+But here we have a slippery slope on what domains within Society should use developmental-sex categories or self-identity categories.
+
+At one extreme, a "Sex is immutable and determined by the presence of a Y chromosome, no exceptions" regime is a stable Schelling point: if you have a lab that can do karyotypes, there would be no ambiguity on how to classify anyone _with respect to_ the stated category system. (It would be cruel to trans people and people with [complete androgen insensitivity syndrome](https://en.wikipedia.org/wiki/Complete_androgen_insensitivity_syndrome), but it would be a Schelling point.)
+
+At the other extreme, "Self-reported self-identity only, no exceptions" is a stable Schelling point: _given_ the self-identity criterion of "Just ask the person what gender they are", there's no ambiguity about how to classify anyone. (This requires us to affirm the existence of ["female penises, female prostates, female sperm, and female XY chromosomes"](https://archive.is/Fpaw3), but it's a Schelling point.)
+
+In contrast, any of a number of "compromise" systems, while potentially performing better on edge cases, suffer from ambiguity and are on that account less game-theoretically stable. It's a lot harder for Society to establish a specific convention of the form "Okay, you can have your pronouns, but you can't use your target-gender {bathroom, locker room, sports league, hospital ward, _&c._} unless you {pass really well, get bottom surgery, have a gender recognition certificate, _&c._}", not only because different factions will disagree on where to draw the line for each particular gendered privilege, but also because any line not drawn on a sufficiently sticky Schelling point will face constant attempts to push it up or down the slippery slope.