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

性别认同和性取向是两个最大的心理性别差异。绝大多数人都是顺性别（即，男性认同为男性，女性认同为女性）并表现出异性吸引力。然而，这些基本模式也有例外。相当多的少数人表现出同性的性吸引力和/或变性，因此，为研究大脑，认知和行为的性别差异提供了独特的人类模型。 激素，遗传和免疫过程被假设影响大脑的性别分化，并导致性别认同和性取向的性别内变异。先前的研究检查了这些生物过程的躯体，认知，行为和人口统计学标记是否一方面区分跨性别与顺性别个体，另一方面区分同性与异性吸引的个体。与变性相关的生物标志物通常也与同性性吸引力有关。然而，由于没有研究直接比较这些生物标志物在同性之间吸引顺性别与跨性别个体，目前还不清楚这两种表型在多大程度上（如果有的话）受到不同生物过程的支持。 为了澄清这个问题，我们必须研究一个人群，在这个人群中，比较同性吸引的顺性别者和变性者是可行的。在西方国家，跨性别是罕见的，很难在专业诊所之外进行研究。因此，拟议的研究将位于泰国，那里的顺性别同性吸引相对普遍（2-5%），由于缺乏对性别表达的社会谴责，变性同性吸引比其他人群多50至250倍。因此，泰国人口提供了一个难得的机会，比较同性性取向和变性作为人类模型的性分化。 泰国同性吸引的顺性别与跨性别个体将在一套全面的相关生物标志物上进行比较。这些包括与产前雄激素暴露有关的几个标志物：大脑结构（例如，皮质厚度，白色物质微观结构），第二至第四指比率，物理尺寸（例如，身高，体重），面部结构，认知能力（例如，心理旋转），和行为特征（例如，偏手性）。为了提供关于遗传和免疫因素的见解，将分别检查跨性别和同性性取向的家庭情况以及参与者的出生顺序。 通过比较同性吸引的顺性别与跨性别个体对一套全面的生物标志物，我的研究计划将改变我们对这些各自的人类模型和生物标志物如何揭示大脑，认知和行为的性别分化的理解。

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.