I had commented that the “brain sex” evidence was conflicting and of very low quality. A pro-gender commenter challenged me to provide references showing that the evidence was conflicting. I can resist temptation but not a challenge, so I provided it. I have added emphases.
In contrast to the main “brain sex” studies on which transgenderism activists hang their hats (Zhou et al 1995; Kruijver et al 2000; Berglund et al, 2008; Garcia-Falgueras et al, 2008) — all conducted in corpses, by the way — Savic and Arver (2011) found the following in their study conducted in living male transgenderists (all erotically attracted to females):
“The present data do not support the notion that brains of MtF-TR are feminized. The observed changes in MtF-TR bring attention to the networks inferred in processing of body perception.”
In their study in living male transgenderists (all erotically attracted to males), Zubiaurre-Elorza and colleagues (2013) later reported:
“In the present report, we studied MtF transsexuals erotically attracted to males that show a feminization of CTh but not in the putamen. Moreover, these findings on the CTh show the same tendency as those reported by Savic and Arver (2011) with respect to the cortical volume of MtFs erotically attracted to females.”
Savic and Arver (2011) had noted enlarged cortical volume, but in view of their findings of deficiencies in networks involved with own-body perception, had speculated:
“[T]the enlargement of the GM volume in the insular and inferior frontal cortex and the superior temporal-angular gyrus could derive from a constant rumination about the own body. Brain tissue enlargement has been detected in response to training, and GM enlargement of the insular cortex has been reported in response to meditation, which involves mental focusing on the own body (Holzel et al. 2008; Luders, Toga, et al. 2009; Vestergaard-Poulsen et al. 2009).”
Anyway, from the perspective of assessing of evidence quality with GRADE (required in WHO guideline development), all of this “brain sex” evidence is pretty worthless, due to very serious problems of imprecision (very small numbers), often serious indirectness (e.g. postmortem studies) and possibly other kinds of bias.
Relative to imprecision, here’s another thought from Button and colleagues (2013):
“A study with low statistical power has a reduced chance of detecting a true effect, but it is less well appreciated that low power also reduces the likelihood that a statistically significant result reflects a true effect. …. The consequences of this include overestimates of effect size and low reproducibility of results. There are also ethical dimensions to this problem, as unreliable research is inefficient and wasteful.”