Hypnotic susceptibility is a measurement of how easily a person can be hypnotized. There are several types of scales used; however, the most common are the Harvard Group Scale of Hypnotic Susceptibility and the Stanford Hypnotic Susceptibility Scales. The Harvard Group Scale, as the name implies, is administered predominantly to large groups of people while the Stanford Hypnotic Susceptibility Scale 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 test scores invalid.
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 In the first few decades of the 20th century, these early clinical "depth" scales were superseded by more sophisticated "hypnotic susceptibility" scales based on experimental research.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 Andre M. Weitzenhoffer and Ernest R. Hilgardin1959. 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.
Based upon the scale developed by Joseph Friedlander and Theodore Sarbin (1938), this form was developed to measure susceptibility to hypnosis with items increasing in difficulty in order to yield a score. The higher the score, the more responsive one is to hypnosis. Following a standardized hypnotic induction, the hypnotized individual is given suggestions pertaining to the list below.
| Item Number | Test Suggestion and Responses | |
| 1 | Postural Sway | |
| 2 | Eye Closure | |
| 3 | Hand Lowering (left) | |
| 4 | Immobilization (right arm) | |
| 5 | Finger Lock | |
| 6 | Arm Rigidity (left arm) | |
| 7 | Hands Moving Together | |
| 8 | Verbal Inhibition (name) | |
| 9 | Hallucination (fly) | |
| 10 | Eye catalepsy | |
| 11 | Post-hypnotic (changes chairs) | |
| 12 | Amnesia |
Form B was designed to be used as a follow-up to Form A when doing experiments involving a second session of hypnosis. The items are similar but are changed somewhat (e.g. the use of the opposite hand in a particular item). The changes were made to "prevent memory from the first exerting too great an influence upon the recall of specific tasks..".[1]
Created a few years after Forms A and B, Form C contains some items from Form B, but includes more difficult items for "when subjects are being selected for advanced tests in which knowledge of their capacity to experience more varied items is required" (pgs v-vi Weitzenhoffer & Hilgard 1962). Following a standardized hypnotic induction, the hypnotized individual is given suggestions pertaining to the list below.
| Item Number | Test Suggestion and Responses | |
| 0 | Eye Closure (not scored) | |
| 1 | Hand Lowering (right hand) | |
| 2 | Moving Hands Apart | |
| 3 | Mosquito Hallucination | |
| 4 | Taste Hallucination | |
| 5 | Arm Rigidity (right arm) | |
| 6 | Dream | |
| 7 | Age Regression (school) | |
| 8 | Arm Immobilization | |
| 9 | Anosmia to Ammonia | |
| 10 | Hallucinated Voice | |
| 11 | Negative Visual Hallucination (Three Boxes) | |
| 12 | Post-Hypnotic Amnesia |
In more modern experiments, a scent such as peppermint has been used in place of ammonia for Item 9.
Ronald Shor and Emily Carota Orne developed the Harvard Group Scale in 1962. It consists of 12 items of progressive difficulty (as defined, psychometrically, by the percentage of subjects in a normative sample that reports experiencing each particular item) and usually takes around forty-five minutes to complete. The items usually consist of motor tasks and cognitive tasks with the motor tasks being easier to complete. The average score is 5 out of 12. The test is self-scored leaving it open to criticism concerning the validity of the scores.
The Hypnotic Induction Profile (HIP)[2] or the eye roll test, first proposed by Herb Spiegel, is a simple test to loosely determine if a person is susceptible to hypnosis. A person is asked to roll his or her eyes upward. The degree to which the iris and cornea are seen is measured. The less of this part of the eye observed, the more hypnotically susceptible a person is. Research has shown that the scale does not carry as strong a relationship with other hypnotic scales as originally thought, with correlations ranging from 0.1 to 0.15 (Orne et al. 1979).
Many other tests are not widely used because they are usually seen as less reliable than the Stanford Scale and Harvard Group Scale. Many professionals think that these tests produce results because they involve concentration and a certain level of concentration is required to be hypnotized.
