Why do most mainstream screenplays–and virtually all good mainstream screenplays–end up fitting into a three-act structure?

The most common answer is, “Because Aristotle says the three-act structure is essential.” There are two problems with this answer:

1. It’s an argument from authority, a logical fallacy of which Aristotle himself would not approve.
2. It’s a lie. Don’t believe me? Read Aristotle’s poetics for yourself. You won’t find the elaborate formula I described in my previous post. The closest you’ll find is this:

Tragedy is an imitation of an action that is complete, and whole, and of a certain magnitude; for there may be a whole that is wanting in magnitude. A whole is that which has a beginning, a middle, and an end. A beginning is that which does not itself follow anything by causal necessity, but after which something naturally is or comes to be. An end, on the contrary, is that which itself naturally follows some other thing, either by necessity, or as a rule, but has nothing following it. A middle is that which follows something as some other thing follows it. A well constructed plot,
therefore, must neither begin nor end at haphazard, but conform to these
principles.

Yes, in very general terms, this applies to the three act structure, but it’s a bit of a stretch. Anyway, if you’re going to treat the Poetics as a screenwriting bible, you’re going to have to start writing your scripts in iambic pentameter, which the great philosopher decrees is “of all measures, the most colloquial.”
At any rate, even if Aristotle didn’t believe in the three-act structure, I do. When I started writing, I found it a very useful way of organizing my scripts, and I was very conscious of my act breaks. Eventually, I absorbed it sufficiently that I don’t really think about it consciously, but my scripts end up falling naturally into three acts.
So why does so much storytelling seem to fall so naturally into three acts? I’d argue that it gets back to that “Manishtana moment” I referred to in my previous post–the moment that answers the question, “Why is this night different than all other nights?” For me, this is the fundamental question of all storytelling. For whatever reason–evolutionary, psychological, or cultural-we humans like stories about the ways in which life can change. In big, plot-driven films, these changes are often huge ones: a moisture farmer gets caught up in a galactic war; an archeologist gets caught up in events that can alter the outcome of a World War. In smaller, character-driven films, these changes are often smaller ones: a wayward brother comes to visit his sister in a small town; a woman becomes dissatisfied with her loveless marriage.
A bit of reflection will show how this leads to the first two parts of the 3 act structure: you can’t depict change (act 2) unless you have a status quo to contrast it with (act 1.) So why, then, do we need a third act?
Because at a certain point, the altered reality that seemed so new at the beginning of act 2 becomes the new status quo. And stories about the status quo are inherently dull.
One of the corollaries of my theory, by the way, the three act structure isn’t inherent to all storytelling; it’s an artifact of the specific length of a feature film. As a story starts to get much longer than 2 hours, that third act starts to feel like another status quo, and you need another change to keep our attention. That’s why many classical plays have more than three acts, and many short films tell a story with only two acts–of which the first is often dispatched in a matter of seconds.
Next up, I’m planning an entry on the how of the three-act structure, which will finally get around to answering the question from Alex Epstein that prompted this series in the first place: just how does a film like The Incredibles fit into the three-act paradigm? I’ll also be taking a look at the limits of the three-act structure, which is, after all, just a tool, and not a divine dictate.