hypnosis and suggestion and thereby encouraged subsequent research into hypnosis by mainstream academic psychologists. Hull's behavioural psychology interpretation of hypnosis, in terms of conditioned reflexes, rivalled the Freudian psychodynamic interpretation in terms of unconscious transference.