This is a study about rats. As always, we don’t know if what works for them is true of humans. The study does, however, point to an important factor to consider in any research on gender identity.
The authors found that they could completely change an observed sex difference in adult rats’ brains by changing their sex hormones. Earlier exposure to sex hormones made no difference.
This means that this observed sex difference was completely caused by adult circulating androgens.
The authors suggest that this might also happen in human brains. They refer to studies of the structures in transsexual brains and suggest that we can’t be sure that observed differences were caused by differences in the early development of the brain. They might have been caused by taking cross-sex hormones.
This study is from 1999, so they are referring to some of the early work on gender identity and the brain by Swaab et al and Zhou et al. Those studies included the brains of trans people who had been taking hormones.
Those studies are probably obsolete; newer studies have indeed found that taking sex hormones changes human brains, including the area Swaab et al and Zhou et al talked about, the hypothalmus.
This study of hormones in adult rat brains would not affect studies if they a) look at trans people who have not yet taken any hormones and b) test people to make sure their hormonal levels are in the normal range for their biological sex.*
It is also possible that some areas of the brain are controlled by circulating hormones and some are affected by earlier exposure to hormones as well as circulating hormones. The authors cite examples where castration dramatically changed areas of the rat brain, but did not completely reverse the sex difference.
The authors also discuss studies that found you could change a bird’s brain by changing its hormones.
They conclude by discussing the ways adult hormones affect the human brain.
“Human behavior is also subject to the activational effects of androgens. Transsexuals treated with cross-sex hormones display sex reversals in their cognitive abilities, emotional tendencies, and libido (34, 35), and sex offenders are sometimes treated with antiandrogens to reduce their sex drive (36). The sociosexual changes observed in these groups most likely reflect structural and physiological plasticity in steroid-sensitive areas within the brain. The volumetric sex reversal reported here substantiates the possibility that hormones in adulthood can dramatically affect the structure of a brain region concerned with sexual behavior. Although the volumetric sexual dimorphism of the MePD is more modest than other animal models [a difference of 150% rather than 400–600% (31)], the extent of the MePD sexual dimorphism in rats in quite comparable to reported sexual dimorphisms in the human brain (1–6) and therefore supports the possibility that sexual dimorphisms of the human brain are caused solely by circulating steroids in adulthood.”
We can’t generalize from a study of rat brains to human brains, but this study does underline the importance of using trans people who have not taken cross-sex hormones if you want to study gender identity and the brain.
*A Japanese study found that many of the female-to-male transsexuals applying to their clinic had polycystic ovarian syndrome (PCOS); PCOS causes high levels of androgens.
(Bold added by George Davis.)