Efficacy of induction and difficulty level in durability of post-hypnotic suggestions

We examined whether participants instructed to reenter a hypnotic state as part of the post-hypnotic suggestion (PHS) show less decay in responding over an 8-week period than participants who do not receive such instructions. We also attempted to replicate Trussell, Kurtz, and Strube's (1996) finding on impact of difficulty level of suggestion on response curve. Fifty-nine highly susceptible participants were selected by the Stanford Hypnotic Susceptibility Scale: Form C (SHSS:C) and were assigned to one of four groups (two levels of Difficulty [easy-hard] x two levels of Condition [hypnotic PHS, non-hypnotic PHS]). Participants were tested for PHS at 1, 3, 6, and 8 weeks. A 2 x 2 x 4 (Difficulty x Condition x Time) factorial ANOVA was conducted, with Time as a repeated-measure. The outcome variable at each time was either pass or fail for relevant suggestion. None of the effects containing Condition as a term were significant indicating there is no advantage to using Berrigan, Kurtz, Stabile, and Strube's (1991) atypical induction technique to influence the durability of PHS. We found a significant Time effect but failed to replicate Trussell et al.'s findings for Difficulty level. The differing results found in these three recent studies (Berrigan et al., Trussell et al., and the current study) suggest the effects for durability of PHS may be quite fragile in spite of rigorous experimental controls used in all three studies.