After testing hypnotic susceptibility with the Stanford Hypnotic Clinical Scale (SHSS), Hilgard, Hilgard and Newman (1961) found 8% of their 220 subjects reported transient experiences of headaches, dizziness and confusion. Hilgard's (1974) study of negative effects in 120 subjects, tested for hypnotizability using the SHSS, demonstrated that 16% showed transient negative effects while another 15% experienced negative effects of greater than one hour duration. Crawford, Hilgard and MacDonald (1982) compared the negative effects reported after administration of the Harvard Group Scale of Hypnotic Susceptibility (HGSHS) with those of the Stanford Hypnotic Susceptibility Scale (SHSS), which has a greater number of cognitive items. The use of HGSHS resulted in 5% of the 107 subjects reporting negative experiences with 1% reporting that these lasted for more than one hour. In contrast, the use of the SHSS resulted in 29% reporting negative effects with 12% of these effects lasting over one hour. There was a tendency for more cognitive distortions to be found in the more hypnotizable subjects. Brentar and Lynn (1989) were not able to confirm this association in a study of 240 subjects using the HGSHS.
Echterling and Emmerling (1987) interviewed 105 students who had attended an 'hypnosis stage show'. Of these subjects, 33% reported negative experiences, although they were generally transitory. Misra (1985) reported 16 of 2000 participants who attended a 'stage hypnotist' were referred for negative effects and again these were mostly transitory in nature. Crawford, Hilgard and MacDonald (1992) reported in their study of subjects involved in hypnosis in the 'entertainment' setting, that approximately one-third of those studied reported mild to severe adverse responses although usually of a transient nature. Anxiety and confusion figure prominently in the reported negative effects.
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Hypnosis has been defined as a state of heightened suggestibility in which the subject is able to uncritically accept ideas for self-improvement and act on them appropriately. When a hypnotist hypnotizes his subject, it is known as hetero-hypnosis. When an individual puts himself into a state of hypnosis, it is known as self-hypnosis.