Biology 202
2004 Second Web Paper
On Serendip

Transsexuality, like homosexuality before it, is defined as a disorder by the American Psychiatry Association. The DSM-IV lists various criteria that one must meet to be diagnosed with what it calls "Gender Identity Disorder." (1) But transsexuality, also known as "gender dysphoria," is not a medical condition. It is not a disease, or a malfunction of the body. The body of a transsexual operates normally, just out of sync with itself. The brain operates as one gender, while the body operates as the other. The result is a fully functional individual who feels trapped in the wrong body. This is quite a phenomenon; for some reason, the brain and body do not correlate. Why? What can this disjunction teach us about the brains and bodies of people who are not transsexual? In what ways can transsexuality inform out thinking and understanding of the brain in general?

Gender and sex are often used interchangeably, but their distinct meanings are important in the study of transsexuality. Sex is a label assigned at birth based upon one's genitalia. It is further physically defined by genetics (XX v. XY) and the gonads: male or female. Gender, or gender identity, on the other hand, is a self-assigned condition. One identifies as a man or as a woman. Usually, the imposed sex and experienced gender are one and the same. A person with a penis and testes identifies as a man whereas a person with a vulva, vagina, and ovaries identifies as a woman. For transsexuals, however, this is not the case. They are one gender trapped in body of the opposite sex.

What does this mean, "trapped in the body of the opposite sex"? It implies that the self is not the body, because the self feels right and the body feels wrong. Self must be the brain, then, the way the body is experienced. If the brain is the right sex, then, does this mean that male-to-female transsexuals (MTF), or individuals with a male sex who identify as women, have female brains? Was the wrong one grabbed of the shelf during assembly?

If only it were that simple. It's not, of course. The brain is overwhelmingly complex, not just in its function but also in its development. Countless factors affect the adult brain, from the womb to the present moment. In the past two decades, however, neuroscientists have begun to uncover conclusive information about the brain's gender that begins to explain transsexuality.

If brain=behavior, then the brain must be sexually dimorphic. Men and women don't act the same, so naturally their brains aren't the same. Overall, the brains of men and women are similar; the dimorphism is of particular structures. A series of studies published between 1985 and 2001 by scientists of the Netherlands Institute for Brain Research document data on the sexual dimorphism of one of these structures, the hypothalamus, and its implication in transsexuality.

In 1985, D.F. Swaab and E. Fliers published their data on the volume and cell number of a region of the human hypothalamus known as the sexually dimorphic nucleus of the pre-optic area, or SDN-POA. Post-mortem analysis of the brains of 13 men and 18 women, between the ages of 10 and 93, concluded that the male SDN-POA is on average 2.5 times larger by volume than its female counterpart and contains an average of 2.2 times as many cells. No function was attributed to the SDN-POA at the time of publication, but the authors noted that "it is located in an area essential for gonadotropin release and sexual behavior in other mammals." (2)

Conclusive studies were published in 1995 and 2000 correlating the size of another hypothalamus region, the central nucleus of the bed nucleus of the stria terminalus (BSTc) with gender identity. The 1995 study determined that, regardless of sexual orientation, the male BSTc was significantly larger in individuals identifying as men than in those identifying as women. For the first time, MTFs were also studied, and their BSTc sizes fell in the female rather than male range. (3) The small sample size (only six M-F transsexuals) and clear results implies that the trend of female-sized BSTc's in MTF is a strong one, as a subtler trend would not appear in a small sample size. (4) The 2000 study confirmed the female-ness of MTF BSTc's. Again, hetero- and homosexual men, heterosexual women, and MTF were studied, this time with attention paid to the quantity of a particular cell type in the BSTc. Again, the MTF data fell within the female range. The study also published the first data of a female to male transsexual (FTM) hypothalamus, whose data fell within the male range. (5) These studies answer, at least in part, the question of what it means to have the right brain but the wrong body: the twelve MTF and one FTM all had BSTc's that correlated with their gender identity rather than their physical sex.

A myriad of important and useful questions are raised by these findings. There are major implications for two central ways that we, as a class, think about the brain. The first of these is the thus-far supported hypothesis that brain=behavior. (6) The second is the existence of an entity known as the I-function, one of the many interconnected "boxes" within the brain. (6)

At first, my thinking was that because psychotherapy has no affect on transsexuality, it is a case in which brain does not equal behavior, or perhaps one in which the brain is larger than the reaches of behavior. This, however, is not the case. The behavioral practices of psychotherapy act on the cerebral cortex, so of course they would have no effect on transsexuality, which lies, we think, in the hypothalamus. Other behavior, however, outside the setting and practices of psychotherapy, has no effect on transsexuality. Often times, transsexual people attempt to assume the gender role, or expected social and cultural practices, associated with their assigned physical sex. Some are able to come to terms with their feeling of gender dysphoria and live in the opposite gender role, while others cannot. I found nothing to suggest that a transsexual person has ever been "cured," or able to change their gender identity. It is always the gender role and/or physical sex that is modified to match the gender identity. Regardless, altered behavior is not changing the brain. Assuming the opposite gender role does not alleviate the gender dysphoria and therefore does not alter the hypothalamus. Why not? Why does brain=behavior not apply here? At this early point in the study of the brain it isn't possible to answer this question.

Another brain/behavior question is one of the chicken and the egg: Did the brain come first or did the behavior? In other words, does the structure of the brain lead to the experience of transsexuality, or does the experience of transsexuality, created by social and other non-biological factors, influence brain structure? (7)) The 1995 and 2000 studies discussed above lend some insight. Brain material of postmenopausal, as well as castrated and non-castrated MTFs was used. These samples did not exhibit statistical differences from their groups, implying that adult levels of sex hormones do not influence the structure of the BSTc. ((2), (5) The structure must then have been determined developmentally, and it was some case of altered exposure or sensitivity to androgens (male sex hormones) in prenatal and early postnatal development that caused the smaller, female-like BSTc in the MTFs. (8), (9)

Our understanding of the I-function is challenged by these studies as well. We recently concluded that the I-function must reside in the neocortex. Animals with neocortexes seem to display some degree of I-functionality, whereas animals without do not. (6) But we never determined what exactly the I-function is. It is the part of Christopher Reeves that cannot move his leg, although the leg can move. But how much of Christopher Reeves is it? Is it all of Christopher Reeves, as a self? The role of the BSTc in (trans)sexuality says "no." The BSTc is part f the hypothalamus, not the neocortex, and so is not a part of the I-function. And yet it is responsible for a huge part of the self, a part so large people undergo sex-change operations to bring their bodies in line with what the BSTc says. So self, that elusive entity, must be more than the I-function.

Transsexuality, though incomprehensible to those of us for whom gender and sex is aligned, is telling us a lot about our aligned selves. From recent studies, we know that our behavior can't always change our reality like many shrinks say. We know that our gender identity is established at a very young age, even to some degree, in the womb. We know that our gender identity, a huge part of who we are, lies at least somewhat in a tiny bundle of nerves in a structure so deep in our brains we can't even point to it. We know that our neocortex, which we so prize as a sign of human intelligence and consciousness, the seat of the psyche and the mind, isn't the sole seat of our selves. Our selves are more spread out, residing in part in a region of the brain that we share with rats, gerbils, and guinea pigs. And while we know these things, we don't know everything. In short, those of us for whom the distinction between gender and sex is not immediately important, for whom the two correlate, take a lot for granted. And we are all a lot more complicated than we think.

References

Cited References

1)Diagnostic and Statistical Manual of the American Psychiatric Association, Fourth
Addition, 1994.

2). "A Sexually Dimorphic Nucleus in the Human Brain.", Swaab, D.F, and E. Fliers. Originally published in Science, New Series, Vol 228, No 4703 (May 31, 1985), 1112-1115).

3)"A Sex Difference in the Human Brain and Its Relation to Transsexuality.", Zhou, J.N. et al. Originally published in Nature 378: 68-70 (1995).

4)genderpsychology.org

5)"Male-to-Female Transsexuals Have Female Neuron Numbers in a Limbic Nucleus.", Kruijver, Frank P.M. et al. Originally published in The Journal of Clinical Endocrinology and Metabolism, Vol 85, 2034-2041 (2000.

6)Grobstein, Paul. Class Lecture/Discussion, Biology 202: Neurobiology and Behavior. Bryn Mawr College, Spring 2004.

7)"The Chicken-and-Egg Argument as It Applies to the Brains of Transsexuals: Does It Matter.", Breedlove, Marc. At genderpsych.org

8)"A Role for Ovarian Hormones in Sexual Differentiation of the Brain.", Fitch, Roslyn Holly and Victor H Denenberg Originally published in Behavioral and Brain Sciences, 21: 311-352 (1998).

9)"Sexuality: Gender Identity.", Ghosh, Shuvo et al.

Other Interesting Resources

Gender Identity Research and Education Society (gires), Britain.

Sex Differences in the Brain, by Dr. Doreen Kimura.

Transsexuality: An Introduciton.

A collection of links to transsexual information, both scientific and personal

Notes on Gender Identity Disorder by Anne Vitale, Ph.D.

"Structural and Functional Sex Differences in the Human Hypothalamus.", Swabb, Dick F. et al. Originally published in Hormones and Behavior 40: 93-98 (2001).


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