M Night Shyamalan’s new movie, Split, stars James McAvoy as a character with 23 different personalities. And, like most screen portrayals of the disorder, it is seen as dangerous and violent. But what’s the truth behind the stigma?

Tom Hanks played six different characters in Cloud Atlas, Eddie Murphy played seven in The Nutty Professor and Alec Guinness notched up eight in Kind Hearts and Coronets. But James McAvoy sets a new benchmark with his new movie, Split. He plays Kevin, a man with at least 23 distinct personalities – not all of them nice. This presents extra challenges for the young women Kevin has abducted and locked in his basement. Every time he walks into the cell, they have to work out who they are dealing with. Is it “Dennis”, the frowny, buttoned-up neat-freak? Is it “Patricia”, the prim, English-accented governess? Could it be “Hedwig”, the nine-year-old Kanye West fan? We don’t get to see all of Kevin’s alter egos, but enough to get the picture and to make this lurid little horror stand out from the crowd.

Split’s writer and director, M Night Shyamalan, professes to having had a lifelong fascination with dissociative identity disorder (DID), formerly known as split personality, or multiple personality disorder, and frequently mislabelled as schizophrenia (which is an entirely different condition). He is clearly not the only one. DID is relatively rare in real life, but we have all heard of it, and we all think we know what it entails because cinema and television seem to be obsessed with it.

You can see the appeal: DID is a condition that lends itself to extremes of behaviour, conflict, torment, secrets and mysteries – everything a juicy drama requires in one character. Unfortunately, those dramas have tended to be horror movies and psychological thrillers, which has not really helped us understand the condition.

So many movies in these genres have used DID as a dramatic driver or a “gotcha” twist, it would be spoiling most of them just to mention their titles in this context (cough Fight Club cough). One conspicuous example old enough to spoil, though, is Alfred Hitchcock’s Psycho, ostensibly based on real-life serial killer Ed Gein but essentially a murder mystery with a DID twist. “He began to think and speak for her, give her half his life, so to speak. At times he could be both personalities, carry on conversations. At other times, the mother half took over completely,” explains a doctor at the end of the movie. Psycho is a horror masterpiece but as a portrayal of a real-life mental-health condition, it’s nonsense. Just as autism in the movies makes you a maths genius, so DID makes you a “psycho”.

The connection between DID and horror was made before cinema was even invented. In 1886, Robert Louis Stevenson published The Strange Case of Dr Jekyll and Mr Hyde, a multilayered literary classic that is, in essence, a study of an extreme DID case: a respectable Victorian gentleman and a bestial monster residing in the same body. Stevenson denied any real-life inspirations for the story, but that same year, Frederic Myers, a founder of the Society for Psychical Research, published an article on what he called “multiplex personality” (a more prescient description than he could ever have known), citing two well-known French cases of the time, Louis Vivet and Felida X. He sent Stevenson a copy. Stevenson’s wife also maintained that the author had read a scientific paper on “sub-consciousness” at the time.

An English actor named Richard Mansfield quickly acquired the rights to Jekyll and Hyde, and within a year was performing the dual role on stage in both the US and in Britain. His performance was apparently so convincing that, for a time, Mansfield was suspected of being Jack the Ripper.

Jekyll and Hyde has been dramatised countless times since, on stage, screen and radio. Fredric March’s 1931 portrayal is still considered the definitive version; others to take on the challenge have included Boris Karloff, Christopher Lee, Anthony Perkins, even Robbie Coltrane. Next up, we have the enticing prospect of Russell Crowe as Dr Jekyll in the Tom Cruise-led reboot of The Mummy.

Jekyll and Hyde’s DNA runs through movies such as Psycho and Split, but, the more you look, DID themes can be found across the entertainment landscape. Movies about demonic possession, werewolves, vampires – all are stories of two personae within one body. And what about superheroes? The Incredible Hulk is essentially a comic-book update of Jekyll and Hyde. Many others live as two discrete identities: Clark Kent and Superman, Peter Parker and Spider-Man, Bruce Wayne and Batman. There’s Tolkien’s Gollum, who struggles to retrieve his former hobbit self, Sméagol. Harry Potter’s psyche is fatefully connected with that of his arch-enemy, Voldemort, who shatters his persona into pieces. Even Eddie Murphy’s Nutty Professor, come to think of it.

Special mention must go to the Farrelly brothers’ Me Myself and Irene, an ill-fated attempt to render DID as comedy. Jim Carrey did his best portraying a nice-guy cop who literally wrestles with his Dirty Harry-like alter ego for the affections of Renée Zellweger, but the laughs proved hard to come by. What’s more, the movie was criticised by mental-health experts for repeatedly referring to Carrey’s condition in such misleading terms as “involuntary schizoid disorder”.

Shyamalan, whose parents were both doctors, has clearly done some reading up for Split. He gets the name right for a start, and roots Kevin’s disorder in childhood trauma – which is often the trigger for DID. He also includes Kevin’s psychiatrist in the narrative, although she is too interested in how Kevin’s condition could “unlock the potential of the brain” to check if he is abducting young women. DID can actually change the chemistry of the human body, she believes, which doesn’t bode well when one of Kevin’s personalities believes he has supernatural powers.

Facebook Twitter Pinterest Anthony Perkins in Psycho. Photograph: Allstar/UNIVERSAL

At least Shyamalan knows he’s going out on a limb. “I wanted to take something scientific and psychologically proven and keep going with it,” he explained recently. “The first two, three steps have been proven, then the next one was not proven, but it’s a question. Do you believe it, what I’m suggesting?”

Movies such as Split can be extremely damaging, argues Dr Simone Reinders, a neuroscientist studying DID at King’s College London in collaboration with universities in the Netherlands. “They make it seem as if patients with DID are extremely violent and prone to doing bad things. This is actually not true and it very badly misrepresents the psychiatric disorder. Individuals with DID definitely do not have a tendency to be violent; more a tendency to hide their mental health problems. I’m very concerned about the effects that the movie will have for patients with DID, and how the general public will now see these patients. There’s already a lot of stigma and scepticism concerning this specific disorder.”

DID is a contested condition. Some professionals have argued it does not exist at all, others that multiple personalities are brought on as a result of therapy. Certainly there have been cases where the condition has been faked or misdiagnosed. Reinders’ research seeks to understand the condition through, for example, neuroimaging – scanning the brains of people with DID. Despite the condition’s worrying “psycho” portrayal, Split’s notions about altering body chemistry turn out to be close to her findings: “With some of my patients, I asked two identity states to listen to a text, and my research has shown that in one state, the blood flow in the brain is different to the other identity state in response to this text. So it is true that the neurobiology is dependent on the identity state that the patient is in.” Some of her patients show different abilities in different personas, such as the need to wear glasses or handedness.

There is another side to DID in the movies: away from the far-fetched genre fiction, a number of film-makers have instead sought to dramatise real-life cases. Intriguingly, where the former category is overwhelmingly dominated by men – usually violent ones – the latter is more of a women’s genre. In 1957, two movies concerning women with DID were released: the pulpy Lizzie, based on a novel by Shirley Jackson (she dismissed the adaptation as “Abbott and Costello meet a multiple personality”), and the vastly more successful The Three Faces of Eve, which made a star of Joanne Woodward. Her character is a demure, dutiful housewife, “Eve White”, but then “Eve Black”, a more flirtatious persona, takes over and destroys her life, before a third persona “Jane” resolves the conflict.

From today’s perspective, the movie says more about postwar attitudes to gender than the human mind, but The Three Faces of Eve was based on a real case study: a woman named Chris Costner Sizemore. The screenplay was co-written by the psychologists who treated her. She revealed her identity in the 1970s, and died last year. When she signed away the movie rights to her story, the studio made Sizemore give three different signatures, one for each persona.

Fifteen years later, in 1973, the US was gripped by the bestselling Sybil, “the true story of a woman possessed by 16 separate personalities”, which detailed her treatment by a psychiatrist named Cornelia B Wilbur. Sybil, whose real name was Shirley Ardell Mason, became something of a celebrity and a controversy. Her case was later alleged to be a total sham, cooked up by Wilbur and Mason, the latter of whom confessed to lying about her multiple personalities. But Sybil sold more than six million copies, and Mason was portrayed very effectively by Sally Field in a hit 1976 TV miniseries. Joanne Woodward played the psychiatrist.

Others have followed in this reality DID vein. In 1990 Shelley Long portrayed Truddi Chase – an abuse victim who developed multiple personalities (the real life Chase reduced Oprah Winfrey to tears when she appeared on her show). Tammy Blanchard played Sybil in a 2007 remake. Halle Berry played both leads in the 2010 movie Frankie and Alice (the former a black stripper; the latter a white racist), again based on a true story.

All of these portrayals have brought DID into the mainstream to the extent where we have even had a primetime sitcom with a twist. In United States of Tara, Toni Collette’s various colourful personae present challenges to family life. Or, in the latest small-screen manifestation (another spoiler alert), Mr Robot – a so-very-now hacking thriller whose protagonist, portrayed by Rami Malek, turns out to have been an unreliable narrator owing to his mental-health issues.

The makers of both United States of Tara and Mr Robot consulted health professionals, and sought to reflect the condition as accurately as possible – with mixed results. Collette’s characters were judged as implausibly showy by real-life DID patients, though they praised the show’s compassion. Another reviewer with DID praised Mr Robot (anonymously, because “patients who have made themselves known to the media have had very negative experiences”, he pointed out) for increasing understanding of the condition. “Where else can I witness thousands of people engaging with the raw struggle that makes up my day-to-day reality?” But he was disappointed by press coverage that used words such as “deranged”, “breakdown” and “damaged”.

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Let’s be honest, though, promoting greater understanding of mental illness has never been the priority for the entertainment industry. Another explanation for the fascination with DID could simply be that actors love it. They could not ask for a better way to demonstrate their range. And it often works. March won an Oscar for his Jekyll and Hyde; as did Woodward for The Three Faces of Eve. Sally Field won an Emmy for Sybil, Toni Collette an Emmy and a Golden Globe, among others, and Rami Malek and Christian Slater have won shelf-loads of awards for Mr Robot. Even Halle Berry won a few minor gongs for Frankie and Alice.

James McAvoy deserves praise for Split, too. Despite its ludicrous premise, he does a fine and fearless job of selling his character’s varied personae. There’s even a showreel-worthy moment towards the end where he cycles between different personae in one scene. It’s a little like the T-1000 at the end of Terminator 2. But there are no special effects here, just acting.

When you think about it, that’s what acting is. It is adopting another persona. Actors feign dissociative identity disorder for a living, for our pleasure. So, in effect, DID isn’t just an obscure, misunderstood condition; it’s the foundation for the whole vast entertainment industry that so often misrepresents it.

Split is released in the UK on 20 January