Mind

Transsexual Brains

By Josie GlausiuszJan 1, 1996 6:00 AM

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Transsexuals define themselves as male or female in spite of what their anatomy tells them. Freud ascribed this sexual disharmony to childhood conflicts within the oedipal triangle; others have attributed it to parents who give children the message that they would be more valued if they were the opposite sex. And indeed, transsexuals themselves often trace their desire to belong to the opposite sex to their very early childhood. This past year, though, a group of Dutch researchers reported that they’d traced the origins of transsexuality even further back--to the womb.

In November neurobiologist Dick Swaab and his colleagues at the Netherlands Institute for Brain Research in Amsterdam announced that their postmortem study of a tiny brain region known as the BSTc--the central subdivision of the bed nucleus of the stria terminalis--had shown that it was, on average, 44 percent larger in heterosexual men than in heterosexual women. More remarkably, the BSTc’s of six male-to-female transsexuals-- whose brains the researchers had painstakingly collected over the course of 11 years--were 52 percent smaller than those of the average man in the study. In fact, they were about the same size as those of the females-from- birth. The researchers chose to study the BSTc because previous research in rodents had shown that it plays a pivotal role in sexual behavior: remove it from a rat and the animal will show no interest whatsoever in sex.

Although five of the six transsexuals had been castrated and all had undergone estrogen treatments to feminize their bodies, the researchers don’t think these procedures affected the size of the brain region. We know from animal experiments that in adulthood you cannot change the size of the nucleus using sex hormones, says Swaab. You can do that only in development. A comparison with the brains of two men who had had their testes removed as a treatment for prostate cancer showed that these nontranssexuals had a BSTc in the normal male size range; a study comparing pre- and postmenopausal women’s brains, meanwhile, showed that the drop in estrogen levels following menopause also did not change the size of the structure.

These findings led Swaab to believe that in humans also, BSTc size is programmed during fetal and neonatal development--perhaps as a result of an interaction between sex hormones and the developing brain--and is probably not the result of parental or social pressures after birth. His research, he says, shows that transsexuals are right. Their sex was judged in the wrong way at the moment of birth because people look only to the sex organs and not to the brain.

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