Dissociation is a partial or complete disruption of the normal integration of a person’s conscious or psychological functioning. Dissociation can be a response to trauma or drugs and perhaps allows the mind to distance itself from experiences that are too much for the psyche to process at that time. Dissociative disruptions can affect any aspect of a person’s functioning. Although some dissociative disruptions involve amnesia, some dissociative events do not. Since dissociations are normally unanticipated, they are typically experienced as startling, autonomous intrusions into the person's usual ways of responding or functioning. Due to their unexpected and largely inexplicable nature, they tend to be quite unsettling.
Different dissociative disorders have different relationships to stress and trauma.
The French philosopher and psychiatrist Pierre Janet (1859–1947) is considered to be the author of the concept of dissociation. He was a pioneering French psychologist, philosopher and psychotherapist in the field of dissociation and traumatic memory.
Contrary to most current conceptions of dissociation, Janet did not believe that dissociation was a psychological defense. Psychological defense mechanisms belong to Freud's theory of psychoanalysis, not to Janetian psychology. Janet claimed that dissociation occurred only in persons who had a constitutional weakness of mental functioning that led to hysteria when they were stressed. Although it is true that many of Janet's case histories described traumatic experiences, he never considered dissociation to be a defense against those experiences. Quite the opposite. Janet insisted that dissociation was a mental or cognitive deficit. Accordingly, he considered trauma to be one of many stressors that could worsen the already-impaired "mental efficiency" of a hysteric, thereby generating a cascade of hysterical (in today's language, "dissociative") symptoms.
Janet began his career as a philosopher, who used hypnosis to explore the dissociative propensities of the human mind. Following his doctoral dissertation in philosophy, he rapidly completed a medical degree and, with the sponsorship of J. M. Charcot, opened a laboratory in Pitié-Salpêtrière Hospital in Paris, where he continued his research into the nature and treatment of dissociative conditions. He was one of the first people to draw a connection between events in the subject's past life and his or her present day trauma, and coined the words ‘dissociation’ and ‘subconscious’. Hypnosis continued to be his investigative tool and therapeutic intervention of choice because it was, in his view, a form of dissociation. "Hypnotism may be defined as the momentary transformation of the mental state of an individual, artificially induced by a second person, and sufficing to bring about dissociations of personal memory". In several ways, he preceded Sigmund Freud.
Jean-Martin Charcot (1825 –1893) was a French neurologist and professor of anatomical pathology. His work greatly influenced the developing fields of neurology and psychology. He was the "foremost neurologist of late nineteenth-century France" and has been called "the Napoleon of the neuroses". Charcot is best known today, outside the community of neurologists, for his work on hypnosis and hysteria. He believed that hysteria was a neurological disorder for which patients were pre-disposed by hereditary features of their nervous system. Charcot's interest in hysteria and hypnotism "developed at a time when the general public was fascinated in 'animal magnetism' and mesmerization'".