What is dissociative identity disorder, the condition affecting James McAvoy's character in 'Split'?

James McAvoy stars in new horror thriller "Split", as Kevin, a man affected by dissociative personality disorder. He has 23 different personalities which he reveals as the movie unfolds.

Multiple personality disorder – now known as dissociative identity disorder or DID – was included in the Diagnostic and Statistical Manual-III in 1980. This standard classification of mental disorders is often referred to as a reference worldwide, so including the disorder in it was a big step in having it recognised and treated. Yet, the question of whether DID really exists and how best to diagnose it has remained persistent in the past decades.

IBTimes UK takes a look at the most common interrogations associated with the condition.

Does DID really exist?

Dissociative personality disorder is probably one of the most controversial topic in psychiatry. It has long fascinated health professionals but also the public. "Split" is only the last example in a series of books of films (think Fight Club or Dr jekyll and Mr Hyde) dedicated to the condition.

Despite having been officially recognised as a psychiatric disorder more than three decades ago, there has been heated debates about DID in the scientific community.

In the 1970's and after the recognition of the diagnosis by the Diagnostic and Statistical Manual, the number of cases in the US steadily grew, to around 4% of the population, raising concerns that the condition was being misdiagnosed, because these figures were not observed elsewhere.

It is actually difficult to estimate the frequency of the disorder in a population especially because diagnosis is hard to establish and people may not go seek treatment or speak out for fear of stigma. The most rigorous studies put it between 0.1 and 1% of the population.

Disbelief remained strong for a long time among psychiatrists. A 1988 study suggested that many of them did not believe the condition existed and had a very strong emotional reaction to it – sometimes refusing to admit patients into hospital. Further research suggested that DID may simply be a by-product of borderline personality disorder, as scientists found few differences in symptoms between the two diagnosis.

While it is now mostly accepted that DID exists as a condition of its own, there are still a lot of questions about what the symptoms are and how best to treat it – if to treat it at all.

Finally, because DID is closely associated to trauma or sexual and physical abuse in childhood, there is sometimes a concern over false memories. Some psychiatrists worry that patients may "remember" abuse that didn't actually happen – with potentially devastating consequences for people they accuse.

How does the condition manifest itself?

When daydreaming, we may experience a form a dissociation - a sensation of being disconnected from the world around us and from ourselves. With dissociative identity disorder, this logic is pushed to an extreme. Patients experience a severe form of dissociation, a mental process which produces a lack of connection in their thoughts, memories, feelings, actions, and sense of identity.

The disorder is characterised by the presence of two or more distinct personality states that can exert control over the person's behaviour. Each of these "alters" may have different age, sex, or race, with different ways of standing, moving, or talking. There is often an inability to recall key personal information or to access memories of what was done by another alter.

How many personalities can a person have?

In "Split", McAvoy's character switches between 23 personalities but in "real-life" cases, psychiatrists rarely see people with that many different alters. They also rarely observe personalities that could be harmful to others.

"Women tend to have on average more alters than men, and from what we have observed, patients have on average between six and seven different personalities. Some people may have more, but in these cases, there are probably personalities that do not manifest themselves much," Peter Tyrer, a professor of community psychiatry in the Centre for Mental Health in the Division of Experimental Medicine, told IBTimes UK.

"Having 23 personalities would be very chaotic and many of these personalities would rarely appear. Also important to note, the people I have seen are not the slightest bit dangerous to others".

What causes people to develop DID?

According to a 2008 research, DID is linked to childhood abuse in 95-98% of cases. This includes physical, as well as sexual abuse. Other factors can pile up on these traumatic experiences, such as lack of family support, making a person more likely to develop the condition.

It is thought that dissociation in the context of chronic, severe childhood trauma, is an adaptive mechanism to escape the traumatic situation and reduce the overwhelming distress caused by it.

"Certainly, the eight people I have seen in my practice had all suffered from physical and or sexual abuse as children and their alters reflected this trauma," Tyrer, who is also a member of the Royal College of Psychiatrists, says.

How can people be treated?

DID cannot be "cured" and there is no specific treatment for it apart from psychological interventions. If patients stay motivated, long-term treatment such as talk therapy or psychotherapy, hypnotherapy, and art or movement therapy can be helpful.

One of Tyrer's patients is a woman who sometimes "becomes" an antisocial teenager. While she still continues to switch between her different "alters", these personalities are not enclosed from one another anymore, thanks to psychological treatment. This is a real progress, because it means one personality now remembers what the other has done.

The problem is patients rarely go out to seek treatment. This can be because they are not unhappy with having different personalities, they might feel it protects them from the trauma they have gone through in childhood.

"The main challenge to treating people is that in many instances people do not want to be treated. DID is a remarkably efficient, self-compensating mechanism to deal with a trauma. People may sometimes find switching to different alters preferable to facing the problem more directly," Tyrer points out.

Usually, personality switches are not as radical as what films portrey. A further obstacle to treatment is that health professionals are not well equipped to identify and diagnose the disorder. People can go on for years without being detected and receiving appropriate care. More research and better training in future years are a priority to help these patients.