As the scientist that I used to be, it occurs to me that it is always the edge cases that decide on whether a theory is potentially describing reality. The default cases are comparably uninteresting. All theories that deserve the word do a good job of describing those, we wouldn't be discussing them otherwise. Sure, crackpot theories that fail even the basic case do pop up from time to time, but I can't say I've seen them in this thread. Creationists' "kinds" don't fail at humans versus pigs, they fail at ring species. Newtonian mechanics don't fail at room temperature and standard earth gravity for objects moving no faster than a high speed train or passenger airplane. And a simplistic theory of sex and/or gender doesn't fail at typical males and typical females, it fails at individuals with intersex conditions. When it falls at those, that still makes it a failed theory though, and basing political decisions on a failed scientific theory sounds like folly to this would-be scientist here.
Alright, let's start from this, because I think you explained it fairly well. But let's expand this premise.
I want to be clear about the specific aspect I'm challenging - that of a "female brain" and a "male brain" being something specifically identifiable and distinctly separate from the sex of the body in which the brain resides. There are two main ways in which we can define "female brain":
1) differences in physical structures within the brain, including different orders of operations for processing perceptual stimuli and abstract concepts
2) differences in behavioral tendencies, preferences, and instinctual reactions
Of course, we could also assume that both exist at the same time.
The context of this entire discussion is based around the asserted notion that transgender people are actually for realsies the sex of their brain, not their body... because of this assumption that the brain can be sexed differently than the body. The underlying assumption involved is that the brain is a better way of identifying someone's sex than their reproductive system, and that as such, whenever there is a "mismatch", we should all be obligated to accept the brain-sex as superseding the body-sex.
We can have intellectual discussions about all kinds of things, but let's never lose sight of the actual premise involved here.
Now then. Let's provisionally accept that brain-sex is a thing, for the purpose of this argument. For any lurkers or those who lack comprehension skill - a provisional acceptance does NOT imply that I actually believe that brain-sex is real in any fashion. It's nothing more than a means by which we can investigate the logic and the consequences of an argument.
So, let's assume for the moment that brain-sex is real in sense (1) - that there are differences in structure. We already know that the structural differences are small, they're not gross. We've had what, a couple of hundred years of brains being dissected and investigated? And during that period, there has not been any obvious and material difference between the brains of males and females. This is in contrast to the differences in both reproductive anatomy and skeletal structure, which we've been observing with extremely high reliability for a long time. That means the differences are small. Where we stand right now are a very few recent fMRI studies that show a few differences in overall volume, and in the proportions of white vs gray matter, something or other about number of connections between different areas of the brain, and levels of activation in specific brain regions.
There have also been rebuttals to a couple of these observed differences. Off the top is the difference in volume, which is directly related to the size of the skull. And it's been demonstrated to be negated by that correlate - men with smaller than average skulls have brain volumes in the "average female" range; women with larger than average skulls have brain volumes in the "average male" range. So the actual material difference in play is the difference in *size* between men and women. The other item that has been repeatedly challenged (and in a couple of cases demonstrated) has to do with specific areas of the brain being activated in different ways, which is almost always related to sexual orientation rather than some ephemeral "sex" of the brain. And unless you want to take the position that gay men are actually women, I think we can set that one aside.
That leaves us with white vs gray matter and connection patterns. Alright. Is this a matter of innate structural differences, or is it an effect of neural plasticity? Is it driven by exposure to different hormones? So far as I know, none of these studies have been done on infants, or even on pre-pubertal children. They've all been done on adults. And by the time we're adults we've been subject to about two decades of behavioral conditioning as well as activation of adult hormones.
But let's leave that weakness aside as well, and let's just roll with it. At this point, let's go back to the premise of this thread, and the context in which "brain-sex" even came up. It came up as an argument that transgender people are *actually* the opposite sex (or some other brand new sex or something completely different or whatever), because their brains are "more like" those of the opposite sex than they are like their own sex. And this leads to a couple of questions that I think are very important.
First off... what does "more like" mean? How much "more like" are we talking about? What's the mean and standard deviations of these various measures, by sex, and where do the brains of transgender people fall? How far apart are the means in the first place? I can't find any numerical information about brain differences that means anything to me, so let's use foot size as an illustration - I know it's not a great analogy, but it should serve to highlight the relational aspect of this that I'm most interested in. We all know that on average, men and women have different sized feet, with men having larger feet than women. The average women's shoe size in the US is about an 8, the average men's shoe is about 11.5, but this is misleading because there's a 1.5 inch difference in the sizing. So a women's size 8 is the same length as a men's size 9.5. Standard deviation is around 2 inches for both sexes. That means that about 90% of women have a foot length between a men's 7.5 and 11.5; it means that about 90% of men have a foot size between 9.5 and 13.5. The difference in the means is less than the standard deviation of each distribution. That presents a situation where there's a material number of people of each sex that falls within one SD of the mean of the range for the opposite sex's shoe size. That's a lot of overlap. If you take it to 2 SDs, it's even more likely to find that any given person's foot size is comfortably within the overlap between the sexes.
This leads me to the second question: So what? If a measure of fine brain structures for transgender people says that they're "more like" the brain-sex of the opposite sex... what does that actually imply in reality? The argument as put forth here is that because of this "more like", that means that transgender people are "really" the opposite sex - we should assume that their "real sex" should be based on whatever their brain is most like, and we should ignore their bodies in terms of policy and social treatment. This, to me, is a prime example of special pleading, because the reality is that there's sufficient overlap in the distributions of these fine structures by sex as to negate the policy ask. If we were to take this approach, if we were to assume that the placement of these fine structures should determine the "real sex" of transgender people, then shouldn't we also assume that the placement also determines the "real sex" of ALL people? Wouldn't that mean that a female who has a brain that is "more like" a male's brain with respect to these fine structures should be treated as if they were all the way male, regardless of their body? But that's not the policy that is being advocated for. What's being advocated for is that transgender people should be treated as the sex of their "brain sex" and not their "body sex", but that everyone else should be treated as their "body sex". This is akin to arguing that if a male has small feet that are still within 2 SD of the male average, they should be treated as if they are a female because they have "female feet" and this overrides their reproductive anatomy completely... but only in the very special circumstance where that male says that they are transgender. If a male has small feet, but doesn't say they're transgender, then we treat them as if they're male in terms of policy. That's pretty clear-cut special pleading.
Now I'll add my additional complaint to this: We're being asked to accept that a transgender person must have a brain-sex that is more like the opposite sex,
without ever bothering to measure it at all. We're being asked to take it on faith that anyone who says they are transgender has a brain-sex that is different from their body-sex, but we're not supposed to check to see if that's true.
Going back to the foot size analogy, the net effect of this ends up being that any female can say that they're transgender, and we are expected to enact policies that treat them as if they are male, based on a study that showed that a small number of transgender people with poorly controlled study criteria had foot sizes that fell in the overlap between the ranges for male and female feet... even if the specific individual has a body-sex of female and wears a size 4 women's shoe. And we're not supposed to look at their feet (that would be rude and an intolerable invasion of their privacy), and we're supposed to pretend that their obvious secondary sex characteristics don't exist (because body-sex doesn't matter).
And that, I contend, makes no sense at all.
So far as I'm concerned, the notion of "brain-sex" is pretty damned nebulous to begin with, based on poorly controlled cherry picked studies, and is inapplicable to policy in any rational way whatsoever.