"Multiple personalities disorder" are entirely the result of suggestion and does not exist as an actual person having multiple "persons" taking turns in the body. It's play-pretend for attention.

It took off when doctors started suggesting and reinforcing the play-pretend and suddenly lots of people wanted that new cool headmates (fake) disorder.

Exactly like the craze of "recovered memories" when a bunch of people started accusing their parents of sexually abusing them as babies and toddlers after multiple sessions of suggestion / reinforcement by their therapist. The poor patients eventually believe those imaginary scenarios they slowly crafted with the therapist are totally real memories.