Although it is very rare, early in my career I met two (different) people with Dissociative Identity Disorder (DID; formerly known as Multiple Personality Disorder). The first person I met with DID was a patient at a clinic in Harvard Square, where I was conducting research. Over the course of my two years there, I watched her slowly unravel from displaying one personality to many. By the time I left, she had the voice and demeanor of a seven year old child. Frankly, witnessing this was quite unsettling.
The second person I met with DID was in the psychiatric emergency room at Massachusetts General Hospital, where I was working on another study. I remember the conversations amongst the staff about her case well - some thought she was faking it, while some thought a diagnosis of DID was legitimate.
In short, this is what I think:
People with Borderline Personality Disorder, which is characterized by self-injury, suicide attempts, extreme mood swings, and stormy relationships also has another very important feature that is often less discussed - identity disturbance. Basically, very different personae are displayed depending on the situation. As Kiera Van Gelder describes in The Buddha and The Borderline, a memoir about recovery from this disorder, she would be a hippie one day and a goth the next if it would get her the sexual attention she wanted. In other words, she would drastically change her personality from day to day (in this case, it was for social approval).
The problem is this: A therapist looking at someone who is a goth one day and a hippie the next might conclude that these are just various forms of the self that need to be expressed. (Yikes!)
While chameleoning can be viewed as adaptive in a sense, the person with BPD / identity disturbance seems to be paying little attention to whether or not they actually enjoy a given activity they are engaging in for social approval. Often, people use these different personae as a tool - and a tool only. The who acts as a hippie today has no real affinity for hippie culture in itself. Expression of one's "hippie side" is resoundingly not to be encouraged.
Introducing certain types of therapy that emphasize different parts of the self to people who already have identity disturbance, may cause further polarization. The patient may be reinforced for behaving in certain ways and expressing themselves from various personalities. Hence, the development of DID. Without a question in my mind, this is what happened with the first patient with DID I watched unravel - she initially had a diagnosis of Borderline Personality Disorder (w/ the identity disturbance feature) - and in two years time (after "parts therapy") she had multiple personalities.
People with identity disturbances crave belonging and unity of their behaviors by definition. Slapping a label of DID on someone who is desperately trying to figure out who they are is dangerous. The person knows that a lot effort needs to be put in to establish a sense of self. These uncharted territories of not living for social approval are scary. This unifying label is an excuse to avoid all the anxiety that will come with establishing a sense of self. It is easier to exaggerate the different forms of self that already exist and diverge further.
In the effort to display many personalities, a unifying role of being an interesting patient is maintained.
While the presentation of someone having several personality disorders is certainly real, I highly doubt this is organic. It seems to me that this disorder is related to the same underlying identity disturbance seen in Borderline Personality Disorder. In fact, I'd be willing to bet that a very large number of DID cases are really Borderline cases gone horribly astray due to treatment with talk therapies that have not been supported by research.
I'd also like to add that after having worked with people who have schizophrenia, there is a definite psychotic flavor to DID as well. My gut feeling is that the core underlying feature is the identity disturbance seen in BPD on top of a psychotic tendency to believe what the mind is constructing. It is these two underlying features that combine to the expression of different personalities. In no way do I think there are actually two different personalities encased in one person. In short, I'd reason that DID is simply a specific form psychosis with the core BPD component of identity disturbance.
In treatment, instead of focusing on expression of various external presentations, focus should be on developing a sense of self. Figuring out what the person likes independent from the approval or opinions of others seems vital.
Lastly, unlike depression or anxiety, there is an ego-syntonic feel to multiple personality disorder. In other words, people like having the disorder to some degree and want to continue any associated dysfunctional behavior. Another example of ego-syntonic disorders would be anorexia, where people typically want to continue restricting food intake. In the case of DID, people want to keep their multiple personalities, as they are potentially useful in different circumstances. As such, the expression of different personalities should be particularly discouraged by all staff members at a facility once rapport has been developed and treatment is underway.