The hypnosis scales at their centenary: some fundamental issues still unresolved

Current approaches to the measurement of hypnotic performance can be traced back to the 19th century. In part because of these early origins and in part because of the nature of hypnotic phenomena, the hypnosis scales are unique psychometric instruments. The classic hypnosis scales are based on the notion of a "performance ladder"; items are scored on a pass/fail basis and can be arranged in increasing order of difficulty. Some of the implications on this "performance ladder" approach are reviewed. The evidence for two-mechanism models of hypnotic performance is reviewed. It is argued that this kind of formulation is at least as plausible as one that argues that the hypnosis scales measure "one thing" or "mostly one thing." If it were the case that the hypnosis scales were tapping two different and distinct processes, the label "hypnotic susceptibility" could not be unambiguously applied to scores on the hypnosis scales. The hypnosis scales would appear well-suited to the investigation of underlying mechanisms, yet no consistent picture of the mechanisms underlying hypnotic performance on the scales has emerged thus far. No resolution is presented, but some of the reasons why such a resolution is so elusive are discussed. The future of hypnosis scales is discussed with respect to multidimensional assessment and alternatives to the "work sample" approach.