The question of possible harmful effects from hypnosis is one worthy of serious consideration, fully as much so as the possibility of beneficial results. Fortunately it is no longer necessary to consider such things as the emanation of the secret power, the control of a weak will by an overpoweringly strong will, the irresistible transmission of thoughts, and similar exploded superstitions. In the light of present-day knowledge hypnotism is looked upon in intelligent circles as a normal though unusual and little understood phenomenon of the human mind, dependent wholly upon the cooperation of the subject, and which can be practiced by anybody willing to learn the psychological principles and technique involved. But there are certain theoretical possibilities of harm which should be considered and studied even though in the writers experience findings in these regards are negative.
It is true that these possibilities are entirely speculative and problematical, but a reasonable answer must be given before there can be a ready acceptance of this method of scientific investigation. The first of these theories of possible detrimental effects centers around the question of the development of hypersuggestibility. The literature is barren of information in this regard. However, there is carefully planned and controlled work under way in a wellknown psychological laboratory, and results so far, are negative. In the writers own experience, upon which it unfortunately will be necessary to a large extent to base the elaboration of these various questions, hypersuggestibility was not noticed, although the list of individual subjects totals approximately 300 and the number of trances several thousand. Further, a considerable number were hypnotized from 300 to 500 times each over a period of years. Also several of the subjects were immediate relatives with consequent intimate daily contact, and they were trained to respond, in experimentation, quickly and readily to the slightest suggestion. Far from making them hypersuggestible, it was found necessary to deal very gingerly with them to keep from losing their cooperation, and it was often felt that they developed a compensatory negativism toward the hypnotist to offset any increased suggestibility. Subjects trained to go into a deep trance instantly at the snap of a finger would successfully resist when unwilling or more interested in other projects. Even when persuaded to give their consent against their original wishes, the induction of a trance was impossible. Nor were those subjects more suggestible to other people, since, when their services were loaned to the authors colleagues, the production of hypnosis in them, despite their extensive training, was just as hard as it had been originally for the author. And the same thing was found true when the author borrowed subjects. In brief, it seems probable that if there is a development of increased suggestibility, it is negligible in extent.
A second question is that concerning the possibility of the alteration of personality. Just exactly what such a question means is difficult to define, but at all events the general significance of the question is comprehensible. As we all know, alterations of personality occur in response to suggestion in ordinary daily life. Hence it is only logical to presume that a state of enhanced suggestibility such as an hypnotic trance would show an increased susceptibility to alteration of personality. In the writers experience, where members of the family were very frequently hypnotized over a period of four years, no alteration was noted in any way attributable to hypnosis. The same thing was observed in the case of friends who were utilized as subjects over a similar length of time, and likewise with subjects met in the laboratory. But an even more forceful and trenchant answer may be derived from the experience of psychotherapists who have deliberately and carefully utilized the method to induce desired alterations in the personality of their patients with disappointing results and usually failed even when such was the goal of their efforts. The inefficiency of hypnosis in the treatment of homosexualism is an excellent illustration of the difficulties of fundamentally altering a personality. Briefly, then, it seems that the conclusion may be drawn justifiably that experimental hypnosis will not cause any fundamental alteration of personality or possibly even any alteration in addition to that which would accrue from ordinary personal contact.