The intrauterine view of gender identity and sexual orientation
The intrauterine theory of gender identity proposes that gender identity is encoded in brain during intrauterine development (e.g., Savic et al. 2011; Swab, 2007). The brain is thought to develop in the male ‘direction’ through a surge of testosterone on nerve cells, likely in the bed nucleus of the stria terminalis (BSTc) in the limbic system (Chung et al. 2002; Krujiver et al. 2000; Zhou et al. 1995), whereas in the female ‘direction,’ this surge is absent.
This view of gender identity has been adapted to explain 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 is possible. Thus, the relative masculinization 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).
The intrauterine theory implies that transsexualism is entirely dependent on a specific and dedicated neuroanatomical brain ‘module’, the BTSc. At a time during the second half of pregnancy, the BSTc comes ‘on-line’, and sexual – or transsexual – identity is thereby formed in the individual.
The intrauterine theory as a maturational theory
As a maturational brain theory, the intrauterine theory assumes functional localization of gender identity as an attribute of a specific brain structure or region (i.e., the BSTc) and its patterns of functional connectivity, rather than its patterns of functional connectivity to other structures or regions, to the whole brain and its external environment (van Rysewyk, 2010). Developmentally, a maturational view assumes establishment of intraregional connections, rather than interregional connectivity. It follows that the intrauterine view implies that transsexualism, at the level of the brain, involves a process of organizing intraregional interactions within the BSTc. The bed nucleus of the STc appears to be critically involved.
Extending the maturational aspect of the intrauterine view to gender development also means that we should observe changes in the response properties of the BSTc during pregnancy as regions within the BSTc interact with each other to establish their functional gender roles. Thus, the onset of transsexual identity during intrauterine development will be associated with reliable changes in several regions in the BSTc.
The intrauterine theory and mind-brain identity theory
Philosophically, the intrauterine view is also highly compatible with mind-brain identity theory, a philosophy of mind and consciousness (van Rysewyk, 2013). Mind-brain identity theory claims that mental states are identical to brain states. This implies that a person’s indubitable sense of gender identity as manifested in real-time feelings, sensations, thoughts and reports to others of being a woman or a man are numerically identical to specific brain states, possibly states of a single brain structure or region.
Are the brain states in question states of one brain structure – the BSTc? It appears not, for Chung et al. (2002) found that significant sexual dimorphism in BSTc size and neuron number does not develop in humans until adulthood. However, most male-to-female (MTF) transsexuals self-report that their feelings of gender dysphoria began in early childhood (e.g., Lawrence, 2003).
Clearly, these important findings are not compatible with the maturation of one brain structure or region, but with inter-regional brain development, of which the BSTc may feature as merely one, but significant, contributor. Thus, following the onset of transsexual identity, there is a reorganization of interactions between different brain structures and regions. This reorganization process could change previously existing mappings between brain structures and regions and their functions. It follows that the same phenomenal sense of gender identity in a person (e.g., recurring feelings of gender dysphoria) could be supported by different neural substrates at different ages during development. This possibility doesn’t necessarily exclude a maturational theory of transsexual identity, since the BSTc may be stimulated to reorganize its intrauterine functional connectivity following appropriate stimulation during postnatal development.
Future experimental questions for the function of the BSTc in gender identity and sexual orientation
1. The extent of BSTc localization in gender identity: how diffuse or focal is BSTc activity that results from gender-identity based stimulation?
2. The extent of BSTc specialization in gender identity: How coarsely or finely-tuned is BSTc activity that results from gender-identity based stimulation?
The inter-regional interaction theory of gender identity assumes that as brain tissue becomes more specialized (i.e., finely-tuned), it will become activated by a narrow range of gender-based experiences. With increased specialization, less extensive areas of brain tissue (BSTc?) will identify with gender-based phenomenology.
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.
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.
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). 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.
van Rysewyk, S. (2010). Towards the the developmental pathway of face perception abilities in the human brain. In: A. Freitas-Magalhães (Ed.), ‘Emotional Expression: The Brain and the Face’ (V. II, Second Series), University of Fernando Pessoa Press, Oporto: pp. 111-131.
van Rysewyk, S. (2013). Pain is Mechanism. PhD Dissertation, University of Tasmania.
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.