Why is such a test necessary? Because color-blind people believe that they are no different than anybody else, this causes them to operate in a "cultural trance" in which they gloss over very real differences in their experiential abilities.
A hypnotic induction and deepening procedure may be thought of as a "hypno-blindness test." A certain percentage of the population, more imaginatively gifted than the rest, is naturally capable of visual and auditory hallucinations, insensitivity to pain, and all of the other phenomena that we associate with high hypnotizability. But, like those who grow up never knowing that they are color blind until they are tested for it, these experientially gifted individuals also operate in a cultural trance which causes them to feel that they are no different than anybody else, and which makes them gloss over real differences in their experiential abilities until they are tested.
What does hypnosis do to change things? A recent thread on a hypnosis discussion forum asked people to describe the most unique induction they knew, After the obviously humorous ones were removed, the only thing the remaining ones appeared to have in common was that they were all suggesting or implying that the subject's consciousness was beginning to function differently. And what does that accomplish? When a hypnotist suggests that someone's consciousness is beginning to function differently, this removes their cultural blinders and, if they are sufficiently able and willing, frees them up to use their imagination in what to the rest of us appears to be an "Alice-in-Wonderland" fashion.
Exceptions do occur, of course, when the cultural blinders are ineffetive, and people manifest hypnotic-like behavior without an induction. But, without social validation for our perceptions, we are usually quite hesitant about appearing to be very much different from those around us. As I have stated elsewhere, if I were to walk up to an imaginatively gifted person, ask him to close his eyes, and suggest, with no previous induction, that by the time I got to the count of five he could open his eyes and see me dressed in a Santa Claus suit and hat, he would usually think that I was crazy. And if such a suggestion should actually happen to work, he would probably think that he was crazy! But if I first suggested that he was "going into hypnosis," using some sort of an induction proxedure to make such a sufggestion sufficiently plausible,then he can use the power of his imagination to do whatever he or she is able and willing to do with these abilities until the session is concluded by suggesting that he is no longer hypnotized.
It has been said that the organisms most frequently experimented on are the laboratory rat and the college sophomore, because they are the most available to academic researchers. The differences in hypnotic responsiveness I have just referred to are reliably obtained when data are gathered under standardized testing conditions such as a college classroom.
The Harvard Group Scale of Hypnotic Susceptibility, Form A (Shor & Orne, 1962) is modeled after the experimental approach originally begun by Clark Hull (1933). It contains a script consisting of a light hypnotic induction, followed by a list of twelve suggestions in increasing order of difficulty, from "easy" ones which almost anyone can pass, to more difficult items such as the inability to shake one's head "no" when challenged, or amnesia for most of the test items until after a prearranged signal has been given. Since its initial publication in 1962, the test has been used in dozens of studies all over the world, in order to give us a greater understanding of individual differences in suggestibility.
Now let's perform a thought experiment. I would like to ask you to imagine that the Harvard Group Scale is being given to a class of introductory psychology students at the American University of Beirut, let us say, when a person dressed in a police uniform bursts into the room and says in a loud, commanding voice, "The city is under biological attack, and a germ cloud is headed this way. Take refuge in the basement immediately and await further instructions!"
Even if such an announcement is a hoax (i.e., a cleverly-designed suggestion) thought up by a dissident student organization to disrupt the orderly running of campus activities, if it were to be conducted in a sufficiently convincing manner, everyone in the class -- including the instructor -- would probably dash for the exits and head for the nearest underground shelter. What happeed to the individual differences in suggestibility which the Harvard Group Scale was supposed to measure? They simply vanished, as everyone took flight!
A high degree of responsiveness to the impostor's suggestions would occur regardless of how an individual student might have scored on the suggestibility test which was currently underway. Notice also that the subjects would probably have been totally involved in the content of the impostor's suggestions: trembling, feeling frightened, weeping, crying out in alarm, and so on. In human society, suggestion appears to be causally related to experiences as diverse as falling in love,, coming under the sway of a totalitarian dictator,being saved in a revival meeting,or turning into an animal (transmogrification),as practiced in Native American culture. Individual differences in responsiveness, if they exist. do not seem to attract much attention.,
Regardless of whether the induction takes you up, down, or sideways, you're hypnotized if you think you are! I use this kind of suggestion-enhanced experience in my psychology practice every day in order to facilitate the acceptance of subsequent therapeutic suggestions which are actualized more easily because the induction has made them more credible..
Lynn, S. J., & Kirsch, I. (2006).Essentials of clinical hypnosis: An evidence-based approach.Washington, DC: American Psychological Association.