And oh my God, I always just believed Hitchcock MAYBE based Norman and his mother's behavior and relationship on BPD (also psychosis, of course), but whoever wrote Psycho 4 thought that too. And wrote the whole Norma character based on BPD, it seems.
Her behavior, specially in one particular "rage" scene when she finds out a highway isn't going to be built near her motel, is a fantastic portrayal of a BPD rage.
Olivia Hussey did a very nice job as Norma Bates. I highly recommend Psycho 4 to anyone who wants to see BPD represented in movies in an astonishingly accurate manner.
I have no concrete information, but I seriously believe the writers and possibly Olivia Hussey really did research in BPD to construct her character.
The movie as a whole, meh, worst Psycho of all, though.