USA  Today  -  March 14, 2001 Sex-identity myths dispelled  By Robert Sapolsky  As a resident of San Francisco, I take great pride in this city's famed tolerance and pluralism. Those traits were extended into new territory last month when it was announced that the Board of Supervisors and Mayor Willie Brown would permit city employees' health insurance to cover sex-change operations.  ''This is a medically diagnosed condition -- gender identity disorder,'' said Supervisor Mark Leno.  Leno is right; this is an officially labeled disorder. But now an extraordinary scientific study should change how we think about what is ''disordered'' about transsexuality. The study suggests that when someone says, ''This isn't the sex that I was meant to be,'' the structure of their brain agrees with them.  Psychologists and psychiatrists have long opined about transsexualism. In some cases, it has been viewed as delusional and treated with drugs that counteract psychosis. In other cases, it has been viewed as an extreme defense against anxieties about one's gender role, or over-identification with the parent of the opposite sex. The groundwork for the recent finding was laid by researchers who found a fair number of differences in the brains of men and women relevant to reproductive hormones, emotion and cognition. Careful exploration showed that some of these sex differences in the brain are pretty impressive, both in magnitude and consistency.  Different counts in men, women The new finding concerns one of those brain regions -- called the BSTc -- that has a large, reliable sex difference. The BSTc is involved in emotions and bodily responses to them. Human males average about twice as many neurons within the BSTc as do females.  During this study, done by Frank Kruijver and his colleagues in the Netherlands and published in The Journal of Clinical Endocrinology and Metabolism, a major publication in its field, researchers counted BSTc neurons during post-mortem examination of brains. In both sexes, transsexuals didn't have the neuron number typical of their gender. Instead, the researchers discovered, they had the number typical of the sex they always believed they should be.  The hormones used when a person undergoes a sex change didn't account for this dramatic difference in the number of BSTc neurons that transsexuals have. The pattern was not only seen in transsexuals who had sex changes, but also in transsexuals who wished they had undergone treatment, but never did. And non-transsexuals exposed to those same hormones for unrelated medical reasons did not show a shift in neuron numbers. Instead, the research suggests something quite new about the source of transsexuality: Your pattern of chromosomes, gonads, genitals, secondary sexual characteristics, the hormones in your bloodstream and the way you are treated by your parents, teachers and society at large may all be agreement that you are of a certain sex. But something as hard-nosed and biological as the number of neurons in a part of your brain may be telling your mind that no, that's not who you are: You are the opposite sex. Imprisoned It's hard to imagine anything more tragic than a sense of being trapped in a body that's the wrong gender. Certainly, it seems pretty reasonable to allow those who feel this way to use their medical-insurance money to try to change.  But a study like this does far more than simply justify insurance payments: It dispels the long-held myths about transsexuality.  Transsexuals are often uncomfortable about research that tries to understand the ''pathology'' of those who think they are of the opposite gender than they actually are. But this study suggests just the reverse: The problem, it finds, is that the bodies transsexuals are born into actually are the opposite gender of who they really are. Robert Sapolsky is a neuroendocrinologist at Stanford University who studies the cellular mechanism of stress-induced diseases.