Critique of the Dutch Brain Studies
1. This study has not yet been replicated on an independent sample. Initially, I thought that this was a very serious concern. However, because a sex difference in the BSTc seems pretty well established (see #2), I’m less concerned. The same Dutch group previously claimed to find sexual dimorphism in another hypothalamic nucleus, SDN-POA (INAH-1); but other investigators were unable to replicate their findings. This group has neither retracted nor even become more cautious in their claims. Read Simon LeVay's comments on this.
2. The sex difference in the BSTc emerges in adulthood, while the sex difference in gender identity emerges during childhood. This would seem to rule out the possibility that the BSTc is the gender identity brain site, per se, although it could be correlated with gender identity for other reasons. Read Anne Lawrence’s take on this.
3. The Dutch group was unfamiliar with (or ignored) the theory of autogynephilia, and did not obtain sufficient information about their transsexuals in order to make confident diagnoses. Based on demographic data (especially age), it is possible that all but one (or even possibly, all) of the transsexuals were autogynephilic. (Read Anne Lawrence’s take, again.) The study is thus irrelevant to the question of whether there are one, two, or more subtypes of transsexuals.
4. The current results cannot rule out the possibility that the brain differences reflect activational hormonal effects, such as those of estrogen (which male-to-female transsexuals would have been given). If the results are due to adult circulating hormones, then they are probably uninteresting with respect to transsexualism. Nor can the current results eliminate the possibility that the difference in the BSTc is a result (and not the cause) of some other difference between male-to-female transsexuals and heterosexual males.
5. I expect that there will be two distinct differences in brain structure or organization between male-to-female transsexuals and heterosexual males, one for each type of transsexualism. One will be related to gender identity and sexual orientation in homosexual transsexuals and in nonhomosexual, nontranssexual men and women. The other will be related to autogynephilic transsexualism. (I would expect a relatively large part of the brain differences distinguishing autogynephilic transsexuals from heterosexual males also to distinguish other paraphilic males from heterosexual males.) Scientists cannot find, or fail to find, such a difference unless they look for it.
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