Because transsexuals are born with bodies that seem perfectly normal to other people, we may suspect that the source of these deep-seated feelings about their bodies arises from their brains.
The recent report from Dick Swaab and his colleagues at the Netherlands Institute for Brain Research confirms this notion (Zhou, J. N., Hofman, M. A, Gooren, L. J. G., & Swaab, D. F. A sex difference in the human brain and its relation to transsexuality. Nature, 378, 68-70, 1995).
Swaab and colleagues examined the brains of many individuals, including homosexual men, heterosexual men and women and six male-to-female transsexuals. They found that a tiny region with the unwieldy name of the central region of the bed nucleus of the stria terininalis (BSTc) was larger in men than in women. Sexual orientation seemed irrelevant to the size of the BSTc because it was as large in homosexual men as in heterosexual men. But the BSTc of the six transsexuals was as small as that of women - about half the volume of the BSTc in other men. Thus the brains of the transsexuals seem to coincide with their conviction that they are women.
Of course such a report seems fantastic for several reasons. We know the brain is the center which controls all our behaviors, so of course differences in our behavior must reflect some differences in our brains. But the brain is a very complex system, and who would suspect that we might discern a difference related to such a rare and complicated condition as transsexuality? On the other hand, perhaps this is no more surprising than the discovery of sex differences in the structure of the human brain, and there have been several such reports over the last decade. If we can discern differences in brain structure between men and women, why not between transsexuals and other genetic males? Is there a simple way to dismiss these findings? Since all the transsexuals had received long-term treatment with estrogen and all but one had been castrated, one concern is whether the brain differences simply reflect the hormone treatments rather than the psychological condition of these people. But among the other men and women are several cases which seem to dispute this view. For example, several of the women were well past menopause and so had seen little estrogen in the years before they died, and two of the heterosexual men had been castrated to treat prostate cancer. Yet the women without estrogen still had a small BsTc and the men without their testes still had a large BSTc. So there does not seem to be any simple relationship between hormone exposure and BSTc size. You can think of other spurious reasons for the correlation, such as body size or brain weight or age, but the scientists have examples that seem to rule out each of those possibilities. So the only variable that seems to explain the small BSTc in the transsexuals is their transsexuality itself.
But there is another aspect of these results that we must consider, and that is the origin of transsexuality and brain differences. When did this difference in the size of the BSTc arise in these individuals - in childhood, in adolescence or in adulthood? We will not know the answer to this soon, because the BsTc is so small that none of the non-invasive imaging techniques provides enough resolution to measure or even detect the BSTc. That means that at present we can measure the BsTc only by removing the brain, which in turn means it can be measured only once in any one individual, after they have died. Thus there remain two alternative explanations for why the BSTc is smaller in transsexuals. Perhaps as babies these individuals were born with a small BsTc (or born with a BSTc that was programmed to grow only a little) and that small feminine BSTc caused them to regard themselves as feminine and to become transsexuals. But on the other hand, it is possible that other factors (such as family structure, peer interactions, or random variation) caused these boys to regard themselves as feminine and grow up to be transsexuals. And those same "other factors' may have caused their BSTc to develop a small size.
At Berkeley, David Krech, Mark Rosensweig and colleagues found that when rats were raised in enriched environments (with toys and other rats) rather than caged alone, the animals showed many reliable changes in brain structure. Shortly after, David Hubel and Torsten Wiesel of Harvard demonstrated that depriving kittens of visual stimulation to an eye would alter connections between the eye and the brain.
Such demonstrations of experience altering brain structure have been extended to monkeys and, in recent years, to humans. For example, a human who had lost his hand as an adult showed clear evidence that the side of the brain controlling that hand was reorganized less than a year after the accident (Yang, T.T., Gallen, 0., Schwartz, B., Bloom, F. E., Ramachandran, V.S., & Cobb, S. Sensory maps in the human brain. Nature, 386, 592-593, 1994 [letter]). As noninvasive imaging techniques are perfected we can expect to see further demonstrations that experience can alter the adult human brain.
Why an I so confident that there will he more such demonstrations? I'm well aware of how much humans can learn, how much they can alter their behavior, and how frequently they do so. All of this behavioral plasticity requires that something in the brain remain plastic, too. But there is another important feature of the recent work with transsexuals that we can all ponder.
Whether these men were born with a small BSTc which caused them to become transsexuals, or whether these men became transsexuals which then caused them to have a small BSTc, the fact remains that their brains are physically different. And that difference is not trivial, because any difference we can detect with our primitive understanding of neuroanatomy is, by definition, not trivial. Thus we might regard transsexuality as a deep, abiding conviction. Presumably these adults could no more set aside their feelings about which sex they are than you or I could. So perhaps the report of Zhou et al. will make it easier for our society to accept and tolerate transsexuality.
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