Biology of Sex Differences

Sex differences in the human brain: a roadmap for more careful analysis and interpretation of a biological reality
A note from the Editor-in-Chief, Jill B Becker
Defining biological sex by gonadal and chromosomal characteristics at birth, multiple large datasets reflecting current best-practice show convergent evidence for small to moderate sex differences in human brain volume above and beyond sex-differences in brain size. For example, M>F in putamen, amygdala, hippocampus, and temporal pole volume, F>M in volume of the cingulate, superior parietal, and lateral prefrontal cortices. In contrast, reviews of past studies obscure this consistency because methodological heterogeneity is not accounted for. We do not yet know if any of the reproducible sex-differences in regional human brain anatomy have any functional implications. Separate studies would be required to address this hypothesis. There also diverse biological, psychological and social factors that could contribute to regional sex-differences in human brain anatomy. Untangling these causes is hard in humans, but may be helped by research in animal models. Finally, the existence of sex-differences in human brain anatomy is based on comparisons between two groups defined by biological sex (often self-reported). This frame of reference does not address intersex conditions, gender diversity or intersectional identities. However - with these boundaries in mind -  we conclude that stratifying people based on biological sex does identify reproducible sex-based group differences in regional brain volume. The best evidence to date suggests that these differences are a biological reality, but that more work is needed to understand their causes and (if any) functional consequences. We also argue that the laden nature of sex and gender in history and society mean that  - despite the idealized objectivity of the scientific method - reports of sex-based differences in the human brain should be disseminated in a pro-actively anti-sexist manner. We offer this Commentary from DeCasien et al regarding sex-based differences in the human brain and argue that it is not sexist to discuss sex differences in the human brain. In fact, it is pro-human to understand where there are and where there are not differences in the human brain based on the sex of the individual.

Read more about our newly launched special series on the topics of Sex Differences in Development and Sex Differences in COVID-19.