Suggestion is the name given to the psychological process by which one person may guide the thoughts, feelings or behaviour of another. For nineteenth century writers on psychology such as William James the words suggest and suggestion were used in senses very close to those which they have in common speech; one idea was said to suggest another when it recalled that other to mind or (in the modern phrase) reproduced it. Early scientific studies of hypnosis by psychologists such as Clark Leonard Hull led to the reclaimation of these words by psychologists in a special and technical sense. The hypnotists of the Nancy school rediscovered and gave general currency to the doctrine that the most essential feature of the hypnotic state is the unquestioning obedience and docility with which the hypnotized subject accepts, believes, and acts in accordance with every command or proposition of the hypnotizer. Commands or propositions made to the subject (they - may be merely implied by a gesture, a glance, or a chance remark to a third person - now regarded as "non-verbal suggestion") and accepted with this peculiarly uncritical and intense belief were called suggestions ; and the subject that accepted them in this fashion was said to be suggestible. As long ago as the beginning of the nineteenth century suggestion and hysteria were linked by Pierre Janet. There has been a recent resurgence of interest in ideas such as these.
Modern scientific study of hypnosis separates two essential factors: trance and suggestion. The state of mind induced by trance is said to come about via the process of a hypnotic induction; essentially instructions and suggestions that an individual will enter a hypnotic state. Once entered into a hypnotic state of mind, suggestions are given which can produce intended effects. Commonly used suggestions on measures of suggestibility (and measures of hypnotic susceptibility) include suggestions that one's arm is getting lighter and floating up in the air, or the suggestion that a fly is buzzing around your head. A 'classic' response to a suggestion is that the subject perceives the intended effect as happening involuntarily.
Suggestions, however, can also have an effect in the absence of a hypnotic 'state'. These so-called 'waking suggestions' are given in precisely the same way and can produce strong changes in perceptual experience. Professor Irving Kirsch has conducted a lot of research investigating such non-hypnotic-suggestibility and found a strong correlation between people's responses to suggestion both in- and outside hypnosis. There are other forms of suggestibility, though not all are considered interrelated, these include: primary and secondary suggestibility, hypnotic suggestibility, which is response to suggestion measured within hypnosis, and also interrogative suggestibility. (Gudjonsson)