From Sex Dimorphism of the Brain in Male-to-Female Transsexuals. Savic I, Arver S. Cereb Cortex. 2011 Apr 5:
Gender dysphoria is suggested to be a consequence of sex atypical cerebral differentiation. We tested this hypothesis in a magnetic resonance study of voxel-based morphometry and structural volumetry in 48 heterosexual men (HeM) and women (HeW) and 24 gynephillic male to female transsexuals (MtF-TR). Specific interest was paid to gray matter (GM) and white matter (WM) fraction, hemispheric asymmetry, and volumes of the hippocampus, thalamus, caudate, and putamen. Like HeM, MtF-TR displayed larger GM volumes than HeW in the cerebellum and lingual gyrus and smaller GM and WM volumes in the precentral gyrus. Both male groups had smaller hippocampal volumes than HeW. As in HeM, but not HeW, the right cerebral hemisphere and thalamus volume was in MtF-TR lager than the left. None of these measures differed between HeM and MtF-TR. MtF-TR displayed also singular features and differed from both control groups by having reduced thalamus and putamen volumes and elevated GM volumes in the right insular and inferior frontal cortex and an area covering the right angular gyrus.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."Both male groups"... oops. That assumes something not actually true, and that there's plenty of evidence against. That MtoF Trans people are "really men". No matter, the data's what's important.
Like HeM, MtF-TR displayed larger GM volumes than HeW in the cerebellum and lingual gyrus and smaller GM and WM volumes in the precentral gyrus. Both male groups had smaller hippocampal volumes than HeW. As in HeM, but not HeW, the right cerebral hemisphere and thalamus volume was in MtF-TR lager (sic) than the left. None of these measures differed between HeM and MtF-TR.In summary, we can't tell someone's sex just by looking at volumes of brain structures.
We knew that before, as I'll discuss later. But what about other variables? Age for example? Were there post-menopausal women in the control group? While the experimental subjects were all gynephillic - lesbian - the sexual orientation of the control groups weren't mentioned in the abstract (all we have at the moment), and neither was experimental group's HRT status - were they pre-op, post-op, on hormones? We know that latter's an important issue when it comes to gross volumes of brain structures from other experiments. We know the sexual orientation has correlations with characteristic brain activities too, though not brain volumes as far as I know.
See Changing your sex changes your brain: influences of testosterone and estrogen on adult human brain structure by Pol et al, Europ Jnl Endocrinology, Vol 155, suppl_1, S107-S114 2006
Methods: Magnetic resonance brain images were made prior to, and during, cross-sex hormone treatment to study the influence of anti-androgen + estrogen treatment on brain morphology in eight young adult male-to-female transsexual human subjects and of androgen treatment in six female-to-male transsexuals.If the Trans women had had little long-term Hormonal treatment, nor surgical intervention with hormonal balance consequences, little would be learned. If, on the other hand, HRT was underway, we'd need to look at the ages of those involved. If all were old, we could tentatively hypothesise that the effects on brain volume of sex hormones decreases over time.
Results: Compared with controls, anti-androgen + estrogen treatment decreased brain volumes of male-to-female subjects towards female proportions, while androgen treatment in female-to-male subjects increased total brain and hypothalamus volumes towards male proportions.
We know that MtoF and FtoM are not simple mirror-images either: from Dichotic Listening, Handedness, Brain Organization and Transsexuality Govier et al International Journal of Transgenderism, 12:144–154, 2010
It was hypothesized that the MtF participants’ dichotic performance would be significantly different from the control males and resemble the control female pattern. This hypothesis was supported. It was also hypothesized that the FtM dichotic pattern would be significantly different from the control females and would resemble the control male pattern. This hypothesis was not supported. Finally, it was hypothesized that there would be significantly more nonexclusive right-handers in both trans-groups. This hypothesis was supported. Taken together, the dichotic and handedness data reported here indicate that the MtF and FtM conditions are not mirror images in terms of the verbal-auditory aspects of their brain organization and neurobiology plays an important role, particularly in the development of the male-to-female trans-condition.
Moving right along: from Regional gray matter variation in male-to-female transsexualism. by Luders et al Neuroimage. 2009 Jul 15;46(4):904-7.
Results revealed that regional gray matter variation in MTF transsexuals is more similar to the pattern found in men than in women. However, MTF transsexuals show a significantly larger volume of regional gray matter in the right putamen compared to men.So it's not as simple as "girl brain in boy body", though that captures the essence of it. In certain ways, Trans people match neither a male nor female stereotype, nor anywhere in between.
Back to the 2011 paper "Sex Dimorphism of the Brain in Male-to-Female Transsexuals":
MtF-TR displayed also singular features and differed from both control groups by having reduced thalamus and putamen volumes and elevated GM volumes in the right insular and inferior frontal cortex and an area covering the right angular gyrus.OK, that narrows things down a bit.
The present data do not support the notion that brains of MtF-TR are feminized.I'd say more than that: I'd say that a simplistic "boy brain" vs "girl brain" hypothesis had been disproved. The brain volumes are different, and hormones make a huge difference anyway. I think we had enough evidence of that already to treat it as a working hypothesis, just as we do that the brains of Trans people are cross-sexed in more subtle ways.
There's something more useful though.
The observed changes in MtF-TR bring attention to the networks inferred in processing of body perception.Bingo. The Body Map. One more variable - were these Trans women all desirous of having sex reassignment? My working hypothesis is that some are not simply because that part of the brain is not affected by the generalised cross-sexing, visible not in brain volumes so much as cell type and grey matter distribution in certain brain structures, but not others. We should only say "boy brain in girl body" etc when telling "lies to children" on the very bottom rungs of Wittgenstein's ladder. There it's the best approximation to the truth, but not higher up.
TLDR version: Brain volumes alone aren't reliable for determining sex: you have to look at the deeper structure, the pattern of activity, and distribution of cell types.
I'll look forward to seeing the full paper: the abstract doesn't tell us enough. I'd also be interested in further work, with both androphillic male and gynephillic female controls, and of different age groups. We may just be detecting sexual orientation, nothing else.
My thanks to OII researcher M.Italiano for bringing this paper to my attention.