Hypnotic susceptibility scales: are the mean scores increasing?

The Stanford Hypnotic Susceptibility Scale, Form C (SHSS:C), developed and named 37 years ago, is arguably the "gold standard" of hypnotic susceptibility scales. However, it has been the impression of several researchers that means obtained on the SHSS:C are higher now than in previous years. The authors comprehensively review studies using the SHSS:C over a 4-decade period. The findings demonstrate a significant linear trend between year and SHSS:C scores, with higher obtained means in more recent work. The authors also report a similar analysis of research with the Harvard Group Scale of Hypnotic Susceptibility, Form A. Although the mechanisms underlying this trend can only be speculated upon at present, these findings underscore the importance of using local control groups in research on hypnotizability.