Testing the masculinization hypothesis in a sample of 23,935 human brains

For over 60 years, the masculinization hypothesis dominates our understanding of sex effects on the brain. According to this view, the male distribution for single brain measures and for the brain as a whole is shifted away from the female distribution. In the last decade this view has been challenged by evidence that sex effects on single brain features may be opposite under different conditions, resulting in brains comprised of unique mosaics of female-typical and male-typical features. Analysis of 289 MRI-derived measures of grey and white matter from 23935 brains revealed only three brain measures for which the masculinization hypothesis was not rejected in favor of the alternative hypothesis that women and men sample from the same two phenotypes. Moreover, at the individual level, sampling was not consistent across brain measures, as some measures were likely sampled from the female-favored phenotype while others were likely sampled from the male-favored phenotype. Last, considering the relations between brain measures, the brain architecture of women and men was remarkably similar. These results do not support the masculinization hypothesis but are consistent with the mosaic hypothesis as well as with other lines of evidence showing that the brain architectures typical of women are also typical of men, and vice versa, and that sex category explains a very small part of the variability in human brain structure.