Hypnotic susceptibility

Hypnotic susceptibility measures how easily a person can be hypnotized. Several types of scales are used; however, the most common are the Harvard Group Scale of Hypnotic Susceptibility and the Stanford Hypnotic Susceptibility Scales.

The Harvard Group Scale (HGSS), as the name implies, is administered predominantly to large groups of people while the Stanford Hypnotic Susceptibility Scale (SHSS) is administered to individuals. No scale can be seen as completely reliable due to the nature of hypnosis. It has been argued that no person can be hypnotized if they do not want to be; therefore, a person who scores very low may not want to be hypnotized, making the actual test score averages lower than they otherwise would be.

Hypnotic susceptibility scales, which mainly developed in experimental settings, were preceded by more primitive scales, developed within clinical practice, which were intended to infer the "depth" or "level" of "hypnotic trance" on the basis of various subjective, behavioural or physiological changes.

The Scottish surgeon James Braid (who introduced the term "hypnotism"), attempted to distinguish, in various ways, between different levels of the hypnotic state. Subsequently, the French neurologist Jean-Martin Charcot also made a similar distinction between what he termed the lethargic, somnambulistic, and cataleptic levels of the hypnotic state.

However, Ambroise-Auguste Liébeault and Hippolyte Bernheim introduced more complex hypnotic "depth" scales, based on a combination of behavioural, physiological and subjective responses, some of which were due to direct suggestion and some of which were not. In the first few decades of the 20th century, these early clinical "depth" scales were superseded by more sophisticated "hypnotic susceptibility" scales based on experimental research. The most influential were the Davis-Husband and Friedlander-Sarbin scales developed in the 1930s.

A major precursor of the Stanford Scales, the Friedlander-Sarbin scale was developed in 1938 by Theodore R. Sarbin and consisted of similar test items to those used in subsequent experimental scales.

The Stanford Scale was developed by André Muller Weitzenhoffer and Ernest R. Hilgard in 1959. The Scale consists of three Forms: A, B, and C. Similar to the Harvard Group Scale, each Form consists of 12 items of progressive difficulty and usually takes fifty minutes to complete. Each form consists of motor and cognitive tasks but vary in their respective intended purpose. The administrator scores each form individually.

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