Suggestion is the psychological process by which one person guides the thoughts, feelings, or behaviour of another. Nineteenth century writers on psychology such as William James used the words "suggest" and "suggestion" in senses close to those they have in common speech—one idea was said to suggest another when it brought that other idea to mind. Early scientific studies of hypnosis by Clark Leonard Hull and others extended the meaning of these words in a special and technical sense (Hull, 1933). The original neuro-psychological theory of hypnotic suggestion was based upon the ideo-motor reflex response of William B. Carpenter and James Braid.
Modern scientific study of hypnosis, which follows the pattern of Hull's work, 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 instructing and suggesting to the subject that they will enter a hypnotic state. Once a subject enters hypnosis, the hypnotist gives suggestions that can produce sought effects. Commonly used suggestions on measures of "suggestibility" or "susceptibility" (or for those with a different theoretical orientation, "hypnotic talent") include suggestions that one's arm is getting lighter and floating up in the air, or that a fly is buzzing around one's head. The "classic" response to an accepted suggestion that one's arm is beginning to float in the air is that the subject perceives the intended effect as happening involuntarily.
Suggestions, however, can also have an effect in the absence of a hypnosis. These so-called "waking suggestions" are given in precisely the same way as "hypnotic suggestions" (i.e., suggestions given within hypnosis) and can produce strong changes in perceptual experience. Experiments on suggestion, in the absence of hypnosis, were conducted by early researchers such as Hull (1933). More recently, researchers such as Nicholas Spanos and Irving Kirsch have conducted experiments investigating such non-hypnotic-suggestibility and found a strong correlation between people's responses to suggestion both in- and outside hypnosis.
In addition to the kinds of suggestion typically delivered by researchers interested in hypnosis there are other forms of suggestibility, though not all are considered interrelated. These include: primary and secondary suggestibility (older terms for non-hypnotic and hypnotic suggestibility respectively), hypnotic suggestibility (i.e., the response to suggestion measured within hypnosis), and interrogative suggestibility (yielding to interrogative questions, and shifting responses when interrogative pressure is applied: see Gudjonsson suggestibility scale.