I won’t comment on the Daphna et. al literature, because it’s still an emerging line of research that has some more philosophical and definitional questions as to what actually constitutes dimorphism (I’ve seen contradicting meanings).
Ummm… this result is in complete agreement with another hypothesis that many transwomen find uncomfortable, one made by Ray Blanchard, in which he hypothesized that late transitioning transwomen would have brain structure differences from both men and women that would NOT be sexually dimorphic; while young (“homosexual”) transsexuals would show shifts in sexually dimorphic structures toward female morphologies. There was an earlier review of previous studies (which I also wrote a post about) that had shown that hypothesis to be supported.
There’s also recent research that shows controlling for sexuality has absolutely no effect on trans brain structures.
From the abstract:
However, controlling for individual estradiol, testosterone, or progesterone plasma levels or for subjects’ sexual orientation did not change group differences
And then in the statistical analyses section they explain their control:
To investigate whether the three sex steroid hormones (E2, T, and P4) or sexual orientation explained group differences, the named variables were added as covariates of no interest within separate ANCOVA analyses. Finally, multiple regression analyses were performed to determine the effects of hormones and sexual orientation on diffusivity maps independent of group membership, i.e., with group in addition to TIV as factor of no interest. Separate models were calculated for each of the independent variables (E2, T, P4, and sexual orientation). Because hormone values over the entire sample were non-normally distributed, they were transformed to ordinal scales based on ranks before inclusion in the analysis. Sexual orientation was entered into analyses in three different ways of coding: (1) as raw values of the Likert scale (1 for attraction toward females; 7 for attraction toward males); (2) as a spectrum from homosexuality (e.g., 1) to heterosexuality (e.g., 7) with reference to the raters’ genetic sex; and (3) as a spectrum from homosexuality to heterosexuality with reference to the raters’ gender identity. The statistical threshold was set at p < 0.05 FWE corrected, using the threshold-free cluster enhancement method to define the clusters (Smith and Nichols, 2009). Voxels showing significant differences were assigned to white matter tracts using the DTI-81 white matter atlas of the International Consortium for Human Brain Mapping as provided by the DiffeoMap software package (www.mristudio.org/wiki/user_manual/diffeomap). Diffusivities based on tract-specific quantification for right and left CST, Fmajor, and Fminor were compared using ANOVA in SPSS. Separate models were calculated for each tract and diffusivity parameter, followed by post hoc pairwise comparisons and correction for multiple comparisons using the Bonferroni’s procedure. Analyses were run with and without TIV as covariate of no interest. Separate correlation analyses were performed to examine the association between diffusivities and age and between volumetric data [gray matter volume (GMV), white matter volume (WMV), CSF, and TIV] and age for each group.
And their results:
Here, we investigated whether sexual orientation associates with diffusivity measures. No effects on our main findings were observed when sexual orientation was regressed out in the ANCOVA design. Moreover, there was no significant effect of sexual orientation on diffusivity parameters in the regression analysis including all subjects and using group as factor of no interest.
In our study, we find robust differences between investigated groups in MD, AD, and RD indicating that biological sex and gender identity both contribute to observed group differences. Moreover, the high positive correlation with adult plasma T levels (controlling for group membership) indicates that group differences cannot be explained by peripheral sex hormone plasma levels. As expected, biological males (MCs and MtF transsexuals) had higher T levels than biological females (FCs and FtM transsexuals), whereas group differences in diffusivity values showed the transition FC > FtM > MtF > MC. Furthermore, group differences were not explained by differences in sexual orientation, narrowing potential determinants for differences in diffusivity parameters to biological sex and gender identity.
diffusivities and age and between volumetric data [gray matter volume (GMV), white matter volume (WMV), CSF, and TIV] and age for each group.
A 2008 study demonstrating that homosexual men and women have sex-atypical brain structures
The connectivity pattern in homosexual subjects was almost reciprocal in relation to the same-sex controls
Second, HoM, just as HeW, displayed connections with the contralateral amygdala, the anterior cingulate, the subcallosum, and the hypothalamus
Group comparisons confirmed these findings: HeW, as well as HoM, showed a greater connectivity with the contralateral amygdala and the cingulate cortex compared with both HeM and HoW.
We also found that the hemispheric ratios, as well as the patterns of amygdala connectivity, were sex atypical in homosexual subjects, with HoM exhibiting more female patterns and HoW showing more male-like features (albeit less pronounced).
(Note that this has been replicated quite a few times)
A TERF or transphobe could easily interpret this data to show that “trans women are just gay men!” and “trans men are just lesbian women”, which is exactly why the neurological argument is intrinsically flawed. But this data yields an alternative interpretation of the (contested) result that lesbian trans women have brains more similar to that of men: they should. Just like lesbian cis women have brains “more similar to that of heterosexual men” (to paraphrase), lesbian trans women do too. My conclusion is that brain structures are not organized by sex or gender, but by who one is sexually attracted to. Male/men-attracted people and female/woman-attracted people differentiate themselves. It’s internally consistent with the data, while the “male brain” / “female brain” hypothesis is not: gay men and lesbian women provide contradicting data. The issue is not one of data, but one of the paradigms we interpret the data through.
Now, this is all moot if we take the data from the study that I cited above that showed that sexual orientation had no impact on brain structures for trans individuals, which just explicates that trans brains are neurologically distinct than cis brains categorically rather than by subgroups
But either way we look at it, her argument is incorrect in some manner: paradigmatically or scientifically.
Searcy is likely to have written her article in an attempt to discount the growing evidence from transgender brain scan research that shows that the two type taxonomy for transwomen is supported.
There’s exactly one study that has shown gynephilic trans women have ‘male brains’, and there is at least one that shows the contrary. I wouldn’t be so eager to run and shout that the two type taxonomy is supported by the research
Where once older transitioning transwomen cherry picked the brain structure research in an attempt to spin it such that all transwomen had female brains. She is spinning the science to lead us to believe that brain structure research is unimportant and should be ignored, first by saying that there is no brain structure sexual dimorphism of any consequence and then say that what differences between transgender folk and nontransfolk is unimportant anyway.
This just gets back into the debate over dimorphism, which as I said I won’t get into those weeds today, but I’d just emphasize that it’s not a simple topic. There are fundamental philosophical disagreements over what constitutes sexual dimorphism between the two camps that can’t be ignored by saying that ‘one side is supported by the evidence’ and that the other isn’t.
I believe it represents a growing fear by autogynephilic transwomen that the brain scan science will undermine their own identity as transwomen if the public were to become aware of what the evidence means.
To me it seems Brown is alluding to her own denial of lesbian trans women’s identities and womanhood, which contradicts her stated “support” of “autogynephilic transwomen”