ARTICLES
By CURT SUPLEE Washington Post
In today's Nature, a team of researchers from Amsterdam reports that they conducted post-mortem exams on the brains of six male-to female transsexuals. Specifically, they studied one particular part of hypothalamus, called the central division of the bed nucleus of the stria terminalis (BSTc).
This area, which is thought to influence sexual behavior, is on average 44 percent larger in men than women. Yet all six subjects had BSTc regions that were female sized, Dick Swaab of the Netherlands Institute for Brain Research and colleagues found.
BSTc volume is not related to homosexuality, the researchers concluded from post-mortem analyses of 36 other brains, including those of homosexual men and het erosexuals of both sexes: "The BSTc was 62 percent larger in homosexual men than in heterosexual women." Nor does it appear to be a factor in sexual orientation in general. Of the six transsexuals, three had been attracted to women, two to men and one was bisexual.
Psychiatrist Sandra Witelson of McMaster University in Hamilton Ontario, said the work adds "to the body of knowledge that has been accumulating in the past five or six years, all of which shows that there are biological correlates to variation in human sexual behavior." Per haps the most celebrated such find ing came in 1991 when Simon LeVay, then at the Salk Institute in California, reported that gay and straight men differed dramatically in yet another area of the hypothalamus.
The authors of today's paper argue that the transsexuals' "feminine" BSTc volume arose from biochemical conditions early in life perhaps during fetal development. The findings, they conclude, "support the hypothesis that gender identity develops as a result of an interaction between the developing brain and sex hormones."
In an accompanying commentary, however, neuroscientist Marc Breedlove of the University of California at Berkeley warns that "this will be far from the final word on the subject." Breedlove notes that of the six subjects, five had had their testicles surgically removed and all had tak en the female hormone estrogen, as well as a drug that blocks the action of male hormones called androgens. Those conditions, he writes, might have affected the size of the BSTc region. Moreover, he writes, it is impossible to know whether the small BSTc size influenced the subjects' transsexuality or vice versa.
Richard Restak, a Washington, D.C., neurologist who has written extensively on brain research, observed that recent studies have shown that "estrogen has an enhancing and modulating effect on neurotransmitters in the brain. Thus, it's conceivable that it might encourage or discourage some change in brain structure."
The researchers acknowledge that possibility but cite the fact that two transsexual subjects who quit taking estrogen shortly before their deaths nonetheless had "a small, female-like BSTc." They also stud ied two other men whose testicles had been removed in the course of treatment for prostate cancer, elim inating their bodies' main source of androgens. Both had BSTc volume "at the high end of the norrnal male range."
The new research "doesn't mean that learning or culture or environment are irrelevant" to transsexuality, Witelson said, "not at all. But it is one more report of a relationship between anatomy and behavior."