Here is an excerpt from Mind Hacks on the rise and fall of the incidence of Multiple Personality Disorder or Dissociative Identity Disorder (DID). This excerpt discusses John Kihlstrom’s ­2005 review article on dissociative disorders:

Dissociative Identity Disorder or DID is a diagnosis that describes where someone manifests various personalities, often of a diverse range of people – from children to adults of either sex.

It is controversial partly because diagnoses seemed to massively increase when two famous films on the disorder were popular.

Kihlstrom makes the interesting point that the increase in the number of people diagnosed with the disorder was also matched by an increase in the number of personalities each person seemed to have.

An interesting feature of the DID “epidemic” is an increase not just in the number of cases but also in the number of alter egos reported per case. In the classic literature, the vast majority of cases were of dual personality (Sutcliffe & Jones 1962, Taylor & Martin 1944). By contrast, most of the new cases compiled by Greaves (1980) presented at least three personalities; in two other series, the average number of alter egos was more than 13 (Kluft 1984, Putnam et al. 1986).

As Kenny (1986) noted, it was almost as if there were some kind of contest to determine who could have (or be) the patient with the most alter egos. The famous Eve, of course, appeared to have three personalities (Osgood & Luria 1954, Thigpen & Cleckley 1954). But when popular and professional interest in MPD was stimulated by the case of Sibyl, who was reported to possess 16 different personalities (Schreiber 1973), Eve replied with her own account of her illness, eventually claiming 22 (Sizemore & Huber 1988).

Despite the almost-infinite number of possible synaptic connections in the brain, one might say that the mind simply is not big enough to hold so many personalities. The proliferation of alter egos within cases, as well as the proliferation of cases, has been one of the factors leading to skepticism about the disorder itself.

In general, dissociative disorders are where one part of consciousness seems to be ‘split off’ or inaccessible to another.

For example, psychogenic amnesia or conversion disorder (‘hysteria’) are more common examples and hypnosis seems to reliably induce the phenomena in some people.

These are still some of the most mysterious processes in psychology and are fraught with controversy, particularly as they’re often linked to repressed memories from abuse or trauma.

This is one of the more difficult areas to study scientifically because it largely relies on self-report, and Kihlstrom notes there is still no convincing evidence that trauma or abuse leads to amnesia for the event.

Link to PubMed abstract of Kihlstrom’s review.
Link to full-text of pre-print.