The diagnosis “multiple personality disorder” or “dissociative identity disorder” is a misnomer because it overlooks the context within which personality occurs. Personality does not happen in a vacuum. It is a component element of a consistent phenomenological world with objects, other people, roles, behavior, conventions and other world-like attributes. Thinking that personalities exist separate from the world they inhabit is a form of latter-day Cartesianism. This being so it stands to reason that if a person has more than one personality, then each inhabits its own world. Not all of them can inhabit the same world, otherwise the case just would be one of an ordinary non-multiple personality disorder (such as borderline personality disorder). For this reason it might be more accurate to call multiple personality disorder, “multiple world disorder.”
A related issue is whether transitions between multiple personalities (read: multiple worlds) are accompanied by changes in physiological state and can be detected by e.g. PET scans, fMRI or changes in neurochemistry. There is little research on the neurological correlates of MPD. Onset symptoms of schizophrenia certainly correlate with profound neurophysiological change, but MPD is a different diagnosis. There also have been neurochemical findings in connection with depersonalization disorder suggesting possible involvement of serotonergic, endogenous opiod and glutamatergic NMDA pathways. Brain imaging studies in depersonalization disorder have alterations in metabolic activity in the sensory association cortex as well as prefrontal hyperactivation and limbic inhibition in response to aversive stimuli. It also has been associated with autonomic blunting and hypothalamic-pituitary-adrenal (“HPA”) axis dysregulation. Simeon, D. (2004), “Depersonalisation Disorder – A Contemporary Overview,” Therapy in Practice, 18(6) 343 – 354. However, as with schizophrenia, depersonalization disorder is not MPD. I think it unlikely in principle that anything conclusive ever will be established because from a neurological perspective a world ordinarily is a stable place. MPD presents with too many (at least, more than one) personalities/worlds. It is not a problem of intra-world instability. A world can become dysregulated, but (as with schizophrenia or depersonalization disorder) that presents a different pathology.
This suggests MPD primarily is a condition of belief or self-attribution, and the contexts in which it occurs. Neurophysiologically these beliefs are like any other beliefs and will have the same PET, fMRI and neurochemical presentations. While advances in brain imaging techniques have enabled us to discern the presence of brain activity and associating certain types of activity with certain regions of the brain, in principle they never will enable us to discern the substantive propositional content of what is being thought. Multiple personalities (and beliefs about other worlds) are precipitates of brain/neurochemical activity in a way that is indistinguishable from the beliefs that someone without MPD holds about the ordinary world.
Philosophical accounts of MPD get bogged down in vexing dilemmas of personal identity, for example, “What is the relationship between the inhabitants of one world and that of another;” “which personality is in charge;” “do their worlds overlap;” “what happens when personality A intrudes into personality B’s world;” and “how can multiple personalities exist within one corporeal body.” Illustrative of the confusion is a paper by Thomas Metzinger, “Why are identity disorders interesting for philosophers?” Metzinger gets bogged down with now-obsolete issues from early-twentieth-century philosophy of mind first introduced by Bertrand Russell. For example he claims the central problem with MPD is a “relation” – for example the relationship between self B to self A or even self A to itself. This transitive identity relationship is not a “property” comprising one aspect of a person’s identity.
The problem with this is that real live pathology does not present as an abstract philosophical concept. Patients with MPD could care less about inter-self relations and clinically they are not all that interesting. What is interesting is the phenomenological texture of the patient’s lived experience and whether it causes impairment to real-world functioning or subjective mental distress. If not then these questions are about a problem that doesn’t exist. Personality A has a set of ascriptive predicates; personality B has a set of ascriptive predicates. Perhaps they overlap to some extent, or maybe not. In any event one only can be one self-set (engage one set of ascriptive predicates) at a time. No one contends a person can be two personalities at once. If we regard personality in this way, as a set of more-or-less consistent beliefs about self, and if MPD is having more than one set of these beliefs (though not simultaneously), then personality B having a belief is an ordinary mental occurrence no different than personality A having a belief. To the extent MPD implicates a theory of mind it is no different than whatever theory of mind pertains in a non-MPD state.
An example is thinking you’ve been the victim of an alien abduction. Or you’re in need of an exorcism. Or you’re a zombie following a Haitian rite of initiation. Or you’re Christian Karl Gerhartsreiter, who imagined he was Clark Rockefeller, related to the well-recognized family of the same name, Belluck, P. & Rimer, S. (Aug. 22, 2008), “Ready-Made Rockefeller, New York Times. All of these are beliefs about a present world outlook or orientation. They may be inaccurate but this does not deprive them of their status as belief-components of a consistently-held world-view in a single, stable-self state. Sometimes even ordinary discourse suggests multiple personalities. For example one might say “I’m of two minds about whether to go see the show” or “I was beside myself with joy.” An utterer of these sentences does not seriously believe they are about to enter into an alternative world. They’re just forms of speech. It is unlikely the neurological presentation for any of these single belief states differs significantly from their exact counterparts, that is, if the exact opposite proposition was asserted, e.g. to go through a Haitian rite of zombie initiation yet conclude at the end one was not a zombie.