What is Dissociative Identity Disorder?
Dissociative Identity Disorder, or previously known as Multiple Personality Disorder until 1994 when the DSM-IV 4th revised edition was published. The essential feature of Dissociative Identity Disorder is the presence of two or more distinct identities or personality states that recurrently take control of behaviour. There is typically one ‘primary’ personality, and treatment for this disorder is often sought out by this primary alter. Often there are two to four personalities by the time that treatment is sought out, but there is a distinct pattern of more emerging during the course of therapy. The personalities within DID are often as complex as any human you would meet on the street, each having their own speech and behaviour patterns and tics, memories, personal relationships, age, gender, range of vocabulary, and general knowledge. All of these facets determine what the personality will do next.
It’s quite common for the personalities present to be very different and even the opposites of each other in many ways, and the extent of their differences can go so far as having a different dominant hand for writing. It has been known that alternate personalities will have different eyeglass prescriptions (and will complain about wearing the wrong one), medication prescriptions (as they will take themselves to the doctor as well), and may even claim to have allergies to things that the dominant personality does not. It is not entirely uncommon for the personalities to be ‘aware’ of the other ones in the sense that they may have the voices of the others echo in their unconscious - but they will not know to whom these ‘voices’ belong. The number of reported identities can range from as low as 2 to more than 100. Half of reported cases include individuals with 10 or fewer identities. Alternate identities have been seen taking ‘control’ in sequence, one often at the expense of another. Many personalities deny the existence of others, but some personalities have been seen to be extremely critical of or in conflict with another. In rare cases, one or more ‘powerful’ dominant personalities will “allocate time” for the others to expose themselves.

DID is not viewed as conscious deception. The issue for DID is not whether it is real, but rather how it develops and is maintained.

A split in the personality wherein two or more fairly separate and coherent systems of being exist alternately in the same person is very different from any recognised symptoms of schizophrenia.

What are the Symptoms of DID?

Who tends to get DID and what causes it?
This disorder is three to nine times more common in women than it is in men, and can begin in childhood and not be diagnosed until adulthood. Females with DID tend to have 15 personalities on average, whereas males with DID often only have 8 personalities on average. There is an average 6 to 7 year gap from first report of DID symptoms to diagnosis of DID. Several studies suggest that DID is more common in first degree relatives of someone with DID than within the general population. This disorder is often comorbid (co-occurring) with disorders like Major Depressive Disorder, Borderline Personality Disorder, and Somatization Disorder.

People with DID have frequently reported a history of severe physical and sexual abuse, most especially during childhood. There is a controversial debate around the validity of these reports, as people with DID are statistically shown to be highly susceptible to suggestive influences, however many of these reports can be confirmed with objective evidence, and as not everyone with sexual abuse in their history develops DID, there is the theory that there is a diathesis which spurs the creation of the disorder. One theory is that people who develop DID have very high levels of fantasy, and that the dissociation from the trauma through fantasy created splits within the persona. Another theory states that DID may be an enactment of learned social roles. This is due to the fact that more alters tend to appear in adulthood and within therapy, typically due to suggestions by the therapist.

It is hypothesized that individuals suffering from DID have an insecure or disorganized attachment style because they were exposed to the chaotic behaviour of their caregiver. A study in Canada confirms that attachment styles has a significant link to rates of dissociative symptom reports. 

How is DID treated?
Treatment for DID is complex and requires heavy use of psychotherapy, where the therapist and the client work in tandem to create a cohesion between all of the personalities if possible.  Psychotherapy will also address the natural issues of anxiety caused by the disorder within the client and will work to prevent the manifestation of a comorbid anxiety disorder. EMDR (Eye Movement Desensitization and Reprocessing) is a therapy which is used most often with sufferers of PTSD but has recently been applied to DID sufferers with positive results. Certain behavioural therapists will go about treating this by only responding to a single personality, though this is generally looked down upon within the psychological community.

All information for this post is based on data from the DSM-IV-TR.