What is popularly termed dual, split, or multiple personality is one form of what psychologists call disassociation. Two or more mental process in the individual can be said to be disassociated if they either coexist or alternate without apparently influencing one another or becoming connected. In the nineteenth century, disassociation described a host of phenomena from dreams to neurotic symptoms. Neurosis was explained as a constitutional weakness in the person that prevented their integrating their personality. Thus daydreaming was condemmed as a symptom of nonintegration. A more extreme example would be what had previously been called spirit possession.
When Freud proposed the existence of an underlying un-conscious, the idea of an underlying constitutional weakness was abandoned in favor of a discussion of various mechanisms by which the ego, the central waking personality, suppressed or isolated unwanted elements and kept them out of the ongoing ego formation.
However, Freudian categories do not handle well the most extreme of disassociation phenomena characterized by the subject maintaining for an extended length of time some action not apparently initiated by the conscious self and the memory of which is not available to the conscious self. Such phenomena includes forms of amnesia, sleepwalking, and post-hypnotic suggestions. It would also include the trance phenomena of a Spiritualist medium or someone engaged in channeling, and the now well-known phenomena of multiple personality, in which the person appears to change from one person to another. This last phenomena challenges some basic assumptions about self identity, that each individual is just that, a single person with a single memory, a more or less unified being.
Sometimes, in trance state, there occurs a split so pronounced that the subject seems to have two or more distinct personalities. The secondary personality may differ from the primary in many ways, and possess entirely distinct intellectual and moral characteristics. The entranced subject may allude to his normal consciousness in the third person, may criticize its opinions and attitude, or even express direct antagonism towards it.
This secondary personality sometimes alternates with the primary in such a way as to suggest that two spirits are struggling to possess the same physical organization. (For an example, see William Sharp. ) Another peculiarity of this state is that whereas the normal consciousness generally knows nothing of the others, the secondary personalities usually have full knowledge of each other and of the normal consciousness.
The more extreme disassociation is by no means confined to the trance state, but may arise spontaneously. Robert Louis Stevenson made effective use of the philosophical implications of dual personality in his science fiction horror story The Strange Case of Dr. Jekyll and Mr. Hyde (1886). In a less horrendous setting, it became familiar to many through the book and movie The Three Faces of Eve. Sometimes the appearance of a dual personality leads to other multiple personalities. In the famous case of Sally Beauchamp, investigated by Morton Prince, four well-defined personalities developed, as described in Prince's book Dissociation of a Personality (1905).
In many cases the emergence of secondary personalities is due to a patient's response to his or her counselor, an attempt to fulfill a real or imagined request.
While much work and discussion has been done on the dysfunctional multiple personality as a disassociation disorder, little effort has been put into understanding mediumship and channeling in the same way. Mediumship differs significantly from multiple personality both in the control of the medium over the appearance of the secondary personality and its nonpathological nature.
Prince, Morton. The Disassociation of a Personality. 1905. Re-print, Oxford: Oxford University Press, 1978.