Hypnosis and Suggestibility: An Experimental Approach

Hull argues that hypnosis is not a unique trance state qualitatively different from ordinary waking experience, but rather an extreme manifestation of normal suggestibility. He rejects both the mystical view of hypnosis as a special power or force and the pathological view that treats it as a form of hysteria. Suggestibility, for Hull, is a measurable trait: the tendency to respond to verbal and ideational stimuli with changes in behavior, imagery, or perception. Hypnosis is the upper end of a continuum of suggestibility that includes everyday phenomena like placebo responses and ideomotor effects. This experimental and quantitative approach was intended to bring hypnosis into mainstream scientific psychology.

Hull and his students developed a battery of standardized suggestibility tests, including measures of body sway, hand levitation, and responses to progressive relaxation instructions. These tests produced quantitative scores that could be correlated with other measures. The experiments revealed large individual differences in suggestibility that were relatively stable over time, supporting the treatment of suggestibility as a trait-like characteristic. They also showed that susceptibility to one type of suggestion predicts susceptibility to others, and that hypnotic responsiveness is not strongly related to intelligence or to personality traits as measured by the instruments then available. This work established the empirical foundation for the later development of standardized hypnotic susceptibility scales.

Hull treats the historical association between hypnosis and sleep as a misnomer. He argues that the hypnotic state is not physiologically similar to sleep and that the term itself perpetuates confusion. Regarding hysteria, Hull examines the claim that hypnosis is a form of pathological dissociation and finds the evidence weak. Some hypnotic phenomena—such as the production of local anesthesia or the induction of hallucinations—resemble hysterical symptoms, but Hull argues that this similarity reflects shared mechanisms of suggestibility rather than a shared pathology. He concedes that individuals with hysterical tendencies may be more suggestible, but he rejects the equation of hypnosis with hysteria as a clinical category.

The book was recognized as the first major experimental monograph on hypnosis in the United States and established Hull's reputation as a rigorous researcher before he turned to behavior theory. Contemporary reviewers praised its quantitative approach and its success in demystifying hypnotic phenomena. However, some clinical practitioners objected that Hull's laboratory methods ignored the therapeutic contexts in which hypnosis is typically used. The book's influence on subsequent research was substantial: it helped shift the study of hypnosis from a clinical specialty to an experimental science, and it provided the empirical foundation for the development of standardized susceptibility scales by Ernest Hilgard and others. Hull's student Milton Erickson, who became a leading figure in clinical hypnosis, diverged from his mentor's quantitative approach.

Hypnosis and Suggestibility (1933) is Hull's major applied work, published before he turned systematically to the construction of behavior theory. The methodological commitments evident in the book—quantitative measurement, experimental control, the treatment of psychological phenomena as lawful and measurable—anticipate the hypothetico-deductive approach he would develop in his later monographs. In the preface to his subsequent work, Hull noted that the research on suggestibility had been undertaken to determine the conditions under which an organism's behavior is under the control of verbal stimuli, a problem that connects directly to his later interest in the stimulus control of behavior. The book is thus a bridge between his early psychometric work and his mature theoretical system.

Hull's central claim—that hypnosis is continuous with normal suggestibility and not a unique state—remains a live position in contemporary hypnosis research, though it competes with state theories that emphasize distinctive alterations in consciousness. His experimental emphasis on standardized measurement and individual differences is fully incorporated into modern research, where hypnotic susceptibility scales are standard instruments. Some of Hull's specific findings, such as the relationship between suggestibility and personality, have been revised by subsequent research with improved methods. The book's lasting value lies in its demonstration that hypnosis can be studied experimentally and in its influence on the professionalization of hypnosis research within academic psychology.