Hypnotic hypermnesia: the empty set of hypermnesia.

Although a long tradition exists suggesting that hypnosis can enhance memory (hypnotic hypermnesia), the experimental literature is quite mixed. When, however, laboratory studies are classified according to the type of stimulus and memory tests employed, a remarkable orderliness of outcomes emerges: Recall tests for high-sense stimuli (e.g., poetry, meaningful pictures) almost always produce hypermnesia, but not recognition tests for low-sense stimuli (e.g., nonsense syllables, word lists). An important methodological issue is whether the recall increments for high-sense stimuli constitute enhanced memory or enhanced reporting (laxer response criteria). Recent laboratory literatures show that, beyond response criterion effects, true memory enhancement (hypermnesia) exists. Experiments conducted over the past decade, however, demonstrate that it is repeated retrieval effort and not hypnosis that is responsible for hypermnesia: Repeated testing without hypnosis yields as much hypermnesia as with hypnosis.