Sex versus gender associations with brain structure

In contrast to sex (a biological distinction), little is known about the associations between gender (a societal construct) and brain structure in the general population. In response to this knowledge gap, we examined the associations of sex vs. gender with FreeSurfer-generated cortical thickness and proportion-adjusted subcortical brain volume regions-of-interest (ROIs) in healthy adults (n = 88) screened for general medical conditions, mental illness, substance abuse, and intracranial pathologies. Gender role endorsement was assessed using the well-established and validated Bem Sex Role Inventory. For our main objectives, we calculated a continuum score as a composite measure of gender. For our secondary objectives, we examined sex-specific associations of the masculine vs. feminine gender role endorsement domains with brain structural outcomes. We found that female sex, independent of continuum scores, was associated with larger proportion-adjusted volumes for the basal ganglia, hippocampus, and ventral diencephalon. Higher continuum scores, independent of sex, were associated with thicker cortical thickness for the left and right superior frontal cortex, caudal and rostral middle frontal cortex, and right pars orbitalis. Female sex and higher continuum scores were independently associated with larger corpus callosum volumes. Post-hoc testing showed sex-specific associations between higher femininity scores and thicker prefrontal cortical thickness for the ROIs in females, but not in males. In conclusion, sex and gender showed semi-independent associations with brain structure in a general population sample. Our research supports the disaggregation of sex and gender to provide a more nuanced perspective on brain structural differences between men and women.