Here is a long blog post about some findings to do with gender identity, it's informative, is not conclusive but also explains why that is.
However science isn't always absolutes, but sometimes a likelihood or probability after eliminating other possibilities is a good marker towards where the truth will be found...
Over time, science bends towards truth. Scientific inquiry isn’t without bias, the very questions we choose to seek answers to can reveal bias on the part of the researchers asking questions. The neat thing about science though is that over time, as we propose hypotheses, conduct experiments (or passively gather data) to test those hypotheses we get data that allows us to ask more informed / better questions, to conduct better research and to expand our learning over time.
The question details say:
I looking for a peer reviewed study that gives evidence that gender is independent of your “birth-sex”.
You won’t find it. The reason you won’t find it is because that’s not how research works. You won’t find a grand unified study that presents that specific conclusion.
What you will find are studies stacked on top of studies stacked on top of studies that build towards those statements. Here, let’s examine an example:
Hypothalamic Response to the Chemo-Signal Androstadienone in Gender Dysphoric Children and Adolescents
Sounds fascinating right? As article titles it’s right up there with Androgen Receptor Repeat Length Polymorphism Associated with Male-to-Female Transsexualism.
If you’re not into biology in a way that makes you want to dive into examining MRI data to critically examine the study methodology then you might be inclined to skim to the bottom of the study so you can read something a little more general audience understandable:
Gender dysphoria has been hypothesized to develop due to an altered sexual differentiation of the body and the brain during early development (30, 31). Here, we investigated a unique data set of individuals with GD at two different developmental stages, in order to determine whether they would respond to androstadienone in accordance with their natal sex, rather than their experienced gender. We found that both, adolescent girls and boys with GD showed hypothalamic activations that reflected their experienced gender. [1]
(Emphasis mine)
So… does the study say that gender is independent of natal sex? Well… no, because that would be a broad conclusion to be drawn from the experiment conducted. Instead we find a statement that the hypothalamus of adolescents with gender dysphoria responded to a stimulus (in this case, an odor) in a way that is consistent with their experienced gender rather than their natal sex.
That’s not a conclusive statement. It is, however, a puzzle piece. In that one puzzle piece we also find two foot notes. Let’s examine those foot notes.
The first foot note points to Sexual differentiation of the human brain in relation to gender identity and sexual orientation. Well, that sounds useful and maybe even a bit more approachable. Let’s look inside:
There is a vast array of factors that may lead to gender problems (Table I). Twin and family research has shown that genetic factors play a part (78,79). Rare chromosomal abnormalities may lead to transsexuality (80), and it was recently found that polymorphisms of the genes for ERα and ERβ, AR repeat length polymorphism, and polymorphisms in the aromatase or CYP17 gene also produced an increased risk (79,81,82).
Abnormal hormone levels during early development may play a role, as suggested by the high frequency of polycystic ovaries, oligomenorrhea and amenorrhea in female-to-male (FtM) transsexuals. This observation suggests early intrauterine exposure of the female fetus to abnormally high levels of testosterone (83). A recent study did not confirm a significantly increased prevalence of polycystic ovary syndrome. However, there was a significantly higher prevalence of hyperandrogynism in FtM transsexuals, also indicating the possible involvement of high testosterone levels in transsexuality (84). A girl with congenital adrenal hyperplasia (CAH), who has been exposed to extreme levels of testosterone in utero, will also have an increased chance becoming transsexual. Although the likelihood of transsexuality developing in such cases is 300-1000 greater than normal, the risk for transsexuality in CAH is still only 1-3% (85), whereas the probability of serious gender problems is 5.2% (86). The consensus is, therefore, that girls with CAH should be raised as girls, even when they are masculinized (24).
Epileptic women who were given phenobarbital or diphantoin during pregnancy also have an increased risk of giving birth to a transsexual child. Both these substances change the metabolism of the sex hormones and can act on the sexual differentiation of the child’s brain. In a group of 243 women who had been exposed to such substances during pregnancy, Dessens et al. (87) found three transsexual children and a few others with less radical gender problems; these are relatively high rates for such a rare condition. On the “DES sons” (diethylstilbestrol, an estrogen-like substance – see later) website they claim that transsexuality occurs in 35.5% and a gender problem in 14% of the DES cases (88,89). This is alarming, but needs, of course, to be confirmed in a formal study. There are no indications that postnatal social factors could be responsible for the occurrence of transsexuality (90). [2]
Holy tap dancing centipedes Batman, that’s a lot of footnotes. Thirteen of them! And that’s just in three paragraphs. The intrepid researcher might be well served by diving deeper to understand the studies that contributed to those paragraphs (by pure coincidence one of the other articles linked earlier is referenced in this block) so they can further understand the conclusions being drawn.
When I read research about transgender issues, I look carefully at the footnotes. Sometimes, after reading a paper and looking at the footnotes I stop and say “huh, given how heavily this relies on [Researcher X], I wonder why the paper doesn’t cite [Researcher Y], given that [Researcher Y] attempted to repeat [Researcher X]’s study with a larger and more diverse sample but failed to reproduce the results… that seems relevant.” Or I might read the foot notes, pull out a highlighter and say “I was aware of the research on AR and CYP17 but I wasn’t aware of the research on ERα and ERβ… I need to run these down later and add them to my reading list.”
See, science and research aren’t easy. It takes time and effort to gain proficiency in a research subject. If you were to chart the levels of expertise in research, you might come up with something like this:
Level 1: Familiar with research from popular media that references the research (Science Daily, The Advocate, etc.) At this level of expertise you’re relying on someone with greater subject matter expertise to tell you what the data says and to help you put it in context.
Level 2: Familiar with limited amounts of primary sources, mainly engages with primary sources for fact checking or to find support for presented arguments. This is what I’d jokingly refer to as the “I want to win arguments on the internet” level of expertise. People who seek out the research that supports their already held positions and cite the research as needed to support their statements. Don’t mistake this for subject matter expertise, it isn’t.
Level 3: Familiar with a significant amount of the published research. Reads the whole study. Capable of putting new data in the context of an existing body of research, evaluating the credibility of data and critically examining both methodology and conclusions. Call this your undergrad level of familiarity with a topic. The kind of thing you’d expect someone holding a Bachelor of Science in Gender and Development to be able to put together.
Level 4: Broad familiarity of existing research, capable of constructing literature reviews, meta-analysis of existing research, etc…
I think you see where this is going so we can stop there knowing full well that this goes on for a while. Let’s bring it back to the question at hand: Is there a scientific basis for transgender identity and further, is there a peer reviewed study that demonstrates it?
Without question there is reason to believe that transgender identity is rooted in biological phenomenon. Wait… that was a really weak statement for a sentence that started with “without a doubt.” But… that’s science for you, especially young science. It’s important for people to remember that research into trans identity is relatively young as a science. While John Money was publishing work proposing that sex and gender were separate things as far back as 1955, you don’t get the steam roller even starting to move with biologists until Zhou’s 1995 publication in Nature. [3] That was just a little over 20 years ago. Since then the research has been piling up. We’ve seen studies in genetics, in psychobiology, neurology, clinical psychology and a number of other disciplines. A majority of the literature moves us towards higher and higher confidence in the statement that transgender identity has a biological etiology.
As always, within the body of research, there are differing opinions. Not all research points in exactly the same direction. Some studies have been conducted only on a very small scale and have never been repeated. Some studies have been repeated but produced different results. I read a study the other day that was sort of tragic: It was published a year or two before some rather significant discoveries about generational differences in the trans population that made the study’s data set difficult to evaluate because it didn’t report data in a way you could sub slice by generation… the study pulled several years of data from a single gender clinic but was published years before significant issues in the clinical practices at that clinic were discovered, ultimately resulting in disciplinary action against the person responsible for the program. So… here we have a study, when it was written, it was a good study using methodology that seemed sound at the time. Now, however, it feels less compelling though not completely invalid. And that sort of thing is why hunt and peck research to argue on the internet shouldn’t be mistaken for expertise. You can find disagreement within the literature, but without genuine breadth and depth of understanding of the research, you can’t evaluate it and you can’t contextualize it.
Science is hard. The science we have on transgender identity bends in the direction that says being trans is something people are born with. It bends in the direction of saying that gender identity isn’t mutable and can’t be changed by post natal interventions. It bends in the direction of saying that gender affirming care is the absolute best way of clinically managing gender dysphoria. The evidence is mounting for all of these things.
But further research will be needed for a long time. I think it’s unlikely that we’ll come to the conclusion that the contemporary consensus around validating transgender identity was in error. I do think there are several places where we just don’t have the data or the research ability to say things like “if you put a person in a n MRI you can tell if they’re trans or not by looking at the results.” We’re not there and we may never get there. We may never be able to give someone a blood test to see if they have the “transgender gene.” But what we do have is a mountain of data that tells us that it’s worth looking for those things because we might be able to find them.
So is there a scientific basis for transgender identity? Best answer you get: There is a significant amount of scientific research that supports validating transgender identity. Make of that what you will.