@Bloodmagic.
"You have provided a fine definition of gender identity, an innate sense of ones own sex."
Thank you. The first point to make is that the post is aimed at the many people on Mumsnet who state that gender identity doesn't exist, or paradoxically state they they have no idea what it means. It is also for all those people who confuse gender identity with gender roles. This is fairly basic stuff to have a handle on if you are going to debate trans issues. What I am pleased about is that quite a few contributors here do seem to be willing to accept that gender identity exists. I think that's progress.
"As other people have pointed out tho, that means that your gender identity always accords with your actual sex... It does not follow from that that it is possible to have a gender identity which doesn't match your birth sex. That could be true, but its a separate supposition that you have provided no evidence for."
We already know that it is possible to have a gender identity that doesn't match the sex you were assigned at birth given the fact that trans people exist. The question is not "if" it happens, but "how" it happens.
"It's important to note that sex reassignment (as was done to the boys in this study) doesn't actually change your sex."
I agree - that's why I put "sex change" in inverted commas. I never changed sex - I was always female - that's why I transitioned. Trans people tend to prioritise the mind over the genitals when determining someone's sex..
"They still had XY chromosomes."
So do CAIS women - human sexual development is rather more complex and nuanced than XX and XY:
www.scientificamerican.com/article/beyond-xx-and-xy-the-extraordinary-complexity-of-sex-determination/
"Their skeletons were still male."
Male and female children's skeletal structure is more or less identical, which is why archaeologists have such a difficult time sexing children's remains. The key differences happen at puberty under the influence of sex hormones. This boys in the study were on estrogen. This is why trans girls who transition with puberty blockers etc develop a fairly typical female skeletal structure.
"Don't you think it's possible that what was identified in this study was not some mysterious 'gender identity', but merely the ability to accurately perceive one's sex regardless of surgical alteration and hormones?"
What were they perceiving? Their liver function?
"For example, if I was somehow unable to perceive my body visually, or by touch, and was never told my sex, I could still deduce my correct sex by noticing how my body's rhythms fluctuate on a monthly cycle"
These boys were without gonads - their hormonal cycles were artificially induced.
"In fact, as the boys in this study continued to perceive their correct sex regardless of surgery, doesn't this undermine the whole concept of sex reassignment as a treatment for people with dysphoria?"
The boys gender identity was immutable as is the gender identity of trans people. The object of transition is not to change your gender identity, but to match your body to the gender identity you already have. That's why transition is so effective:
whatweknow.inequality.cornell.edu/topics/lgbt-equality/%20what-does-the-scholarly-research-say-about-the-well-being-of-transgender-people%20/
"How could it cure them when people generally maintain the ability to internally perceive their sex regardless?"
You are mistaking gender identity for an ability to perceive the sex of your body rather than an ability to perceive the sex of your core identity. The boys in the study had no way of knowing they were physically male, their knowledge of their sex was innate and separate from genitalia etc. And as we know from trans people, even if you are fully aware of the shape of your genitals, and the sex you were assigned at birth, this has no effect on your gender identity.
"What does your definition imply for gender fluid and non-binary people?"
It's an interesting question. One thing that the science suggests is that gender identity doesn't have a simple aetiology. So imagine, for the sake of argument, that gender identity was distributed across say 6 regions of the brain. If your brain developed 3 regions in the male range and 3 in the female range, you could possibly expect a non-binary identity. We will see how science unravels that one.
"or to have that innate sense reverse completely on a daily or hourly basis?"
I suspect that if your gender identity is balanced in the centre, it's quite easy for your perception of it to fall either side from time to time. But again, we will have to wait for the science on that.
"Or do you discount those people's gender identities?"
Not at all - I fully support them. Nor do I discount those people who seem either to not have a gender identity or have an inability to perceive it (agender) many of who are on this forum. It's only a shame that so many of the agender people here have made the leap from "I have no gender identity" to "Therefore no-one has a gender identity".
Also, gender identity can be quite a difficult thing to unravel - it's not always absolutely clear. May trans people for example know their is something wrong at the age of 3 or 4 and that it is something to do with boys and girls, but it can take a long time for the penny to drop. You can see an example of of this in a real life cloacal exstrophy case here:
www.dailymail.co.uk/femail/article-3191600/The-agony-learning-ve-raised-wrong-sex-rare-condition-meant-doctors-weren-t-sure-sex-Joe-born-support-Princess-Diana-finally-accepted-girl-came-shattering-discovery.html
Apologies for posting a Daily Mail article.
"In addition, your definition of gender identity seems to rely heavily on body dysphoria which is not widely accepted in the trans community."
I work with trans women. I can assure you that's not the case.
"It is possible to be transgender without having any issues at all with the sex of one's body."
That's not quite true - it's more a case that there are some trans people who are not troubled by their body being in opposition to their gender identity. That's also OK.
"This is a belief which is in contradiction with accepted neuroscience - that the brain is not an especially sexed characteristic and that it is malleable."
It is certainly true that although there are statistical differences between male and female brains, there is also a huge overlap. However, we are talking about a very specific property of the brain - gender identity. We certainly know that there are brain anomalies in trans people as you can see from the 26 peer-reviewed articles linked to here:
www.cakeworld.info/transsexualism/what-is/brain-similarities
And that is just up to 2015. Or to get an overview of the scientific consensus based on just a few of those studies, see this Scientific American article:
www.scientificamerican.com/article/is-there-something-unique-about-the-transgender-brain/
"Importantly, you can't have a 'gender identity' if you don't believe that the brain or mind is a sexed characteristic"
Gender identity is not a matter of belief - it is a matter of observation. The cloacal exstrophy boys didn't need a complex analysis of current thinking in the neuroscience of brain sex - they simply observed that they felt male despite no physical or social evidence to support that observation.
"so gender critical people are never 'cis',"
Most gender critical people appear to be agender, and are therefore not the "c word" which I am not allowed to use.
So that leaves the question of how trans people occur. Evidence at the moment is pointing quite strongly to genetics:
www.abstractsonline.com/pp8/#!/4592/presentation/578abstract
And here's an interesting one. As I am sure you know, CAIS people are born with vaginas (and sometimes wombs) despite XY chromosomes. So we know that it is possible for an XY foetus to develop a typically female physical characteristic. Some CAIS people seem to have an issue with the AR gene:
en.wikipedia.org/wiki/Androgen_insensitivity_syndrome#AR_mutations
It's not the only cause, and it's not a simple cause, but interestingly, trans women have also been found to have anomalies on this gene:
www.cakeworld.info/transsexualism/what-is/transsexual-genes
Of course, that's not the last word, and I am not suggesting the question is solved, it's merely one example of one possible cause for a person's gender identity not matching the sex they were assigned at birth. We will see how the science progresses, but so far, none of it seems to be moving away from the observation that there is something biological going on behind gender identity.