Iām flinging enrichment into your enclosure:
Talk about Norman and his three part system. Be free
YAY YIPPEE THANK YOU!!! So fur context to any unfamiliar onlookers, in the Psycho film franchise, Norman's system has two stated alters, being Norman and Mother (here named as such to distinguish her from the living Norma). In the original novel, the system is explicitly a trifecta, being Mother, the adult Norman (dubbed Normal), and a child Norman.
First of all, I want to lay out the function of novel!Norman's system in more depth. We have a scenario of three (known) alters, with two of them essentially identifying with each other, yet still evidently separated enough to not simply be an age slider or age regression. (Of course, the distinction between age distortion in flashbacks vs age regression vs alters of different ages is largely a semantic debate, as they all relate to the basic mechanism of dissociation and distortion of time.) This makes direct communication between the two of them in some manner less effective or at least literalized than the dialogue between them and Mother. They also seem to have a lower amnesia barrier between each other as opposed to Mother. Normal also notes feeling more adult when reading a book or otherwise not around Mother, which indicates that the little Norman interacts more with her. This, with the considered role of Normal, implies to me that the little Norman is also a trauma holder.
So. Why the change from three to two? Certainly Norman's connection to childhood in the film is maintained through his limited life and childish room, and even expanded on with a prominent stutter & favor of simple foods such as sandwiches, milk, and candy corn. (One can note a line between Norman's autism-coding and his portrayal as infantile, but that's another topic.) Not much is changed by the removal of the little Norman, so why do so?
In part, I suspect it has to do with the filmic shift. While age regression can certainly be portrayed in film (the Psycho sequels make this especially explicit), it is far easier to hide and foreshadow systemhood and psychosis in literature, where we are directly given access to the character's thoughts. I've said before that literature has a far easier time navigating liminality, and this particularly stands out with hallucinations; the Psycho novel hardly shows Mother, but we have plenty of Norman imagining her actions or exchanging pure dialogue such that we don't really need to see her. Disguising the physical existence of Mother as an alter or Norma as a corpse is far easier in a medium where we don't have direct visuals. In fact, Hitchcock noted the overhead shot of Mother attacking Arbogast was because it was a natural way to hide the face. Little!Norman essentially has the opposite problem, where he's too close in identity to Normal to be easily separated from the audience's perspective, and thus wouldn't make as compelling of a reveal as opposed to the novel where Norman constantly talks about a separate inner child.
I think in part it's because of a larger issue the Psycho franchise has with anti-system ableism. The evil alter trope is an obvious example, but I think it's only one branch of the larger base, where singlethood is viewed as the natural and stable way of being.
The voices had exploded when triggered into fission, but now, almost miraculously, a fusion took place. So that there was only one voice. And that was right, because there was only one person in the room. There always had been one person and only one.
When the mind houses two personalities, there is always a battle. In Norman's case, the battle is over... and the dominant personality has won.
Collapsing the two Normans into one furthers the normativity (ha) of singlethood, as there isn't the question of which Norman is "real", and as such, a singlet audience may feel a greater sense of loss, as the "rightful owner" of the body-- the one that aligns most with it-- is the one that is lost, rather than the ambiguity of two.
(For what it's worth, I kind of prefer this tragedy in the novel; Norman never formed a whole identity, there is always the struggle between two archetypes, the ideal child and ideal adult, that struggle to synchronize their goals, so eventually the most focused personality, an introject of their abuser, the person they exist in large part to protect, takes over their mind, body, and life.)
There is nuance to this, of course. To characterize Psycho as a piece that exists exclusively to demonize mental illness is to miss a major reason the psychological horror is so compelling: Norman is intentionally relatable. As Lila Crane says in the novel:
"And right now, I can't even hate Bates for what he did. He must have suffered more than any of us. In a way I can almost understand. We're all not quite as sane as we pretend to be."
Part of the reason the novel is able to keep the dissociative nature of the little Norman hidden for so long is because our society does understand a layered self! Everyone will feel ambivalent impulses at some point, or feel themselves torn between two identities. The film echoes this with the expanded role of Marion, who (non-literally) hears voices as she drives, both repeats of past conversations and imaginations of the present, much as Norman hallucinates conversations with his dead mother.
So, I think the change of Norman's system from three to two is largely clarity of film, an attempt to play fair with the mystery. Certainly the age regression is maintained, and arguably brought more to the forefront in sequels! Still, it is sad that it often goes unknown or at least unacknowledged, because I think the tension between the two Norman alters adds a real depth to the system!