Multiple Personality Disorder/DID is a fake illness perpetuated by social contagion. These people don't have multiple personalities, they are often very young, teenagers and young adults, who will grow out of it and admit later on that it was made up.

I highly recommend the book "Sybil Exposed: The Extraordinary Story Behind The Famous Multiple Personality Case" it goes over the psychology behind faking an illness like this, and the social contagion, how it spreaded.

Here are subreddits dedicated to tracking these fakers.

https://www.reddit.com/r/DIDcringe/ https://www.reddit.com/r/fakedisordercringe/

Multiple examples can be found there of the (mostly) teens admitting they were making it up.

I am very interested in social contagions and how they relate to the trans movement. Especially ones involving self-medicating, like trans, DID, anorexia. Suicide however is the most widely known social contagion, where schools, news agencies, etc will be careful of the way they report a suicide so as to not spark a trend of suicides.