While I would agree with your general premise, I should probably point out that my peers and I were quite familiar with the idea of multiple personality disorder, often called split personality at the time, all the way back to the early seventies. Nor was our understanding new. The 1950s bestselling book and blockbuster film “The Three Faces of Eve" probably had more to do with popular understanding of multiple personality disorder than anything published in the DSM.

Much of this popular understanding was incorrect, but long before the DSM definition came out, people not only presumed the disorder existed, but actually talked about it quite a lot.

My friends and I did, even in elementary school. It was a regular part of urban mythology.