An exceptionally skillful operator can individualize their approach and thereby reduce the number of 'unhypnotizable' or 'resistant' subjects quite a bit, but there are still some people that respond much more easily than others to hypnotic suggestion, especially with regard to 'deep trance' phenomena. This responsiveness appears to show high test-retest reliability, even after many years.
There are 12 standard tests in the SHSS (Stanford Hypnotic Susceptibility Scale) which measure how well a subject conforms to the behavior of a classically hypnotized person. By these scales, about 5% of people are classically unhypnotizable, most people show moderate scores, and about 10% are hypnotizable to extreme depths and show the classical deep trance phenomena such as somnambulism, visual and auditory hallucinations, and ability to remain deeply in hypnosis with eyes open.
As mentioned, hypnotizability does not appear to show any obvious correlation with any of the usual personality traits or characteristics. Not only is gullibility not directly correlated, but gender, extraversion/introversion, and neurotic tendencies have also been shown not to correlate well with hypnotizability.
There is some tentative evidence that physiological response to suggestion is influenced by certain forms of sensory deprivation or isolation. For example, see Barabasz and Gregson, "Antarctic wintering--over, suggestion and transient olfactory stimulation: EEG evoked potential and electrodermal responses." in Biological Psychology. 9(4):285-95, 1979 Dec.
EEG evoked potential and electrodermal responses to real and suggested olfactory stimulation were recorded on a team of nine men who wintered -over at Scott Base, Antarctica. Multi-variate analysis of variance findings indicated some consistent trends despite adverse conditions and marked inter-individual differences. Consistent with studies of secondary afferentation olfaction-related EEGs were evidenced in the occipital area (O1 and O2) as well as the temporal area (T3 and T4). Skin conductance (SC) showed significant responses for real and suggested odorants pre and post wintering-over. Suppression of EEG amplitudes for real and suggested stimuli was evidenced prior to wintering-over. Following wintering-over experience suppression of EEG amplitudes for real stimuli showed a decrease while suppression increased for suggested stimuli. The implications of the suggestion findings are discussed in possible explanation of the apparent conflict between different sources of information about human responses to isolation in the Antarctic environment.
4.2. The 'Fantasy Prone Personality'
T.X. Barber and his colleague Sheryl Wilson did some interesting research
where they apparently identitified some loose correlates to hypnotizability,
and which appear to enhance an individual's capacity to respond to hypnotic
Called the 'fantasy prone personality,' (FPP) these correlates do not seem to form a unitary personality type, but represent a diverse group of naturally imaginative and visionary individuals.
Josephine Hilgard and other researchers have also found similar results, that some people have particularly rich inner fantasy lives and cultivate a lifetime of vivid imagery experience corresponding to an openness to unusual experience, extraordinary memory in many cases, capacity for intense concentration, sharp sensory acuity, and unusually strong somatic responses to mental imagery (such as response to placebos).
FPP may also describe the people who most frequently report various psychic phenomena, and 75% of FPP subjects in one study reported having experienced orgasms from fantasy alone. 65% reported fantasies of hallucinatory intensity.
This helps support Barber's earlier contentions about hypnotizable subjects also experiencing similar kinds of phenomena without specific hypnotic induction.
See Wilson and Barber, "The Fantasy Prone Personality: Implications for understanding imagery, hypnosis, and parapsychological phenomena," in Imagery, Current Theory, Research, and Application, from Wiley Press.
4.3. Can hypnotizability be modified?
In spite of the fact that hypnotizability frequently remains constant over
time, the capacity to be hypnotized can be changed to some degree with
specific training of various kinds. The most general requisite capacity, as
we saw in the description of the FPP above is the ability to focus
concentration internally and become extremely absorbed in imaginative
One general review of this work can be found in Michael Diamond's "Modification of hypnotizability: A review," in Psychological Bulletin, 81: 180-198. Diamond examines several experiments where music, silence, psychedelic drugs, biofeedback, sensory deprivation, hypnotic behavioral training, operant conditioning, and relaxation training were used to attempt to modify response to hypnosis.
Another review can be found in Wicramasekera's 1976, Biofeedback, Behavior Therapy, and Hypnosis, which proposes that imagination training, suggestions for adventurousness, use of psychedelic drugs, sensory deprivation, and biofeedback training all can have a lasting effect on hypnotizability.
A different approach is that taken by Gorassini and Spanos, 1986, "A sociocognitive skills approach to the succesful modification of hypnotic susceptibility," Journal of Personality and Social Psychology, 50: 1004-1012. They propose the CSTP, a program for enhancing certain responses under hypnosis by training individual's in the individual behaviors used to measure hypnotic susceptibility. There are likely various different ways in which suggestion-related responses can be produced, and we already have seen that objective measures for distinguishing hypnosis from simulation have not yet been developed, if indeed they are possible or practical at all. Subjective experiential response is apparently not modified with the CSTP, at least not to the degree that 'imagination training' helps modify subjective response to hypnosis. Spanos suggests that further research in this area should make more clear distinctions between compliance, reinterpreting and reclassifying experience, and changes in sensory experience in order to further delineate what is going on in hypnotic responses.
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