Braid made a rough distinction between different stages of hypnosis which he termed the first and second conscious stage of hypnotism, he later replaced this with a distinction between “sub-hypnotic”, “full hypnotic”, and “hypnotic coma” stages. Jean-Martin Charcot made a similar distinction between stages named somnambulism, lethargy, and catalepsy. However, Ambroise-Auguste Liébeault and 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 In the first few decades of the 20th century, these early clinical “depth” scales were superseded by more sophisticated “hypnotic susceptibility” scales based on experimental research.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. Andre Weitzenhoffer and Ernest R. Hilgard developed the Stanford Scale of Hypnotic Susceptibility in 1959, consisting of 12 suggestion test items following a standardised hypnotic eye-fixation induction script, and this has become one of the most widely-referenced research tools in the field of hypnosis. Soon after, in 1962, Ronald Shor and Emily Carota Orne developed a similar group scale called the Harvard Group Scale of Hypnotic Susceptibility (HGSHS).

Whereas the older “depth scales” tried to infer the level of “hypnotic trance” based upon supposed observable signs, such as spontaneous amnesia, most subsequent scales measure the degree of observed or self-evaluated responsiveness to specific suggestion tests, such as direct suggestions of arm rigidity (catalepsy).

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