After leaving this comment on jimhines‘ Facebook post, it’s kind of taken on a life of its own, so I thought I’d share it here, too.
I am not a neuroscientist, but had to study the neuroscience behind gender identity quite a bit as part of my Behaviour degree. Rambly science flailing ahead!
The thing is, humans like to sort things into boxes. Way back in the day, the ability to quickly decide "this thing is poisonous and this is not" or "this is a thing that will eat me and this is not" contributed to our survival, so it makes sense that our first inclination is to sort things into box A or box B. But nature is not about boxes. Nature is about spectra.
In some ways, it's true that you're born male or female (or neither or both), but it's not locked in to sex the way some people believe it is, nor is it as white or black. Brain differentiation in the part of the brain that controls self-perception (including gender) happens in-utero in response to a hormone dump, months after the hormone dump that starts the differentiation of the bits between the legs (which is ALSO a spectrum with the results usually clustered around two loci, but in both brain and body structure, there are outliers and points in between). Often, the people with brain formation A (or close to it) have body part A, and people with brain formation B usually have body part B. It happens often enough that this is why we have the heteronormative ideas of "male" and "female" that we have used throughout history to whack anyone who doesn't fit squarely on the loci over the head with. Though this doesn't even touch on the extremely messed-up fact that society pretty much arbitrarily decides how AA and BB act and dress, or that AA likes blue and BB likes pink, until the mid-20th-century when they arbitrarily decide again that, no, it's the other way round. Society is kind of a dick about shifting the goalposts that way.
But not everyone is smack on either locus, either in their brain structure or in their body configuration, and sometimes people get different hormone dumps that result in brain formation on or around locus A but body parts on or around locus B. It's a less common combination than AA or BB, sure, but no more "abnormal" than someone who, for example, has an uncommon combination of pigments (like dark skin and blond hair). You can find it happening all the time across species. Just because it's not common doesn't mean it's not "normal." It just means that the person got a different dose of hormones in the womb.
That's not opinion. That's science. There are many peer-reviewed articles on the topic. Saying otherwise is like saying "it's my opinion that rocks are soft like feather pillows." That's nice for you, but it doesn't change the fact that if you jump off a roof onto a pile of rocks, you're gonna get messed up. The rocks don't care if you think they're soft. See, we can see inside of forming brains now. These findings are repeatable. Saying that you don't believe in it is like saying you don't believe in gravity. Again, that's nice, but I'm probably not going to trust you to fly a plane.
So if someone has brain formation A, and I see the "person" as being in the brain and not in the bits (because hey, we talk to faces, not crotches -- if you do the latter on a regular basis, you might want to talk to someone about that), and I am a thinking human-type-person, I have no problem with using pronoun A and not B (though it may take me a while to stop falling back into habit if I'm not thinking about it). Ditto if they weren't on either the A or B brain locus and wanted a pronoun to reflect that.
And the fact that I'm a human being capable of reason and compassion (and not an animal who needs to classify something as predator or prey in order to not be eaten) is why I'd still use a person's preferred pronouns even if there was no science to back it up. Because in the end, it hurts no one and helps many. Wheaton's Law.
(Have a Franklin D. Roosevelt in his adorable dress and pretty patent leather shoes, which was perfectly "normal" until that goalpost got shifted).