Atavistic regression is a hypnosis-related concept introduced by the Australian scholar and psychiatrist Ainslie Meares.
Meares coined his term from the English atavism, which is derived from the Latin atavus, meaning a great-grandfather's grandfather and, thus, more generally, an ancestor.
As used by Meares, for example, his 1960 work A System of Medical Hypnosis, the term atavistic regression is used to denote the tendency to revert to ancestral type:
“The atavistic hypothesis requires… a regression from normal adult mental function at an intellectual, logical level, to an archaic level of mental function in which the process of suggestion determines the acceptance of ideas. This regression is considered to be the basic mechanism in the production of hypnosis.”Meares held the view that when in hypnosis, the higher (more evolved) functions of the subject's brain were switched off, and the subject reverted to a far more archaic and far less advanced (in evolutionary terms) mental state; something which significantly altered the subjects' cognitive processing so that they readily accepted internally consistent, literal logic without any of the normal filters and verifications against the objective facts of the real world.