Watching movies and TV shows, it would be easy to think that dissociative identity disorder is something that exists only in the realm of thrillers and superhero stories. Previously known as multiple personality disorder, DID has been sensationalized to a fantastical degree in everything from “Sybil” to “Split” and Marvel’s “Moon Knight,” and it’s easy to see why.

Multiple personalities in one body: the idea lends itself to dramatic storytelling. But for some people, including folk legend Joan Baez, it’s not just material for Hollywood drama but a reality.

So, what is DID? William Sharp, an associate teaching professor of psychology at Northeastern University and a practicing therapist, says it is a severe personality disorder that happens when the brain, in response to trauma, figuratively splits, creating separate, autonomous personalities, or “alters.”

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