An Evolutionary Biodevelopmental Neuroscience of Same-Sex Sexual Orientation and Transgender Identity

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My research program aims to achieve an integrated evolutionary biodevelopmental neuroscience of sexual orientation and gender diversity. Same-sex sexual orientation (relative to birth-assigned sex) is thought to be influenced by biological processes that apply across populations and is expressed as either cisgender (cis; i.e., gender identity/role and birth-assigned sex align) or transgender (trans; i.e., gender identity/role and birth-assigned sex do not align). The proposed research consists of three projects designed to test the hypothesis that cis vs. trans expressions of same-sex sexual orientation have distinct biological bases. A critical aspect of the research design is that these projects are conducted in Thailand. Given most prior work on the biology of same-sex sexual orientation focused on Western populations, examining Thai participants provides strong tests of the idea that biological processes influencing sexual orientation apply across populations. Moreover, due to the high visibility and social acceptance of sexual and gender diversity in Thailand, it is one of the few populations in which it is feasible to recruit sufficiently large samples of cis and trans same-sex attracted individuals for rigorous studies. Project 1 tests several individual hypotheses regarding the biodevelopment and evolution of same-sex sexual orientation. Specifically, it consists of studies that examine markers of genetic, hormonal, and immune processes thought to underpin same-sex sexual orientation biodevelopment. It also tests two evolutionary hypotheses regarding same-sex sexual orientation: kin and balancing selection. Project 2 evaluates the possible existence of same-sex attracted subgroups. Genetic, hormonal, and immune processes are predicted to each apply to a subgroup of same-sex attracted individuals. Based on prior research, it is expected that these subgroups will be delineated by gender expression and that different subgroups will show evidence of distinct evolutionary origins. Project 3 tests the idea that biological processes influence sexual orientation and gender identity via their impacts on sexual differentiation of the brain. Thus, this project compares sex-differentiated aspects of brain structure (i.e., cortical thickness and surface area, white matter microstructure) in relation to sex, sexual orientation, and gender identity. This project will also explore brain-biodevelopment-evolution links. Together, these projects will elucidate the biological bases of sexual orientation and gender diversity. In so doing, they will also inform understanding of sexual differentiation of the brain and behaviour. In addition, this program of research will furnish unique training opportunities for highly qualified personnel and provide a model for how to integrate evolution, biodevelopment, and neuroscience to achieve comprehensive understanding of complex psychological traits.