Abstract
Previous research has reported many sex differences in cortical and subcortical anatomy, but only a subset of findings is consistent across studies. Here, we used improved Freesurfer-based automated methods to analyze the properties of the cortex and seven subcortical structures in young, right-handed subjects (69 male and 69 female), carefully matched in age and education. Significant sex differences were observed. Females had greater gyral complexity (i.e., greater bending energy). In contrast, males had greater unadjusted cortical surface area (+10.3%), but area differences were reduced (to +2.8%) when area was adjusted for total intracranial volume (ICV). There were no significant omnibus sex differences in cortical thickness. Males showed larger unadjusted subcortical gray matter structural volumes, as well as larger ICV-adjusted volumes in the amygdala. These results help to resolve some of the inconsistencies in previous studies of sex differences in brain anatomy.
Highlights
Brain structure has been reported to differ between male and female brains (Bostan et al, 2013; Inano et al, 2013; Kim et al, 2012; Koolschijn & Crone, 2013; Ruigrok et al, 2014), but locations showing sex differences have varied across studies
Substantial differences in volume remain even when accounting for body size difference (Sacher et al, 2013), it should be noted that no body size covariate has been yet found to be fully adequate to account for overall brain size within most human adult populations (Peters et al, 1998)
Isolating the scale at which we find differences or the lack thereof can help with confirming possible mechanisms of sex differences: e.g. an overall brain size difference without further relative differences on finer scales is consistent with a single-factor that may influence early development such as sex hormones or genetic sex differences
Summary
Brain structure has been reported to differ between male and female brains (Bostan et al, 2013; Inano et al, 2013; Kim et al, 2012; Koolschijn & Crone, 2013; Ruigrok et al, 2014), but locations showing sex differences have varied across studies. By establishing consistent sample characteristics, image scan quality, segmentation, and corrections for overall intracranial volume, we were able to identify sources of inconsistency in previous studies, and provide a clearer picture of sex differences in brain anatomy. Sex differences in overall brain size are consistently observed in all studies: adult males have 7.4%–11.5% greater gray matter brain volume than females and show a 10.4%–13.6% increase in intracranial volume (ICV) (Good et al, 2001; Ruigrok et al, 2014). Substantial differences in volume remain even when accounting for body size difference (Sacher et al, 2013), it should be noted that no body size covariate has been yet found to be fully adequate to account for overall brain size within most human adult populations (Peters et al, 1998)
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