One of the key developments in 20th century hypnosis research was the the development of hypnotic susceptibility scales for the reliable measurement of hypnotic susceptibility. These allow researchers to understand more about how & why hypnosis works, and some clinicians use them to judge how best to help clients. A person's susceptibility to hypnosis is usually gauged as 'high', 'medium', or 'low'. Approximately 80% of the population are medium, 10% are high and 10% are low. Being able to reliably measure hypnotic susceptibility has allowed researchers to study hypnosis and its correlates in the laboratory.
Hypnotic susceptibility scales normally begin with a quick discussion to allay any fears or misconceptions participants may have, then then proceed to a hypnotic induction which encourages participants to enter a hypnotised 'state'. The scales then consist of a number of test suggestions which participants can either pass or fail. Suggestions are either for motor (movement) or cognitive (thought) effects, and either aim to produce or inhibit an effect. There is a full list of scales on the hypnotic susceptibility scales page.
Susceptibility scales typically contain a number of test suggestions which the participant will either pass or fail. The suggestions chosen are designed to test a range of hypnotic domains. Test suggestions are either 'motor' or 'cognitive' and also either 'positive' or 'negative' in nature. A positive motor suggestion would be trying to produce a motor effect, e.g. a suggestion that the participant's arm will raise all by itself. A negative motor suggetion would be aimed at inhibiting a willed motor action, e.g. a suggestion that the participant will not be able to open their eyes (traditionally called eyelid catalepsy). Cognitive suggestions are aimed at producing or inhibiting perceptions or sensations. An example of a positive cognitive suggestion would be a visual hallucination of someone's best friend. A negative cognitive suggestion might be where it is suggested that a participant cannot hear (hypnotic deafness).
The 'classic suggestion effect' (Weitzenhoffer, 1980) posits that for a suggestion to be genuinely experienced it must be experienced involuntarily. That is: it should feel like it is happening by itself. For example, one item might give a suggestion that your arm is getting lighter and is beginning to float up into the air. If your arm feels like it is floating effortlessly and actually moves up in the air you are said to have passed the item. The subjective involuntariness with which a suggestion is experienced is measured by a number of scales.
There is currently some debate as to what 'hypnotizability' scales actually measure. Kirsch & Braffman (2001) have pointed out that traditional scales (e.g. Harvard, Stanford) measure responsivity to suggestion in hypnosis. However, this does not take into account the fact that people also respond to suggestions without a hypnotic induction. Kirsch proposes that responsivity to suggestion without hypnosis be termed 'imaginative suggestibility', that responsivity to suggestion following a hypnotic induction be termed 'hypnotic suggestibility', and that the difference between the two be termed 'hypnotizability'. Logically this distinction makes a lot of sense, although some have noted that this system would allow the possibility of 'negative hypnotizability' - responding to fewer suggestions when hypnotised. Currently, the distinction between imaginative and hypnotic suggestbility is not universally accepted in the research community (for example, see [Barnier & Nash, 2008]).
Hypnotic susceptibility is generally thought to be a trait which remains stable over time. High levels of test-retest reliability have been observed over periods of 10 (Hilgard, 1965) and 25 years (Piccione et al, 1989). A study assessing the hypnotizability of monozygotic (MZ) and dizygotic (DZ) twins found a correlation of 0.52-0.63 for MZ, but only 0.08-0.18 for DZ twins, indicating the presence of a genetic component to hypnotisability (Morgan, 1973). Some preliminary results have suggested that the presence or absence of two sub-types of the COMT gene can predict hypnotic susceptibility, although these results only held for men and remain to be replicated (Lichetenberg et al, 2000). Similarly, a preliminary finding has been reported of an increased volume of the rostrum, a region of the corpus callosum, in high hypnotizables (Horton et al, 2004). Some researchers argue against the idea that hypnotizability is a trait, and present evidence demonstrating that hypnotic responsivity can be modified (they argue that hypnotic responsiveness is stable unless attempts at change are made). There is disagreement about the magnitude to which hypnotizability can be affected (e.g. Benham, 1998; Spanos, 1986). For more information on enhancing hypnotizability see the page on modifying suggestibility (and Gorassini, 2004).
Ever since the development of measures of hypnotic suggestibility researchers have looked for other personality characteristics which might predict how susceptible someone will be to hypnosis. A strong correlate would allow us to predict hypnotisability without performing a lengthy test, and might also tell us more about what hypnosis is. In a typical correlation study subjects will complete a number of questionnaires measuring varying personality characteristics and will then undergo an assessment of hypnotic suggestibility (commonly a group assessment - correlational studies require a large N and it is time consuming to administer a large number of assessments).
The table below illustrates a variety of measures which have been examined in correlational studies.
|Measures which may correlate with hypnotic susceptibility||Measures which do not correlate with hypnotic susceptibility|
Fantasty proneness 14
Reaction time on a go/no-go task 15
Five factor model of personality (Extroversion-Introversion, Neuroticism, Agreeableness, Conscientiousness, Openness) 19
Minnesota Multiphasic Personality Inventory (MMPI) 22
One factor to be aware of when reviewing these studies is that the assessment of hypnotic susceptibility is usually conducted in the same session as the assessment of other personality characteristics. This methodology has been found to artificially inflate the relationship between the two measures, and demonstrates what is known as the 'context effect' 23, 24. To get around this problem the assessments of the two measures must be done separately without research participants knowing the true nature of the study. Correlations controlling for the context effect have often been reported to be smaller or nonsignificant.
One concept which is often used clinically, but which does not currently seem to have so much traction with researchers, is hypnotic depth. Clinicians often talk about clients being in 'light trance' or 'deep trance', but what does these terms mean?
Traditional hypnotic susceptibility scales, which are a mainstay of hypnosis research, measure responses to suggestion. Participants are said to score more highly the more suggestions they respond to, and responses are often observable. Implicit in their logic is the assumption that hypnotic ability is something which varies between individuals: some people are more highly hypnotisable than others.
Hypnotic depth on the other hand, is a concept which looks for variation within individuals. Someone may go lightly into hypnosis to begin with, but may then choose to enter a deeper trance.
One difficulty with the concept of hypnotic depth is that there is no reliable independent marker for it. Studies of brain activity have so far failed to find a marker of hypnotic depth. Subjective self-report measures are currently the best way to measure hypnotic depth. A selection of measures are listed below.
Hypnotic depth scale (Lecron, 1953)
Lecron, L. M. (1953). A method of measuring the depth of hypnosis. Journal of Clinical & Experimental Hypnosis, 1(2), 4-7.
Long Stanford Scale (Tart, 1970)
Tart, C. T. (1970). Self-report scales of hypnotic depth. International Journal of Clinical and Experimental Hypnosis (18), 105-125.
Davis-Husband Scale (Davis & Husband, 1931)
Davis, L., Husband, R. (1931). A study of hypnotic susceptibility in relation to personality traits. Journal of Abnormal and Social Psychology, 26, 175-182.
Friedlander-Sarbin Scale (1938)
Friedlander, J. W., Sarbin, T. R. (1938). The depth of hypnosis. Journal of Abnormal and Social Psychology, 33, 453-475.