Dissociative Identity Disorder (DID) used to be referred to as Multiple Personality Disorder (MPD), but was renamed when psychologists realized that DID was more accurately causing a fragment or fracture in an individual's sense of self, rather than growing secondary or new personalities (as MPD implies).

Much of the way Tyler Durden's DID is portrayed in Fight Club is largely accurate, according to the National Institute of Mental Health.

• He experiences two distinct and separate identities.
• The disruption in identity involves a change in sense of self, sense of agency, and changes in behavior, consciousness, memory, perception, cognition, and motor functions. ("I look like you want to look, I fuck like you want to fuck, I am smart, capable, and most importantly, I am free in all the ways that you are not.")
• Frequent gaps are found in memories of personal history, including people, places, and events, for both the distant and recent past. (Edward Norton's character has either absent or inaccurate memories of Fight Club and Operation Mayhem.)
• These symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning (this is well played out in the final confrontation between the two). 

One key difference is that Edward Norton's character did not seem to experience any significant childhood trauma, which is noted as one of the leading causes of DID. The implication here is that the culture of consumerism enacts that trauma on all American men. However, this is where the narrative philosophy of the film (and book) does start to stray from psychology. It also somewhat makes the condition seem "cool" by giving Edward Norton's character a sexy anarchist gangster as his alter-ego, possibly downplaying the remarkably difficult struggle it is to live with DID. 

In the movie Fight Club, how does the portrayal of Dissociative Identity Disorder accurately or inaccurately reflect the condition?

In David Fincher's adaptation of Chuck Palahniuk's satirical novel Fight Club, an unnamed narrator (Edward Norton) suffers from insomnia and depression in a materialistic world. He is weak and submissive with no life direction. When he "meets" the charismatic macho-man Tyler Durden (Brad Pitt), he begins to feel free. Durden is everything he is not: confident, masculine, and free-spirited. As it turns out, Durden is simply another personality of the narrator that the narrator imagined and mentally projected. When the narrator falls asleep, Durden takes over. The accuracy in the film's portrayal of Dissociative Identity Disorder is minimal. The narrator's creation of his other personality as a coping mechanism is realistic, for most cases of DID are acts of desperate coping. Yet, what Fight Club lacks is a severe mental trauma to cause the narrator to need Durden. In most cases, DID stems from some sort of childhood trauma that becomes too difficult to cope with. In Fight Club, the only traumas the narrator experiences are insomnia and existential depression.

One way in which the film—as many others similarly do—mocks mental illness is through its portrayal of Pitt's Tyler Durden as a violent psychopath. In addition to the brutal medieval-style violence of the fight clubs, Durden starts a terrorist organization called Project Mayhem. Mental illness in film and television frequently stigmatizes mental illness as being inherently violent or something for the general public to be afraid of. This is wholly unfair to those diagnosed with mental illnesses, for the categorization and dehumanization strays from offering actual help.

Another instance of making a mockery of mental illness comes from the physical manifestation of Tyler Durden—in other words, the fact that the narrator communicates with and sees Durden as another tangible being outside of his body. At the end of the film, the narrator begins to recognize Durden to simply be a split personality of his own, another fictionalized account, for few, if any, diagnosed with DID suddenly snap into the realization of their own illness. The narrator shoots himself in the mouth, "killing" Durden. Once again, the illness is trivialized, fictionalized, and mocked in order to provide a satisfying ending. Cures for mental illness such as Dissociative Identity Disorder are not as simple as recognizing the illness and causing self-harm to "kill" the other identity.

While an entertaining and iconic film, Fight Club certainly doesn't stand as a medically or psychologically accurate testament to a real mental disorder.