Structural connections in the brain in relation to gender identity and sexual orientation

TLDR

Transgenderism and homosexuality are biological phenomena thought to arise from distinct sexual differentiation of the brain, yet they differ fundamentally, with transgenderism involving a mismatch between physical sex and experienced gender that is not seen in homosexual individuals. The study seeks neural correlates of transgenderism and homosexuality by examining fractional anisotropy of white matter connections. Fractional anisotropy was compared across 40 transgender men, 27 transgender women, 29 male and 30 female homosexual, and 40 male and 40 female heterosexual cisgender controls. Cis‑heterosexual groups displayed expected sex differences in FA while cis‑homosexual groups did not; after adjusting for sexual orientation, transgender groups showed sex‑typical FA except for the right inferior fronto‑occipital tract, suggesting transgenderism is linked to self‑body perception areas and that homosexuality is associated with reduced cerebral.

Abstract

Both transgenderism and homosexuality are facets of human biology, believed to derive from different sexual differentiation of the brain. The two phenomena are, however, fundamentally unalike, despite an increased prevalence of homosexuality among transgender populations. Transgenderism is associated with strong feelings of incongruence between one's physical sex and experienced gender, not reported in homosexual persons. The present study searches to find neural correlates for the respective conditions, using fractional anisotropy (FA) as a measure of white matter connections that has consistently shown sex differences. We compared FA in 40 transgender men (female birth-assigned sex) and 27 transgender women (male birth-assigned sex), with both homosexual (29 male, 30 female) and heterosexual (40 male, 40 female) cisgender controls. Previously reported sex differences in FA were reproduced in cis-heterosexual groups, but were not found among the cis-homosexual groups. After controlling for sexual orientation, the transgender groups showed sex-typical FA-values. The only exception was the right inferior fronto-occipital tract, connecting parietal and frontal brain areas that mediate own body perception. Our findings suggest that the neuroanatomical signature of transgenderism is related to brain areas processing the perception of self and body ownership, whereas homosexuality seems to be associated with less cerebral sexual differentiation.