"As observations have accumulated, it has become more and more probable that hypnosis is not a single, unitary thing. It appears, rather, to be a more or less loosely related group of phenomena (p. 23)."
Over 70 years ago, the experimental psychologist Clark L. Hull (1884-1952), known perhaps best for his learning theories and research, wrote his seminal work on Hypnosis and Suggestibility: An Experimental Approach
(1933). It is one of the true classics in experimental hypnosis research that remains quite relevant today and can easily be savored by those individuals interested in clinical or experimental hypnosis work. Thus, it is a joy to find this classic republished now for the second time with a new introduction by Michael Yapko. Even though some of Hull's findings may now be controversial or even incorrect, as Dr. Yapko states in his introduction to this reprint, one of Hull's major goals and contributions was to stimulate high quality hypnosis research by subsequent researchers.
Drawing on his many studies, first done at the University of Wisconsin and later at Yale University, Hull's purpose in writing this book was to present a body of experimental material, involving normal rather than pathological subjects, to the public. Hull was assisted by 20 research associates to whom his book was dedicated "in remembrance of our united efforts to establish hypnotism on a secure experimental basis." Guided by John Stuart Mill's (1919) "method of difference," he presents many controlled experiments with sound methodology designed to investigate what effects hypnosis produced, if any, beyond that which could be produced in non-hypnotic conditions. Hull managed to give the field a place of respectability in the scientific community and literally set the stage for future researchers to investigate hypnosis. He further stated that the applied use of hypnosis would benefit from scientific experimentation. Due to conflict with Yale University's psychiatry department and administration over his research program, upon the completion of his book, Hull sadly left further hypnosis work to be done by others (Hilgard, 1987).
For the readership of the American Journal of Clinical Hypnosis, there are two chapters of particular interest that are intriguing yet controversial even by contemporary standards: "The Recovery of Lost Memories in the Hypnotic Trance" and "Hypnosis and the Dissociation Hypothesis." Even though Hull reports a case of seemingly accurate hypnotic recall of an accident for which the subject had almost complete amnesia, he also reports a case of memory fabrication in recalling an early memory and cautions that subjects may be fabricating recalled memories. In addition to prior experimental research, he reviewed his associate Huse's paired-associate recall paradigm in which she found that recall was slightly better in the normal state than in the hypnotized state. Hull concludes that hypnosis does not usually aid in the recovery of recently acquired material. He goes on to conclude "There is some striking experimental evidence which, while not absolutely convincing, tends strongly to confirm the clinical observations that hypnosis facilitates the recall of childhood and perhaps other remote memories" (p.127).
The chapter on hypnosis and the dissociation hypothesis reviews Pierre Janet's influential conception of dissociation, Morton Prince's attempt to test the dissociation hypothesis, and Burnett's studies of carrying out two simultaneous tasks when both tasks were conscious or one was "subconscious." This set the stage for the well-known experimental studies of the dissociation hypothesis by his laboratory associates Messerschmidt and Mitchell. He concluded that their results suggest rather strongly that the whole concept of dissociation as functional independence is an error. It is to be hoped that the situation is now sufficiently clarified that the near future will see a series of well controlled, large-scale investigations which will completely remove the uncertainties which at present becloud this extremely important problem (p. 191).
Due to World War II and the lack of interest in scientific hypnosis research, we waited much longer than Hull would have liked! Ernest Hilgard at Stanford University picked up his interest in dissociation, and he and his colleague's work is detailed in Divided Consciousness: Multiple Controls in Human Thought and Action
(Hilgard, 1977). And, still we have not removed all of the uncertainties that Hull referred to, and more research is still needed on this extremely important problem.
Hull begins his book by briefly reviewing the history of hypnosis from its unscientific beginnings with Mesmer, through its progression to the current status existing in the early 1930s. It was in 1923 that Hull became interested in hypnosis. According to Milton H. Erickson (1961), he had an impact on Hull when Erickson, still an undergraduate at the University of Wisconsin, was invited by Hull to provide demonstrations to a seminar of graduate students. As an historian of American psychology. Hilgard (1987) explored this apparent association between Erickson and Hull, and was perhaps surprised to find that Hull attributed his beginning interests not to Erickson but rather to his hypnotizing a student with a phobia. "The only mention of Erickson is of a tooth extraction reported to Hull by Erickson after he already had his medical degree, a case in which Erickson had served as the hypnotist" (Hilgard. 1987, p. 824). Who was correct we will never know, but these little stones humanize our hypnosis ancestors.
Hull devotes one chapter to the broad phenomena of waking and hypnotic suggestibility and provides wonderfully rich and detailed demonstrations. He spends another chapter on direct waking suggestions; which is reminiscent of work by Gheorghiu (Gheorghiu & Reyher, 1982) and others. Hull shows experimentally that suggestion can influence differentially the behavior of individuals in both waking and hypnotic states. He highlights but does not investigate systematically individual differences to the degree that the field does today. Hull investigated suggestibility as correlated to other fundamental variables such as sex, age, intelligence, character, psychoneurotic tendencies, psychoses, delinquency. and drug influence. As with all his conclusions, Hull cautiously reports his findings with the full knowledge that he is relying on statistical probability, and that future research could support or contradict his findings.
In the chapters on hypnotic suggestibility and the transcendence of voluntary capacity, Hull emphasized that hypnotized subjects were more responsive to suggestion and that they experienced analgesia and were less susceptible to pain during experimental procedures, two main findings that are still supported in hypnosis research today. He also found little if any enhancement of motoric strengths or resistance during hypnosis.
Influenced by his interest in learning and the history of facilitation of hypnosis by successive repetitions of the hypnotic state, Hull designed a series of experiments to determine if hypnosis was a habit. He identified six characteristics of habituation, detailed in Chapter Twelve, and proceeded to investigate the extent to which hypnosis conformed to those characteristics. Additionally, Hull tested the effects of repetition on habituation in the waking state and on tonic immobility in a fowl. The experimental results revealed that repeated hypnosis inductions resulted in high conformity to the characteristics he detailed for habituation. Hull concluded that "such a remarkable and detailed conformity of the phenomena of hypnosis to the known experimental characteristics of ordinary habituation can hardly be accidental and without significance. The indication would seem to be that, whatever else hypnosis may be, it is-to a considerable extent, at least -a habit phenomenon..." (p.347). Hull found, to a lesser degree, parallel results for the waking condition.
In the last chapter Hull goes into some detail describing what hypnosis is not. Hypnosis is not inherently rapport or essential catalepsy, is not a form of true sleep, does not inherently involve heightened sensitivity, is not pathological, and does not necessarily involve a state of dissociation. He finally concluded that:
the only thing which seems to characterize hypnosis as such and which gives any justification for the practice of calling it a "state" is its generalized hypersuggestibility. The difference between the hypnotic state and the normal is, therefore, a quantitative rather than a qualitative one (p.391).
We anticipate that all clinicians and researchers interested in hypnosis will place this book on their bookshelf, and that the pages will show evidence of their being read and enjoyed over the years. We agree with Hull's reflection on his own book: "I believe.. that the book itself has been worth doing from the point of view of the advancement of science. I believe that it is an important contribution, that it may mark a new epoch in that form of experimentation, and that it will be read and quoted for a long time, possibly a hundred years (Hull, 1962, p.852, cited by Hilgard, 1968 p. xv). The year 2033 is not far off and we hope Hull's worthwhile book will still be in print.
Erickson, M. H. (1961). Historical note on the hand levitation and other ideomotor techniques. American Journal of Clinical Hypnosis, 3, 196-199
Gheorghiu, V. A., & Reyher, J. (1982). The effect of different types of influence on an "indirect-direct" form of a scale of sensory suggestibility. American Journal of Clinical Hypnosis, 24, 1 91 - 199.
Hilgard. E. R. 1977). Divided consciousness: Multiple controls in human thought and action. New York, John Wiley & Sons.
Hilgard, E. R. (1987). Psychology in American: A historical survey. Forth Worth, TX: Harcourt Brace Jovanovich College Publishers.
Hull, C. L. (1933/1968). Hypnosis and suggestibility. New York: Appleton-Century.
Hull, C. L. (1962). Psychology of the scientist: IV. Passages from the "idea books" of Clark L. Hull. Perceptual and Motor Skills, 15, 807-882.
Mill, J. S. (1919).A system of logic. London: Longmans, Green & Co., 1919.