Dissociative Identity Disorder :) :(

The modern term for this is dissociative identity disorder, or DID for short. It is a very controversial diagnosis, because it is so rare, and many times the disorder turns out to be iatrogenic (that is, created by the therapist. For example, your therapist plants in your mind the idea that maybe your symptoms are caused by DID, so you start manifesting different personalities to please him/her, etc.). And also because it lends itself to malingering ("I didn't kill him! That was my evil personality!")

DID was once known as... multiple personality disorder.

We don't know a whole lot about it, because real cases are so rare. The main theory is is that DID is an extreme response to some terrible trauma in childhood, like horrific abuse. The child then dissociates (a feeling of being out of your body) as a coping mechanism to deal with the abuse. This leads to the creation of a seperate personality. It is the seperate personality who actually experiences the abuse, while the primary personality can live a "normal" life without experiencing the abuse. In this way, it is a highly specialized coping mechanism that becomes maladaptive later in life, because the seperate personality never "leaves" the person, but takes over from time-to-time. When the split personality takes over, the person acts, thinks, and even talks, very differently than they normally are. So, the person has streches of time when they don't know what they did.

Treatment consists of reintegrating the personality (or personalities) back into the consciousness, so that the person is whole. This also involves acknowledging and dealing with the original trauma.