(CNN) Psychiatrist Dr. Garrett Marie Deckel received a "moving" email last week from a patient with multiple personalities, a condition known as dissociative identity disorder, or DID.

"There's a new movie out about a person with DID. It's a thriller/horror movie," her patient wrote, referring to M. Night Shyamalan's latest movie. "Do I ever scare you?"

In Shyamalan's latest film, "Split," actor James McAvoy plays the villain, a man with 24 personalities who abducts and torments three girls in a windowless bunker. The film opened at No. 1 this weekend with more than $40 million in domestic sales . Though McAvoy, best known for his work in the "X-Men" prequels, has received praise from critics for his performance in "Split," the Hollywood film has faced criticismfor how it portrays mental illness.

Mental health advocates warn that the film stigmatizes dissociative identity disorder and may directly impact those living with it.

"You are going to upset and potentially exacerbate symptoms in thousands of people who are already suffering," said Deckel, a DID specialist at Mount Sinai's Icahn School of Medicine, immediately after seeing the film.

A coping mechanism

Amelia Joubert, 18, of Fort Mill, South Carolina, was diagnosed with DID three years ago. She lives with 11 other personalities, or alters. Even before she was diagnosed, the way she spoke would change throughout the day: sometimes with a Southern accent, sometimes like a small child.

Like most patients with DID, Joubert suffered ongoing trauma as a young child.

According to Deckel, people who have been chronically abused, typically in situations with no viable escape, may "reconfigure the mind" into different parts or personalities. Some of these parts can step in to handle traumatic or stressful situations, while other parts dissociate , or escape, from reality.

For an introverted Joubert, this may mean calling on her energetic alter, Scarlet, in large crowds.

Since beginning therapy, she has been able to better control and agree on the switches with her alters. For Joubert and many others with DID, the goal of therapy is not always to "integrate" the different parts back into one, but to learn to function and work together.

"She has been able to use them very effectively," said her therapist, clinical psychologist Bilal Ghandour. "There's no reason to disrupt that system, as she calls it."

"It really is a survival or a coping mechanism," said Joubert, who does not plan to see "Split."

In contrast with McAvoy's character, Deckel said, people with DID, who may represent over 1% of Americans, are rarely violent. Research has shown that they are far more likely to hurt themselves than to hurt others.

But movies tend to portray only "the most extreme aspects" of the disorder, she said. This can misrepresent a form of mental illness that is not well understood by the lay public, and even some psychiatrists, she said.

"In my residency, I don't even think DID was brought up," said Deckel, who believes that a lack of training and research is one reason why even some mental health professionals approach the disorder with skepticism.

Joubert, who has been hospitalized in residential and acute care facilities, said she often felt that her doctors did not understand or even believe in her disorder.

"I was tired of hearing this and feeling like I had to be ashamed of something ... that helped me survive trauma as a child," she said.

A community reacts

Joubert maintains a YouTube channel and manages a DID support group on Facebook with nearly 4,000 members. Many are people living with DID, but some are family members and clinicians.

"People are upset" about the film, she said. "They're feeling discriminated against ... but this is nothing new."

Like the patient who emailed Deckel, Joubert said people have asked whether she was dangerous. She works as a nanny and said one online commenter suggested that she should not be working with children.

Joubert said she thinks "Split" is having a larger impact for younger people with DID; they're less familiar with older films, such as "Psycho" and "Identity," that also contain violent characters with multiple personalities. She is afraid that "Split" may deter young people from coming out and seeking help.

"This is the first big movie they've experienced that has a stigma to it," she said. "It's hitting hard for that reason."

There are some more positive portrayals in the media, she said, such as "United States of Tara." In the Showtime dramedy about a suburban American family, Toni Collette played a mother who is diagnosed with DID. The series, which Joubert said was "overexaggerated, but had a lot of relatable things," ran from 2009 to 2011.

"At this point, any movie that doesn't villainize us is a win," she said.

Leah Peterson, 46, of San Diego, California, has written about her experience with DID and was brought on as a consultant on all three seasons of "United States of Tara."

When the trailer for "Split" came on in a movie theater, Peterson "had to get up and leave."

"It's not about the acting or the people who wrote it," she said. "But are you doing a disservice to the people you're portraying?"

has said he was unable to speak to someone with DID in preparation for the role. Instead, he Scottish actor McAvoy, whose representatives did not respond for comment,has said he was unable to speak to someone with DID in preparation for the role. Instead, he told "Today Show " host Matt Lauer that he "spent a bit of time with some medical professionals" and watched YouTube diaries made by people with DID.

"I couldn't find anybody that would sit and talk with me, unfortunately," McAvoy said.

Hoping to shape the conversation

Clinical psychologist Bethany Brand , a professor at Towson University, was contacted in late 2014 by Shyamalan, who was hoping to learn more about DID. They met twice in early 2015, once at his home in Pennsylvania.

"I understood it was a big gamble," Brand said, adding that she was not paid to speak with Shyamalan. "I hoped that I would be able to influence the movie."

Brand said she offered to help introduce people with DID to Shyamalan, who did not respond to CNN's request for comment. She did not speak with McAvoy or Joaquin Phoenix, the film's original lead.

Shortly after the trailer was released in July, Brand emailed the filmmaker to express concern.

"The trailer for Split is causing outrage among trauma and general therapists," she wrote in emails she shared with CNN. "Do you plan to do anything to help the patients you are portraying as dangerous?"

Shyamalan responded that he and Universal Pictures were interested in promoting information and support for those with DID.

"When the film opens we will work ... together and raise awareness," he wrote.

Brand was also put in touch with a representative from Universal Pictures. She described their conversation as encouraging.

"And then, crickets," said Brand. "There was nothing."

Shyamalan told Yahoo Movies for a story published last week that he has seen no backlash over the film's treatment of mental health issues among people who have seen the final product.

"We've had no issues from people that have seen the finished film, just zero," he said.

Joubert sent her own petition to Shymalan's production company, Blinding Edge Pictures, in September. With more than 16,000 signatures, it called on the actors to publicly affirm that violence is rare among those with DID in the form of a PSA. A similar petition calling for a boycott of the film was signed by more than 20,000

Joubert did not receive a response to her petition.

"I feel like the DID community is being ignored," she said.

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Joubert said that she is not against people seeing the film but that she hopes for more education about the realities and misconceptions of DID.

The movie portrays the negative side of something in which many have found positives, according to experts and people living with DID.

"The brain is amazing that it's able to do this," said Peterson, a mother of four.

Though Peterson no longer has alters, she credits them with getting her through traumatic periods in her life and said she understands the role they play for many people with DID.

"They are some of the strongest, most big-hearted people you will ever meet," she said. "And you know some of them."