Neurological Correlates of Transsexuality
In class we have discussed the theory that brain structure determines functional output, or behavior. Certainly, the brain plays a major role in determining behavior, but can the brain explain all behavior? The Harvard Law of Animal Behavior states that animals will do whatever they please regardless of efforts to control their environments. How can we reconcile these two theories when they seem to be fundamentally opposed? If “brain equals behavior” but animals continue to create outputs without inputs, aren’t both theories missing a piece of the puzzle? Perhaps we give the brain too much credit when we attribute all behavior to it. The environment also plays a critical role in behavioral output. In the case of transgendered people, the behavior exhibited by the individual does not directly reflect the genotypic brain. A “brain equals behavior” perspective does not allow for transgendered people to exist in its realm of causal interactions between inputs and outputs. Unless the brain is found to be responsible for the underlying mechanisms of gender identity, the “brain equals behavior” model will not support the idea of transgender issues.
Several researchers have attempted to determine the biological underpinnings of gender identity in the brain. A group of researchers in Amsterdam studied the post-mortem brains of forty-three subjects. These forty-three brains were from nine gender-sex concordant males, ten gender-sex concordant females, nine homosexual males, six male-to-female (MTF) transsexuals, six people (three male and three female) with sex hormone disorders, a nontreated male with cross-gender identity feelings, and a female-to-male (FTM) transsexual. The researchers, Kruijver et al. (2000), found that the neuron number in a specific region of the brain seemed to correspond with gender identity/expression. Specifically, it was found that “the number of neurons in the BSTc of MTF transsexuals was similar to that of females” (Kruijver, et al., 2000). The area of interest is the central subdivision of the bed nucleus of the stria terminalis (BSTc), which is a bundle of nerves that acts as a pathway between the amygdala and the hypothalamus. It is interesting to note that the amygdala and the rest of the limbic system is thought to be primarily involved in processing emotional stimuli and memory, as well as emotions/pleasures related to survival, including sexual behavior and eating. If this area of the brain partly determines an individual’s sexual behavior, it makes sense for regions in this particular area to be sexually dimorphic. This topic holds further implications to the role of emotion in gender identity and expression. That research could be further expanded to determine a possible correlation between gender identity and sexual orientation.
Kruijver et al. (2000) acknowledged that most of the transsexuals had received hormone treatment, but maintained that hormone levels could not explain the significant differences found in neuron number in the BSTc because of the data obtained from the six men and women with sex hormone disorders and the one man who had “strong cross-gender identity feelings” but had never been treated with hormones. However, this claim is based solely on the data from seven individuals. In future studies, more people with sex hormone disorders should be included to determine whether sex hormones are responsible for altering neuron number and regional size of sexually dimorphic areas in the brain.
Dr. Anne Lawrence who challenges the brain-sex theory of transsexuality reviewed the work conducted at the Netherlands Institute for Brain Research. The brain-sex theory, which is akin to the “brain equals behavior” idea from class, assumes that transsexuality is a result of “a neurological intersex condition” (Lawrence, 2007). In other words, a sexually dimorphic region, or multiple regions, of the brain are responsible for an individual’s cross-gender identification because of the inconsistency between genetic sex and neurological sex. Dr. Lawrence explains that the brain-sex behavior of transsexuality does not account for the “two subtypes of MtF transsexuals, homosexual and nonhomosexual” (Lawrence, 2007; Blanchard, 1989a, 1989b, 2005). Dr. Lawrence refutes the Kruijver et al. (2000) claims that the difference in neuronal numbers of the BSTc between heterosexual/homosexual men and MTF transsexuals could not be due to sex hormone levels in adulthood. She cites Chung, De Vries, and Swaab (2002), asserting that sex hormones play an influential role in the proliferation of neurons in the BSTc of all people because “significant sexual dimorphism in BSTc volume and neuron number does not develop in humans until adulthood” (Lawrence, 2007; see also Chung, De Vries, & Swaab, 2002). Even though the development of sexual dimorphism of the BSTc area does not occur until adulthood, I do not believe this is evidence against the brain-sex theory. It could probably be argued that the delayed sexual dimorphism is one of the main reasons why there sometimes exists a discrepancy between an individual’s genetic and neurological sex.
The issue of delayed sexual dimorphism of certain brain structures implies that there could be some temporal limit involved in the sexual differentiation of neurons in the BSTc. Perhaps there is only a window of opportunity for the plasticity of the brain in regard to the sexual dimorphism of the BSTc. Or perhaps the variations in the number of BSTc neurons could be a result of the presence of a certain amount of sex hormone. The individuals with sex hormone disorders might not have produced enough endogenous sex hormone (discordant to their genetic sex) to exhibit a quantifiable difference in their BSTc neuronal number. Future research could look into the “threshold” of gender identity to more accurately determine when the transition from one’s genetic sex to their perceived neurological sex is complete, according to the brain-sex theory.
The brain-sex theory corroborates the “brain equals behavior” discussions from class and validates cross-gender identity issues from a scientific perspective by associating them with specific neurological structures. Dr. Lawrence notes that a major weakness of the Kruijver et al. (2002) study was the fact that all transsexuals had received sex hormones to transition from their genetic sex to their neurological sex. In fact, she maintains that the results of the Dutch study only portray the results of the sex hormone treatments. Luckily, a study was developed to compare MRI scans of the brains of twenty-four nontreated transsexual individuals with those of sixty nontranssexual (or gender-sex-concordant) control subjects (30 male, 30 female). This study was a major breakthrough because the brains were examined in vivo as opposed to post-mortem brain slices. The inclusion criterion, in regards to the absence of sex hormone therapy, also makes this study a seminal work.
The results of the study, led by Luders et al., implied that “regional gray matter variation in MTF transsexuals is more similar to the pattern found in men than in women” (Luders et al., 2009). However, Luders et al. (2009) did find that MTF transsexuals had a higher volume of gray matter than gender-sex-concordant men in the right putamen, which is part of the basal ganglia. Even though the BSTc neuronal number may be able to change based on sex hormone therapy, there are some regions of the brain that may be innately sexually dimorphic without the implementation of sex-affirming treatments (i.e., sex-hormone therapy or sex-affirming surgery). The results of the Luders et al. (2009) study certainly support the brain-sex theory and the “brain equals behavior” discussion from class.
Both studies provide intriguing new observations because it was previously believed that, at least in the case of “vertebrates in the animal kingdom, sex determination is usually a fixed characteristic in terms of life history” (Rice, 1999). Dr. Aaron Rice is a professor in the Biology Department at Davidson College. He studies the sex-change behavior of reef fish. Fish are believed to be unique in their ability to physically alter their bodies to switch their sex. Usually, fish alter their sex to meet the evolutionary needs of the entire school of fish. Perhaps, the observed physical differences between the brains of transsexuals and nontranssexuals could imply that transsexuality is an evolutionary response. Further research could begin determining what elicits that evolutionary response.
The observations from the Luders et al. (2009) study lend credibility and support to the notion that sex determination may indeed be fixed from conception. However, as Kruijver et al. (2000) observed, external stimuli, such as sex hormone therapy, can change the neurological structures that may also determine neurological sex or feelings of gender identity. The brain-sex theory remains a highly-debated issue which should engender more research and interesting new perspectives on gender identity and its proposed neurological correlates. The brain-sex theory validates the occurrence of gender identity issues through the “brain equals behavior” lens.
Bell, V. (2004). Imaging the transgendered brain. Retrieved from http://www.mindhacks.com/blog/2004/12/vaughan.html
Kruijver, F. P. M., Zhou, J., Pool, C. W., Hofman, M. A., Gooren, L. J. G., and Swaab, D. F. (2000). Male-to-female transsexuals have female neuron numbers in a limbic nucleus. The Journal of Clinical Endocrinology & Metabolism, 85(5), 2034-2041.
Lawrence, A. A. (2007). Transsexual Women’s Resources. A critique of the brain-sex theory of transsexualism. Retrieved from http://www.annelawrence.com/twr/brainsex_critique.html
Rice, A. N. (1999). The physiology of sex-change in coral reef fish. Retrieved from http://www.bio.davidson.edu/Courses/anphys/1999/Rice/Rice.htm