The concept of gender, or the concept of passports? For the latter, there's a lot of small differences you only even notice by moving across a border, because

The concept of gender, or the concept of passports? For the latter, there's a lot of small differences you only even notice by moving across a border, because the "common sense" you grew up with conflicts with the "common sense" in the new home. My own experience moving to Germany from the UK had this, even: some countries (e.g. Germany) require everyone to have ID, but not all (e.g. UK); while I approve of it being as easy to move from Somalia to America* as from South Dakota to Arkansas, we're not actually ready for it yet. Would be a huge benefit to all of us if we were, and we should work towards it. For the former, the absence of distinction on legal records doesn't mean people won't treat you differently anyway; consider how ginger and blonde hair have stereotypes, even though hair dye is trivial to get and use. Thing is, there are even legit reasons to care about gender, unlike for hair — it's just… how do I put this… you know how you sometimes see people (IMO in bad faith) demanding to know what "the definition of a woman is"? In my experience, such people never like that all possible "definitions" are extremely domain-limited, and absolutely not universally applicable despite the correlation. To give some common examples and why they aren't universal: • If you're a surgeon, you'll care about the current actual anatomy, not the chromosomes — I can't get ovarian cancer if I don't have ovaries, doesn't matter if that congenital or from a oophorectomy. • If you're a geneticist, you care about the chromosomes (of which there are many more than 2 groups even in humans), not the anatomy. • If you're dating casually (and, at risk of me making the typical mind fallacy, if you're not bisexual like me), you care about the exterior bits. You do otherwise use contraceptives for sex while casually dating, right? So it doesn't matter if the bits outside are connected to anything functional inside, right? • If you want to start a family with someone, you care if they're fertile… which not all cis people are, while some trans people are. • If you're an animal biologist, you care about which has the biggest gamete… hence why in seahorses it's the males who get pregnant. • If you're trying to keep sports interesting, you care about how much testosterone is floating around, and wouldn't you believe it, that's something you can take as a supplement even without being trans. • You want to know how to address someone? Sure, in English polite forms have "Sir/Madam" variations, and many other languages have stronger variations between male and female forms in language (e.g. Pavlov/Pavlova), but that's why people list their pronouns now, and those don't have to have anything to do with gender anyway (I know several with the title Doctor, both men and women). etc. * Replace with your own country here, I'm picking on the USA just because it's where I think most HN readers are