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For instance, early studies reported an almost two-fold volumetric increase of the suprachiasmatic nucleus of the hypothalamus 16 but twice smaller volumes of the third interstitial nucleus of the anterior hypothalamus (INAH-3) in homosexual (HoM) relative to heterosexual men (HeM) and no volumetric differences in the INAH-3 between heterosexual women (HeW) and HoM 17. The same regions have been identified when investigating the association between sexual orientation and brain morphology and function. Meta-analytic findings confirmed the involvement of these structures in sexual behaviors 15. Contrastingly, secondary rewards (i.e., monetary incentives) are represented in ontogenetically newer brain regions, like the prefrontal cortex 14. Moreover, this system is involved in behaviors like value-based decisions 11, reward, appetitive-seeking or avoidance behaviors 12 across species (see review 13,).
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Evidence for the involvement of these regions comes from studies where already in the phylogenetically oldest group of vertebrates (i.e., cyclostomes) it was observed that basic components of the basal ganglia in lampray are similar to those of mammals, and that the circuit features, molecular markers, and physiological activity patterns were conserved 10, 11. Reproductive behaviors (i.e., sexual or parental behavior) along with self-preservation behaviors (i.e., eating and drinking) are intrinsically rewarding and encoded in phylogenetically old subcortical structures including the thalamus, hypothalamus, amygdala and the striatum, with each region playing specific roles, such as reward expectation and evaluation, affective processing, guiding to foraging, etc. Morphological and functional bases of sexual behavior, however, are scarcely investigated. While these findings may imply a greater probability of innate encoding of reproductive and self-preservation behaviors (including sexual orientation), we cannot rule out the contribution of plasticity in shaping sexual interests and orientation in that, for instance, steroid hormone profiles can shape the development of male versus female neural phenotypes 8. Differences are reported in total brain volume, gray and white matter proportions and volumes, as well as in tissue densities of several phylogenetically old structures including the insula, hippocampus, thalamus, putamen, amygdala, and the cerebellum 3, 4, 5, 6, 7. Sex differences in brain morphology have been extensively documented in neurocognitive and behavioral domains 1, 2. Developments in neuroimaging methods allowed us to push research on this topic further and investigate the functional and structural neural phenotype underlying gender and sexual behavior and, in particular, sexual orientation. In the last couple of decades, there has been intense debate over sex and gender and over how societies perceive and categorize sexual roles and sexual behaviors. Furthermore, our findings provide important new insights into the brain morphology underlying sexual orientation and likely have important implications for understanding brain functions and behavior. The results emphasize the need to include or control for potential effects of participants’ sexual orientation in neuroimaging studies. This study shows that sexual orientation is reflected in brain structure characteristics and that these differ between the sexes. Heterosexual men exhibited more GMV in the thalamus than homosexual men. We found significant interactions between biological sex and sexual orientation, indicating that the significant effect for the putamen cluster was driven by homosexual women, whereas heterosexual women had increased precentral gyrus GMV. We observed a significant effect of sexual orientation for the thalamus and precentral gyrus, with more GMV in heterosexual versus homosexual individuals, and for the putamen, with more GMV in homosexual + than heterosexual individuals.
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Gray matter volumes (GMV) were compared with respect to sexual orientation and biological sex across the entire sample using full factorial designs controlling for total intracranial volume, age, handedness, and education. We examined potential cerebral structural differences linked to sexual orientation in a group of 74 participants, including 37 men (21 homosexual) and 37 women (19 homosexual) using voxel-based morphometry (VBM). While sexual orientation (hetero- versus homosexuality) has been similarly linked to functional differences in several phylogenetically-old brain areas, the research on morphological brain phenotypes associated with sexual orientation is far from conclusive. Biological sex differences in brain function and structure are reliably associated with several cortico-subcortical brain regions.