Sex differences in cortical thickness and their possible genetic and sex hormonal underpinnings.

Although it has been shown that cortical thickness (Cth) differs between sexes, the underlying mechanisms are unknown. Seeing as XXY males have 1 extra X chromosome, we investigated the possible effects of X- and sex-chromosome dosage on Cth by comparing data from 31 XXY males with 39 XY and 47 XX controls. Plasma testosterone and estrogen were also measured in an effort to differentiate between possible sex-hormone and sex-chromosome gene effects. Cth was calculated with FreeSurfer software. Parietal and occipital Cth was greater in XX females than XY males. In these regions Cth was inversely correlated with z-normalized testosterone. In the motor strip, the cortex was thinner in XY males compared with both XX females and XXY males, indicating the possibility of an X-chromosome gene-dosage effect. XXY males had thinner right superior temporal and left middle temporal cortex, and a thicker right orbitofrontal cortex and lingual cortex than both control groups. Based on these data and previous reports from women with XO monosomy, it is hypothesized that programming of the motor cortex is influenced by processes linked to X-escapee genes, which do not have Y-chromosome homologs, and that programming of the superior temporal cortex is mediated by X-chromosome escapee genes with Y-homologs.