+But that was just my _guess_ at how to apply reductionism to describe the atmosphere of women-only spaces using lower-level criteria—and it was probably a _bad_ guess. (For one thing, [TODO: lesbians]) A woman who benefits from women-only spaces and knows more about psychology than me might say something different, and we should listen to _her_, not me. The "more than −1σ in both of these two dimensions" threshold was completely arbitrary; maybe she would prefer some other function. Maybe she [doesn't like the Big Five model](https://en.wikipedia.org/w/index.php?title=Big_Five_personality_traits&oldid=868909816#Critique). Maybe (in fact, almost certainly) multiple such women wouldn't _agree_ on the exact criteria.
+
+But even if such a committee of female psychologists _could_ agree on such criteria, I think most people would say that reorganizing the group as the "not-exclusively-androphilic people in our-favorite-hobby whose results on this-and-such personality battery match the following 1 KiB description ..." is not a particularly very appealing proposal.
+
+It would seem that in a world where psychological traits can't be cheaply, precisely, and verifiably measured, discrimination on the basis of [easily-observable traits that happen correlate with harder-to-measure traits that we actually care about](/2018/Feb/blegg-mode/) turns out to be a practical bright-line Schelling point for people to coordinate around.
+
+_Not_ an infinitely-thin, infinitely-bright line,[ref]As it is said: what about masculine women and feminine men (whose share of the population depends on where you set your sex-atypicality thresholds)? What about trans people (0.3%–[TODO] of the population, depending on how you define your categories and whose statistics you trust)? What about people with [5α-Reductase deficiency](https://en.wikipedia.org/wiki/5%CE%B1-Reductase_deficiency) or any of a dozen other specific intersex conditions?[/ref] but a line thin _enough_ and bright _enough_ that the forces of social evolution have coughed up some institutions and other cultural practices that take the line into account for _functional_ reasons.
+
+My goal in writing about this is certainly not to argue for more sexism—I'm looking forward to the postgender lesbian transhumanist future of Total Morphological Freedom as much as anyone else. (I already have my new name and outfits picked out!) If we can invent _new_ institutions and practices that serve more people more effectively, we should _do it_. But because I am a rationalist, because I cannot _unsee_ the cold, cisheteronormative logic of [Chesterson's fence](https://en.wikipedia.org/wiki/Wikipedia:Chesterton's_fence), I have to speak out when people are using clever word games to obfuscate the function of the existing fences.
+
+In response to the argument that women's restrooms function as safe havens that women can retreat to and exclude scary or threatening men from, Ozy writes:
+
+> I do not understand the relationship between this and psychological gender differences. It seems quite obvious that the relevant category here is "people who look like the vast majority of street harassers" versus "people who do not look like the vast majority of street harassers." The former group uncontroversially includes some trans women (closeted trans women) and some trans men (Buck Angel) and has nothing to do with psychology anyway. No matter how female-typical a trans man's psychology is, if he has muscles like Chris Hemsworth and a beard like a lumberjack, he belongs in the men's room.
+
+It has to do with _probabilistic predictions about_ psychology in a world where [male violence against females is _older than humanity itself_](https://en.wikipedia.org/w/index.php?title=Sexual_coercion&oldid=866576906), and with [defensible Schelling points](https://www.lesswrong.com/posts/Kbm6QnJv9dgWsPHQP/schelling-fences-on-slippery-slopes).
+
+Certainly _most_ men are nice, civilized people who don't harrass women—and occasional Hemsworthlike, lumberjack-bearded androphilic trans men with a feminine personalities, present even less of a threat. But when designing the social norms for a safe space for the modal cis woman, false positives (including someone who shouldn't be included) are probably going to be worse than false negatives (excluding someone who shouldn't be). If "Does this person look male?" is _easier to assess_ than "Does this person-of-whatever-sex look like a potential threat to my safety, comfort, and privacy?"—and possibly more importantly, is easier for third parties to _agree on_ when third parties are called in to enforce the rules—then the rule ends up being "no men" (or more precisely, "no male-looking people", with corresponding consequences for trans men and non-passing trans women), because "no suspicious-looking people" is nearly impossible to enforce in a non-arbitrary way.
+
+Depending on your values, this may not be the best rule! This is (despite everything) not a politics blog. I should hope to help clearly identify the trade-offs inherent in the objective reality of a situation, rather than champion one trade or the other; it's not for me to decide what kind of spaces people should demand, or what false-positive and false-negative rates they should accept.
+
+[TODO: transition sentence (no pun intended)]
+
+[TODO (somewhere): self-identity is itself a Schelling point]
+
+When the _Times_ of London filed some freedom-of-information act requests, they found that [almost 90% of harrassment/assault/voyeurism incidents in changing rooms took place in the minority of unisex facilities](https://www.thetimes.co.uk/edition/news/unisex-changing-rooms-put-women-in-danger-8lwbp8kgk).
+
+[TODO: explicitly acknowledge that I'm not trying to shift goalposts; locker rooms are different from bathrooms, everyone deserves to pee]
+
+Ozy continues—
+
+> Similarly, early-transition trans women can be placed into the former category. In our culture, it is generally very stigmatized for men to wear dresses, skirts, makeup, and other signifiers of womanhood. In particular, catcallers and sexist harassers essentially never do: if you're a catcaller or a sexist harasser, it is probably because you are invested in a particular style of masculinity that is completely incompatible with wearing a skirt. Therefore, allowing all dress-wearing people to use the women's bathroom has minimal risk of allowing catcallers in. In the event that men wearing dresses and makeup is completely destigmatized to the point that even sexist assholes do so, I am happy to reexamine this statement.
+
+Although I lack relevant lived experience, I suspect this is _wildly_ overestimating the _ideological_ component of women's discomfort around men. I agree that certain very overt kinds of harrassment (the kind that involves yelling slurs or obcenities) can be attributed to sexist subcultures of _machismo_ and toxic masculinity. Unfortunately, I fear the [threat model](https://en.wikipedia.org/wiki/Threat_model) is a little bit subtler and more expansive than that.
+
+[TODO: expand on the expanded threat model, marginalism]
+
+Maybe you want to bite that bullet. But you should at least _acknowledge_ the existence of the bullet, once someone points it out. And if you want to call yourself a rationalist, maybe try to proactively look for the bullet
+
+-----
+
+I think there's a more general lesson underlying these kinds of discussions. If you want to get through life without _verbally_ acknowledging the concept of biological sex, then you _can_ get away with it.
+
+If the object of discussion is a large, undeniable, binary sex difference, you can always say, "Oh, that's a mere policy question that can be handled on the basis of more specific details of that particular use-case." So, for example, we can agree with people with prostates should get prostate cancer screenings as they get older, without necessarily reifying that category of people as 'men' or 'males'.
+
+And if the object of discussion is a small, statistical sex difference, you can always point out (correctly! importantly!), "Some cis people of that gender are like that, too!"
+
+Does the fact that it's possible to scrupulously rephrase any individual sentence to elide sex imply that the corresponding mental _representation_ of the concept of 'sex' is of little practical use? I don't think so, and I've tried, within the limits of my time and my writing ability, to explain why. But if some readers still aren't convinced—well, maybe I can live with that.
+
+[...]