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Introduction

According to an influential neuroscientific theory, gender identity is encoded in the brain during intrauterine development. The brain is thought to develop in the male ‘direction’ through a surge of testosterone on nerve cells; in the female ‘direction’ this surge is absent (e.g., Savic et al. 2011; Swab, 2007). Call this the ‘standard view of gender identity’. The standard view of gender identity offers a plausible explanation of transsexualism. Since sexual differentiation of the brain occurs in the second half of pregnancy, and sexual differentiation of the sexual organs occurs in months 1-2 of pregnancy, transsexuality may occur. The relative masculinzation of the brain at birth may not reflect the relative masculinization of the genitals (e.g., Bao & Swab, 2011; Savic et al. 2011; Veale et al. 2010). According to the standard view, transsexualism is entirely dependent on, and thereby reduces to, specific neurophysiological changes that occur during intrauterine growth in two interconnected organ types (i.e., brain and genitals).

The reductive nature of the standard view of gender identity is compatible with  mind-brain identity theory in philosophy of mind and consciousness. Mind-brain identity theory claims that mental states are identical to brain states. Concerning gender identity, mind-brain identity theory claims that a person’s gender identity is identical to neurophysiological mechanism. A strong and profound implication of this view is that a person’s indubitable sense of being a woman or a man is nothing more than the operations of neurophysiology encoded during intrauterine growth. Mind-brain identity theory contrasts with philosophies of mind which propose that minds are dependent but still somehow ‘more than’ the body on which they depend.

Brain-Sex Theory of Transsexualism and Mind-Brain Identity

According to the strong version of ‘brain-sex’ theory of transsexualism,  transsexualism is nothing more than (one and the same as) a specific neuranatomical (i.e., structural) intersex type, in which one or more sexually dimorphic brain areas are incompatible with bological sex. The theory therefore assumes that the relationship between transsexualism and neurophysiology is one of identity. Gender identity reduces to neurophysiology. Thus, there is a specific neuroanatomical type for female gender identity in male-to-female (MTF) transsexuals, and a specific neuroanatomical type for male gender identity in female-to-male (FTM) transsexuals. The most compelling neuroscientific evidence in support of an identity view of transsexualism comes from Kruijver et al. (2000) and Zhou et al. (1995).

Neuroscientific Evidence for Brain-Sex Theory of Transsexualism

Zhou et al. (1995)

Zhou et al. (1995) observed that a group of neurons in the hypothalamus, the central subdivision of the bed nucleus of the stria terminalis (BSTc), was sexually dimorphic in humans. Zhou et al. found that the average volume of the BSTc in postmortem males was roughly 44% larger than in females. However, in 6 male-to-female (MTF) transsexuals who had feminizing hormone treatment, the average volume of the BSTc was within the typical female range. The authors found that the 6 transsexuals they investigated varied in their sexual orientations and inferred that there was no relationship between BSTc size and the sexual orientation of transsexuals. I assume that this assertion implies that transsexual sexual orientation and BSTc size are not type identical; that is, they are not the same type. Finally, further postmortem investigations conducted in a small number of nontranssexual patients with abnormal hormone levels, led Zhou et al. to reason that the small volume of the BSTc in MTF transsexuals cannot be explained by adult sex hormone levels” (p. 70). Thus, there appears to be a relationship of identity between transsexualism and small BSTc volume. They are one and the same.

Kruijver et al. (2000)

Kruijver et al. (2000) conducted a follow-up study in which they investigated the number of neurons in the BSTc rather than its volume. The authors examined tissue from the same 6 MTF transsexuals studied by Zhou et al. (1995). They also studied nerve tissue from one female-to-male (FTM) transsexual and from an 84-yr-old man who “had very strong cross-gender identity feelings but was never . . . sex-reassigned or treated . . . with estrogens” (p. 2039). The authors found that BSTc neuron number was even more sexually dimorphic than BSTc volume; namely, the average BSTc neuron number in males was 71% higher than in females. Once again, the 6 MTF transsexuals showed a sex-reversed identity pattern, with an average BSTc neuron number in the female range. BSTc neuron number was also in the female range in the untreated gender dysphoric male and was in the male range in the FtM transsexual. Again, the putative sexual orientation of the MTF transsexuals appeared to make no difference. In contrast to the claims of the standard view of gender identity, data from the few nontranssexual patients with abnormal hormone levels led Kruijver et al. (2000) to conclude that “hormonal changes in adulthood did not show any clear relationship with the BSTc . . . neuron number” (p. 2039).

Neuroscientific Objections to Brain-Sex Theory of Transsexualism

Chung et al. (2002)

Brain-sex theory of transsexualism faces several neuroscientific challenges. Chung et al. (2002) found that significant sexual dimorphism in BSTc size and neuron number does not develop in humans until adulthood. However, most MTF transsexuals self-report that their feelings of gender dysphoria began in early childhood (e.g., Lawrence, 2003). Since MTF transsexuals have not yet become sexually dimorphic by the time cross-gender feelings have become obvious, it is unlikely that BSTc volume and neuron number can be a neuroanatomical signature identifiable with gender identity. However, Chung et al. (2002) speculate that foetal or neonatal hormone levels could influence gender identity and could also produce changes in BSTc synaptic density, neuronal activity, or neurochemicals that may not affect BSTc volume or neuron number immediately, but may do so during adulthood. I am not aware of any evidence in support of this hypothesis. In any event, mind-brain identity theory can agree with Chung’s et al. (2002) speculation. Mind-brain identity theory is neutral on whether ‘brain characteristics’ will be macro or micro, or both, or what their specific developmental effects will be. Gender identity might be a state of the entire brain, synapses, or multiple, interacting physiological systems. Macro/microreductionism is optional, not required. Finally, Chung et al. (2002) speculate that inconsistency between an individual’s gender identity and biological sex might likely affect adult BSTc size and neuron number by some yet unknown mechanism or mechanisms. Given that neuroscience is in a very early stage of understanding gender identity, the implication that more time is needed to understand transsexualism appears prudent.

Joel (2011)

Joel (2011) challenges an implicit assumption in the standard view of gender identity; namely, human brains are one of two types –  ‘male’ or ‘female’ – and that the differences between these two types subserve subtype differences between men and women in gender identity and transsexualism. According to Joel (2011), this assumption is true only if there is robust correspondence (i.e., high statistical correlation) between the ‘male’/’female’ type of all of the brain characteristics in a single brain. It turns out there isn’t. As Joel points out, concerning most documented sex brain differences, there is overlap between the distributions of the two sexes (e.g., Juraska, 1991; Koscik et al. 2009). Neuroanatomical data also reveal that sex interacts with other factors during the intrauterine period and throughout life to determine brain structure (e.g., prenatal exposure to psychoactive drugs, early handling, rearing conditions, maternal separation, acute and chronic postnatal stress). Human brains therefore are a dynamic heterogeneous mosaic of ‘male’ and ‘female’ brain characteristics that cannot be type identified on a simple continuum between a ‘male type brain’ and a ‘female type brain’ (Joel, 2011). Thus, brains are not type sexed, but type intersexed; sexually multi-morphic rather than dimorphic.

Joel’s theory is compatible with brain-sex theory of transsexualism since both theories claim that transsexualism is intersexual, but incompatible because it denies what brain-sex theory asserts; namely, in transsexualism, one or more sexually dimorphic brain areas are incompatible with bological sex. Thus, Joel’s view rejects the stronger claim that gender is type identical with the sexually dimorphic brain. Accordingly, we cannot predict the specific properties of ‘male/female’ brain characteristics of an individual based on her/his sex. However, Joel’s view implies the weaker consequence that, on average, we can predict that females will have more brain characteristics with the ‘female’ type than with the ‘male’ type (vice versa for FTM transsexuals), and males will have more brain characteristics with the ‘male’ type than with the ‘female’ type (vice versa for MTF transsexuals). Whether two individuals are similar or not is dependent on the similarity in the details of their brain mosaic; not on the quantity of ‘male’ and ‘female’ characteristics. This means that two similar individuals share characteristics of the same ‘brain mosiac’ type – they have the same type. Brains of the same type must possess the characteristics and properties typical of the type, but that does not imply that they all be exactly similar to one another. This implication is compatible with mind-brain identity theory.

References

Bao, A. M., & Swaab, D. F. (2011). Sexual differentiation of the human brain: relation to gender identity, sexual orientation and neuropsychiatric disorders. Frontiers in neuroendocrinology, 32(2), 214-226.

Chung, W. C., De Vries, G. J., & Swaab, D. F. (2002). Sexual differentiation of the bed nucleus of the stria terminalis in humans may extend into adulthood. Journal of Neuroscience, 22, 1027-1033.

Hines M. (2004). Brain Gender. Oxford: Oxford University Press.

Koscik, T., O’Leary, D., Moser, D. J., Andreasen, N. C., & Nopoulos, P. (2009). Sex differences in parietal lobe morphology: relationship to mental rotation performance. Brain Cognition, 69, 451–459.

Kruijver, F. P., Zhou, J. N., Pool, C. W., Hofman, M. A., Gooren, L. J., & Swaab, D. F. (2000). Male-to-female transsexuals have female neuron numbers in a limbic nucleus. Journal of Clinical Endocrinology and Metabolism, 85, 2034-2041.

Joel, D. (2011). Male or female? Brains are intersex. Frontiers in integrative neuroscience, 5, 57.

Juraska J. M. (1991). Sex differences in “cognitive” regions of the rat brain. Psychoneuroendocrinology 16, 105–109. doi: 10.1016/0306-4530(91)90073-3.

Lawrence, A. A. (2003). Factors associated with satisfaction or regret following male-to-female sex reassignment surgery. Archives of Sexual Behavior, 32, 299-315.

Savic, I., Garcia-Falgueras, A., & Swaab, D. F. (2010). 4 Sexual differentiation of the human brain in relation to gender identity and sexual orientation. Progress in Brain Research, 186, 41-65.

Swaab, D. F. (2007). Sexual differentiation of the brain and behavior. Best Practice & Research Clinical Endocrinology & Metabolism, 21(3), 431-444.

Veale, J. F., Clarke, D. E., & Lomax, T. C. (2010). Biological and psychosocial correlates of adult gender-variant identities: a review. Personality and Individual Differences, 48(4), 357-366.

Zhou, J. N., Hofman, M. A., Gooren, L. J., & Swaab, D. F. (1995). A sex difference in the human brain and its relation to transsexuality. Nature, 378, 68-70.

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Simon van Rysewyk

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