The preprint of a summary of measures of brains in 5,216 donors to the UK's Biobank program was released on bioRxiv this past week. As a summary, they found that male brains were slightly larger in most structures, on the average, than female brains. This should not be surprising given that human males are on average slightly larger than human females. After controlling for average brain volume, though, there was still frequently a 0.7 to 0.8 Cohen's D value effect size of biological gender on many brain measures, with male brains usually larger on such measures, except, as has been noted before, female brains had a slightly thicker cortex and a larger corpus callosum.
The chart above is to show the degree of overlap for such differences in an ideal normal distribution case. The charts below reflect the actual current study under review's data. It's important to note that such size differences do not generally influence social aspects of gender. For example, persons who identify themselves more with their opposite biological sex (the trans-gendered) have scans that reflect on average measures that reflect their biological sex, not their psycho-social preference for gender. A further point: variances between individuals were larger than variance between men and women, reinforcing that we need to understand a mutiplicity of each individual's own personal qualities in order to better understand them and their abilities, much more than we need to rely on a single fact such as gender. As the paper says, "Overall, for every brain measure that showed even large sex differences, there was always overlap between males and females (p. 8)."
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ABSTRACT
Sex differences in the adult human brain: Evidence from 5,216 UK Biobank participants
Stuart J. Ritchie1,2*, Simon R. Cox1,2, Xueyi Shen3, Michael V. Lombardo4,5, Lianne M. Reus6, Clara Alloza3, Matthew A. Harris2,3, Helen L. Alderson7, Stuart Hunter8, Emma Neilson3, David C. M. Liewald1,2, Bonnie Auyeung1, Heather C. Whalley3, Stephen M. Lawrie3, Catharine R. Gale2,9, Mark E. Bastin2,10,11, Andrew M. McIntosh2,3, Ian J. Deary1,2
bioRxiv preprint first posted online Apr. 4, 2017; doi: http://dx.doi.org/10.1101/123729.
SEX DIFFERENCES IN THE HUMAN BRAIN
Summary
Sex differences in human brain structure and function are of substantial scientific interest because of sex-differential susceptibility to psychiatric disorders [1,2,3] and because of the potential to explain sex differences in psychological traits [4]. Males are known to have larger brain volumes, though the patterns of differences across brain subregions have typically only been examined in small, inconsistent studies [5]. In addition, despite common findings of greater male variability in traits like intelligence [6], personality [7], and physical performance [8], variance differences in the brain have received little attention. Here we report the largest single-sample study of structural and functional sex differences in the human brain to date (2,750 female and 2,466 male participants aged 44-77 years). Males had higher cortical and sub-cortical volumes, cortical surface areas, and white matter diffusion directionality; females had thicker cortices and higher white matter tract complexity. Considerable overlap between the distributions for males and females was common, and subregional differences were smaller after accounting for global differences. There was generally greater male variance across structural measures. The modestly higher male score on two cognitive tests was partly mediated via structural differences. Functional connectome organization showed stronger connectivity for males in unimodal sensorimotor cortices, and stronger connectivity for females in the default mode network. This large-scale characterisation of neurobiological sex differences provides a foundation for attempts to understand the causes of sex differences in brain structure and function, and their associated psychological and psychiatric consequences.
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