[INTRO ♪].

In 1955, a woman named Shirley Mason visited the psychotherapist she'd been seeing for a few months. When the therapist asked her how she was doing, she said, “I'm fine but Shirley isn't.

She was so sick she couldn't come, so I came instead... I'm Peggy!” Through continued therapy, the therapist discovered what seemed like 16 personalities living inside Shirley's mind. She later wrote a now-famous book about Shirley, titled "Sybil"— using a different name to protect her patient's privacy.

And that book put a rare syndrome, then called multiple personality disorder, on the map. Before Sybil, fewer than 100 cases had ever been reported, but after the book came out, diagnoses exploded into the thousands. In a surprising twist, Shirley's specific case might not have been real, and she eventually wrote her therapist claiming she'd made it all up.

But researchers are still looking into the disorder as a whole. Today, we call Shirley's diagnosis dissociative identity disorder, or DID. It often occurs alongside other psychiatric disorders, making diagnosis very difficult.

And it's still very controversial among psychologists and psychiatrists. But what's not controversial is that the way Hollywood portrays it is way off. To be diagnosed with DID, a patient needs to have two or more so-called “identities”, or personality states.

This doesn't necessarily mean a whole other person is living inside their mind. In many cases, it's more like their overall personality is fragmented, and different parts of their history and self-image can take control. For example, only one personality state might be able to speak a language learned during a semester abroad in college, while another doesn't remember that experience.

Or if they'd previously been a very religious person, they might have one fragment of them that's devout, and another that isn't. Sometimes these fragments present themselves as multiple people with different ages or genders, but not always. And while these fragments can be complicated, with their own wants and aspirations, they aren't there to fulfill, like, evil or criminal desires.

So the idea that people with DID are ruled by the worst parts of themselves, like in Fight Club or Split, is entirely fictional. The other major symptom is memory loss or amnesia. This could be missing personal information about themselves, or missing memories from times when another personality fragment was in control.

Also, it doesn't count if the personality shift or memory loss are chemically-induced. If everyone tells you that you turn into someone else when you're drunk, but you can't remember what you said or did … you do not have dissociative identity disorder. And, parents: imaginary friends?

Totally cool. They're really common—like, 2/3 of kids have them— and this is not a symptom of your child harboring another secret personality. But if a patient has multiple identities, their memory is shaky, and they're distressed or impaired by these symptoms, then a psychiatrist might diagnose them with DID.

Many, though, wouldn't. It's not that their condition isn't real— there's no denying that patients have these symptoms. But experts disagree on where the symptoms come from— whether DID is a stand-alone disorder, or whether it's better explained by other, well-established disorders.

Surveys of psychiatrists found that most agree it's a valid diagnosis, but many think it's commonly misdiagnosed. That's partly because DID is so rare, most studies on it have very small samples. But it's also because it's very unlikely that patients get just a DID diagnosis.

It's usually tacked on to other diagnoses, and this is one reason why many psychologists think that it's better understood as an extreme version of other disorders. For example, DID can look a lot like a dissociative fugue state. This is a kind of temporary memory loss where people behave impulsively, leave their homes and wander around, and do things that they would never otherwise do.

So it might seem like a shift in personality. These people also often lose their memories of who they are, so they sometimes invent a new identity to fill in the gap. And although a lot of these fugue cases are a bit of a mystery, some are explained by substance use or neurological conditions.

DID also overlaps a lot with post-traumatic stress disorder, or PTSD. That's especially interesting, because the psychologists who most strongly believe DID is a distinct disorder think it stems from childhood trauma or abuse— which could also cause PTSD. And DID similarly shares symptoms with borderline personality disorder, which is characterized by impulsive behavior, as well as unstable relationships and identity.

One study of 33 patients in the 1980s found that 70% of patients with DID could have just as easily been diagnosed with borderline personality disorder instead. But, the researchers concluded that because 30% of the cases were distinct, DID should be considered a stand-alone disorder. DID is also controversial because many of the new diagnoses seem to be coming from just a few psychiatrists, and some suggest these doctors may be overzealous because they're eager to find new cases.

But the doctors themselves counter that by saying that they're just better at seeing the signs and identifying the disorder. Regardless, the good news is that treatment for DID with tailored therapy— at least for the symptoms related to identity fragmentation— is pretty effective. Which is great, because although there's a lot of controversy surrounding the disorder, psychiatrists all agree that real people are suffering and need help.

And like anyone else, they should be shown compassion, and not made into movie monsters. Thanks for watching this episode of SciShow Psych. If you want to learn more about the psychology of trauma, you might be interested in our episode on how trauma can be inherited. [OUTRO ♪].