Dissociative identity disorder (DID), is a mental disorder. 
It was called multiple personality disorder (MPD). It is listed in the mental health diagnosis manual DSM IV.  

This is one of the most controversial mental disorders because people disagree about what causes it. The main symptoms of DID are a person showing two or more "identities" or "personality states". The person behaves differently depending which identity is in control.  The second important symptom is when a person forgets important, personal things which people normally do not forget.  

A person is not diagnosed with DID if the symptoms are caused by drugs, illness or (with children) pretending to play with imaginary friends.  Doctors must also rule out pretending to have DID for attention or sympathy, and malingering (pretending to have DID for some personal gain).  Most patients with DID are also diagnosed with other mental disorders.

Definitions
The DSM IV uses the terms "identity", "personality states" and "alternate identities" when talking about DID and other people use the term "alters" to mean the same thing. These words can be confusing since not everyone uses the same definition. Professionals have not agreed on a specific definition for a personality; other terms, including dissociation and amnesia, may also be defined differently by different professionals. When talking about DID, saying someone changes personalities or alters means the person speaks and acts differently, and usually responds to a different name.

Dissociation is a symptom.  Most people experience normal dissociation, where they stop paying attention to what is happening around them.  People with DID have pathological dissociation, which is much more serious.  It involves problems with memory and attention that make it hard for people to work, shop and have relationships with other people.  Everyone can dissociate but some people can do it much more easily and to an extreme that can be unhealthy.  Some doctors think trauma causes the dissociation that is seen in disorders like DID.

Signs and symptoms
According to the DSM-IV, the symptoms of DID are "the presence of two or more distinct identities or personality states" (which are often called "alters"), as well as forgetting things that people normally don't forget.  Also, the symptoms cannot be caused by drugs or normal childhood play.  People can report having a lot of alters, or just a couple.  Most people diagnosed with DID have less than ten, though some have said they have several thousand. A person diagnosed with DID cannot be diagnosed with another dissociative disorder.  Because people with DID hear voices of different alters, it may be confused with schizophrenia.  However, the two problems are very different and doctors can usually tell if a patient has DID or schizophrenia by asking the right questions.  DID is always diagnosed by what patients tell their doctors and what the doctor thinks this means.  There are no objective tests using blood or machines that can tell if someone has DID.  Though people have tried diagnosing and studying DID using medical imaging or brain scanning.

Diagnosis
The DSM uses the following criteria to diagnose DID:
 At least two "distinct identities" that alternate control of behavior
 Inability to recall "important personal information too great" to be attributed to "ordinary forgetfulness" 
 None of the above are due to the direct physiological effects of a substance or a general medical condition

The International Classification of Diseases-10 (ICD-10) uses the term multiple personality disorder instead of dissociative identity disorder.  It classifies DID in section F44.8, "Other dissociative [conversion] disorders".

Causes
There are two main ideas about what causes DID, but there is still disagreement about which is correct.  The trauma model says that DID is caused by parents or family that abuse or neglect their children.  Some of these children deny the abuse is happening or pretend it is happening to someone else, and this eventually becomes a different identity or alter.  Most people diagnosed with DID say they remember being abused by their parents or other caregivers when they were children.  When asked about their childhood, patients with DID are more likely to say they were abused or neglected than people with any other diagnosis.  However, the idea that DID develops in childhood is disputed because childhood memories are not very reliable and it is not clear if the abuse actually happened.  Though DID has been diagnosed in children, there are reasons to believe the diagnosis is because of the influence of parents and doctors rather than abuse.  There is not enough good scientific evidence about DID in childhood to be sure DID is actually caused by abuse or something else.  

The iatrogenic or sociocognitive model (SCM) of DID says it is created during psychotherapy when the therapist creates false memories and patients become convinced they have multiple personalities. Some patients may be more likely to develop DID in these circumstances because they are naturally more likely to accept their therapists are right about DID.  SCM supporters also think that patients have seen DID in movies and books, and this shows them how people with DID are supposed to behave, which makes it easier for them to change their behavior when they are supposed to be a new alter.  They also say the criteria used by the DSM are unclear which makes it hard to agree whether a patient has DID or another diagnosis.

A middle position says that trauma may change how the brain remembers things which makes it easier to remember things that did not actually happen.  It has also been suggested that early trauma may make children more likely to use their imaginations to pretend abuse or other painful situations are not happening, and if they enter therapy when they are older, it is easier for therapists to convince them they have DID.

Treatment
There is no treatment approach that has been scientifically proven to work.  Most therapists who treat patients with DID use several different approaches.  Therapy usually lasts for several years, and does not always work.  Some therapists try to reduce or eliminate the alters so the patient only has one personality, while others do not.  The ISSTD recommends first improving a patient's ability to deal with symptoms and live more normal lives, then trying to deal with traumatic memories.  The final step is bringing all the identities together so the patient can access all of their memories.

Prognosis
Sometimes patients with DID will get better without any help, but this is not common.  Patients with more diagnoses than just DID often take longer to improve.

Epidemiology
There have been no large-scale studies to learn how common DID is. The ISSTD says DID occurs in between 1 and 3% in the general population, and between 1 and 5% in groups of patients in hospitals.  DID is more common in women than in men but this may be caused by men with DID being sent to prison instead of hospitals.  The number of people diagnosed with DID increased from about 200 in 1970 to about 20,000 in the 1990s.  According to the ISSTD this is because doctors did not have the training or experience to recognize DID.  Instead they diagnosed people with depression, PTSD or borderline personality disorder.  People who support the SCM say the increase in diagnoses was because a small number of doctors diagnosed a large number of people and because a large number of therapists began using types of psychotherapy that made people think they had DID.

History and culture

Before 1900 it was thought DID was caused by spirits or ghosts who controlled people's behavior.  The first case of DID described in medicine was in 1646 by the Swiss doctor Paracelsus.  DID was very uncommon until the 1970s.  Between 1980 and 1990 the number of cases reported in medicine went from about 200 to more than 20,000.  In 1994, the 4th edition of the DSM was published, which changed the name from "multiple personality disorder" to "dissociative identity disorder".  The DSM also changed how DID was diagnosed.  Many scientific articles were written about DID during the same period, though after 1995 the number of articles published every year decreased. Today there is little research on DID.

Even though DID is a very rare, there are a lot of books, plays and movies about people with DID.  In 1886 Robert Louis Stevenson published the short book The Strange Case of Dr. Jekyll and Mr. Hyde which was very popular.  People later thought that Dr. Jekyll and Mr. Hyde were examples of someone with DID.  The Three Faces Of Eve was a movie made in 1957 about a woman with DID.  Sybil, a popular book about a woman with DID, was published in 1973.  It was made into a movie in 1976, and a second time in 2007.  A survey done in 2001 of American and Canadian psychiatrists found that there was a lot of disagreement and skepticism about diagnosing DID and if the diagnosis was based on enough good quality science.

DID in the legal system
Lawyers and doctors who work with the law also consider DID to be very controversial.  Since the 1990s, DID has become more common in courts. People have tried to avoid going to prison for such crimes as murder and serial rape by claiming to have DID, and lawyers and judges are concerned about people pretending to have DID to avoid going to prison.  The first time this happened was in 1976, and since that time most people who commit crimes and claim it was because of DID have gone to prison.

References

Mental illnesses
Psychology
NPOV disputes