Of course, Psycho II posited that after being revealed as the killer of Marion , Arbogast and others, Norman was RETURNED to a mental institute(this time, for the criminally insane) for 22 years and then released to his house and motel again. And Psycho III ended with Norman AGAIN sent back to the institution, likely for good Psycho IV had him out again, married to a psychiatric nurse and living in a tract home, but that was written as if Psycho II and III had never happened. But in the final analysis, Psycho II, III, and IV "never happened." They were "fan fiction" based on a work of art.
Now, even with all of this found in the Bloch book, nothing of this nature is said in the movie, so all the movie really gives us is the sneaky hunch that Norman HAS been in a mental institution for some reason, or perhaps his mother was, or perhaps another relative. He knows too much about them -- "the laughter and the tears, the cruel eyes studying you." And that "cruel eyes studying you" remark sounds in the many eyes studying people in Psycho: Marion is studied by the cruel eyes of Cassidy, the cop, California Charlie, and Norman himself(at the peephole); Norman is studied by the cruel eyes of Arbogast, Sam, and the psychiatrist, and, noted Hitchcock himself, by the cruel eyes of the stuffed birds who watch Norman and "appeal to his masochism.". (This is part of what I call "the effortless symbolism" of Psycho.)
In addition to the revelation about Norman's institutionalization, other things found only in Bloch's book but not in Hitchcock's movie include:
Arbogast's first name(Milton.)
What's in that book that Lila opens(pornography.)
The name of Mrs. Bates' lover ("Uncle" Joe Considine.)
Thank you, ecarle. The movie seems to have the right balance between explaining things from the book and allowing the audience to infer.
Sure. Thank you for reading! And..I agree.