Hypnotic susceptibility or trance capacity refers to the ability of a subject to achieve a given level of hypnotic trance. This in turn makes two assumptions:
1. There is such a phenomenon as a trance state.
2. This state can be meaningfully measured along a depth scale from shallow (hypnoidal) to deep (somnambulistic).
With respect to the first issue, Sutcliffe (1961) has oriented theoretical views about the nature of hypnosis on a scale ranging from "credulous" to "skeptical." At the credulous end of the scale are the "hypnotic state" theorists, who regard a trance state as a phenomenon that enhances the suggestibility of a hypnotized subject. At the skeptical end of the scale are theorists like Barber, who take the view that a hypnotic state is neither a necessary nor sufficient condition to produce the classic effects of hypnosis, or Gibbons, who refers to the notion of a trance state as a "shared delusion."
Barber points out that well-motivated subjects who have not been hypnotized can produce all of these phenomena, while some subjects in a deep trance cannot.
Implicit in the idea of measuring hypnotic susceptibility is the notion that it is a stable personality characteristic as opposed to a situational variable. If susceptibility is, in fact, a stable characteristic of a person, questions arise about what factors cause some people to develop a markedly greater capacity for hypnosis than others and how readily this capacity can be modified.
Does repeated experience with hypnosis improve a subject's ability to achieve a deeper state, in the sense of being able to do things under hypnosis that he formerly was incapable of achieving?
In order to be hypnotized a subject not only has to have the trance capacity, but must also want to be hypnotized and must actively cooperate in the process. It is conceivable that a person with a lot of ability as a hypnotic subject may be afraid of being hypnotized, react negatively to the hypnotist, or be suffering from some physical or mental distraction at the time of an original attempt at hypnosis. The subject will thus appear to be a poor subject.
If after repeated hypnotic sessions these fears abate, the subject's rapport with the hypnotist improves, or his motivation to be hypnotized increases, he may achieve a much deeper trance. This result may give the illusion that the practice has improved the subject's basic trance capacity when in fact it has not. It is clear that the best subject cannot be hypnotized unless he wants to be.
Thus, tests of hypnotic susceptibility are valid only when the tester is certain that the subject is will motivated and doing his best.
To avoid semantic confusion the term hypnotic susceptibility or trance capacity will be used when referring to the stable or long-term ability of a subject to be hypnotized, and the term hypnotizability will denote the net effect of susceptibility plus any operative situational factors affecting the hypnotic ability of a subject at a given time.
Unfortunately, this distinction is not generally made in the literature, and usually the terms susceptibility and hypnotizability are used interchangeably, resulting in a great deal of confusion in research dealing with the issue of whether susceptibility is modifiable.
It is a common experience that subject exposed to repeated hypnotic sessions tend to enter the trance state more rapidly on successive sessions and often appear to develop greater depth. It is for this reason a good idea not to give up therapeutic efforts on what may seem like a poor subject without at least a few trials. (Fortunately many therapeutic applications do not require a very deep trance.)