Testing Biological Theories of Same-Sex Sexual Attraction and Transgender Identity: Somatic, Cognitive, Behavioural, and Demographic Markers

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Gender identity and sexual orientation are two of the largest psychological sex differences. The vast majority of people are cisgender (i.e., males identify as men, females identify as women) and exhibit opposite-sex attraction. Yet, exceptions to these fundamental patterns exist. Substantial minorities of people exhibit same-sex sexual attraction and/or transgenderism and, thus, provide unique human models for studying sexual differentiation of the brain, cognition, and behaviour. Hormonal, genetic, and immunological processes have been hypothesized to influence sexual differentiation of the brain and lead to within-sex variation in gender identity and sexual orientation. Previous research examined whether somatic, cognitive, behavioural, and demographic markers of these biological processes differentiate transgender vs. cisgender individuals on the one hand, and same- vs. opposite-sex attracted individuals on the other. Biomarkers associated with transgenderism are often also associated with same-sex sexual attraction. Yet, because no studies directly compared these biomarkers across same-sex attracted cisgender vs. transgender individuals, it is unclear to what extent, if any, these two phenotypes are underpinned by different biological processes. To clarify this issue, one must examine a population in which comparing groups of same-sex attracted cisgender vs. transgender individuals is feasible. In Western countries, transgenderism is rare and difficult to study outside of specialty clinics. Hence, the proposed research will be situated in Thailand where cisgender same-sex attraction is relatively common (2-5%) and, due to a lack of social censure regarding gender expression, transgender same-sex attraction is 50 to 250 times more common than in other populations. Thus, the Thai population provides a rare opportunity to compare same-sex sexual orientation and transgenderism as human models of sexual differentiation. Thai same-sex attracted cisgender vs. transgender individuals will be compared on a comprehensive set of relevant biomarkers. These include several markers linked to prenatal androgen exposure: brain structure (e.g., cortical thickness, white matter microstructure), second-to-fourth digit ratio, physical size (e.g., height, weight), face structure, cognitive abilities (e.g., mental rotation), and behavioural characteristics (e.g., handedness). To provide insight regarding genetic and immunological factors, familiality of transgenderism and same-sex sexual orientation and participants' birth orders will be examined, respectively. By comparing same-sex attracted cisgender vs. transgender individuals on a comprehensive set of biomarkers, my research program will transform our understanding of how these respective human models and biomarkers shed light on sexual differentiation of the brain, cognition, and behaviour.