The idea that people can fragment into multiple personalities (Herschel apparently has 12) has always generated feverish fascination among the public and experts alike. Unfortunately, an authoritative review published in 2005 reveals that our scientific understanding of the condition remains severely lacking (abstract here).
The prevalence of DID/multiple personality has tended to rise and fall dramatically depending on its profile at any given time – always a bad sign from a scientific perspective. The best known case is probably Sybil, with 16 personalities, whose condition was documented by the journalist Flora Schreiber in the 1970s.
Today DID is recognised by psychiatrists as one of a cluster of conditions including depersonalisation disorder (feeling unreal), dissociative fugue (forgetting yourself) and dissociative amnesia (forgetting specific autobiographical episodes).
The name change from multiple personality to DID reflected the fact that experts shifted their focus from the splitting of personalities to the presence of disorders in memory and consciousness. The diagnosis is only made in the absence of any identifiable organic cause.
A popular perspective is that DID emerges as a response to trauma experienced in childhood (Herschel says he was bullied at school). This view of DID makes intuitive sense but has been hard to verify scientifically as accounts are always based on patients’ retrospective report.