Measurement of hypnosis

One of the key developments in 20th century hypnosis research was the the development of scales for the reliable measurement of hypnotic susceptibility. 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 assess hypnotic susceptibility has allowed researchers to study hypnosis and its correlates in the laboratory.

Hypnotic susceptibility scales

Hypnotic susceptibility scales consist of a number of test suggestions which participants can either pass or fail. Suggestions are either for motor (ideomotor) or cognitive effects, and either aim to produce or inhibit an effect. Examples of a number of hypnotic susceptibility scales were available at hypnoscales.org but the site currently seems to be down.

A key point is that a 'classic suggestion effect' (Weitzenhoffer, 1980) posits that a suggestion must be experienced as involuntary, that is, it should feel like it is happening by itself.

There is currently some debate as to what 'hypnotizability' scales actually measure. Kirsch and 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 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'.

Suggestions

Susceptibility scales typically contain a number of test suggestions which the participant will either pass or fail. 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).

Types of hypnotic suggestion

Scales

The 'gold standard' of hypnosis scales is currently considered to be the Stanford Hypnotic Susceptibility Scale. This scale is administered individually and consists of a hypnotic induction followed by twelve test suggestions. In research situations a group scale is often used to test large numbers of people, best practice is then to follow up high and low scorers with an individually administered scale.

Many suggestibility scales have been developed, with varying properties. The most common scales are listed below:

Stanford Hypnotic Susceptibility Scale: Forms A and B (SHSS: A and B)
Stanford Hypnotic Susceptibility Scale: Form C (SHSS: C)
Stanford Profile Scales of Hypnotic Susceptibility: Forms I and II
Revised Stanford Profile Scales of Hypnotic Susceptibility: Forms I and II
Childrens Hypnotic Susceptibility Scale (CHSS)
Harvard Group Scale of Hypnotic Susceptibility: Form A (HGSHS: A)
Stanford Hypnotic Clinical Scale for Adults (SHCS: Adult)
Stanford Hypnotic Clinical Scale for Children (SHCS: Child)
Stanford Hypnotic Arm Levitation Induction and Test (SHALIT)
Barber Suggestibility Scale (BSS)
Creative Imagination Scale (CIS)
Hypnotic Induction Profile (HIP)
Carleton University Responsiveness to Suggestions Scale (CURSS)

Properties of hypnotic susceptibility

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 (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 (Lichtenberg 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 et al, 1998; Spanos, 1986). For more information on enhancing hypnotizability see Gorassini, 2004).

Correlates of hypnotic suggestibility

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. 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 used in correlational studies.

Measures which may correlate with hypnotic susceptibility

Measures which do not correlate with hypnotic susceptibility

Absorption (Tellegen & Atkinson, 1974; de Groh, 1989)

Fantasty proneness (Lynn & Rhue, 1988)

Reaction time (Braffman & Kirsch, 2001)

Empathy (Wickramasekera II & Szlyk, 2003)

Self-directedness (one component of the Temperament and Character Inventory, Cloninger et al, 1993) (Laidlaw et al, 2005)

Five factor model of personality (Extroversion-Introversion, Neuroticism, Agreeableness, Conscientiousness, Openness) (Green, 2006)

Dissociation (Measure: DES) (Kirsch & Council, 1992; Faith & Ray, 1994)

Minnesota Multiphasic Personality Inventory (MMPI) (Deckert & West, 1963)

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' (Council, 1993; Council et al, 1996). To get around this problem the assessments of the two measures must be done separately. The resulting correlations have often been reported to be smaller or nonsignificant.

For more detail on this topic, correlates of imaginative suggestibility and hypnotisability have been well reviewed by Kirsch & Braffman (2001), Barnier & McConkey (2004), and de Groh (1989).