8. Finding sex differences does not rule out gender influences as the two are entangled in humans. Also, we map and annotate anatomical sex differences in the brain, but we do not directly test causal effects, or probe potential functional consequences (of which there may be none).

7. I thank all study participants and the team! @bogglerapture.bsky.social, @konradwagstyl.bsky.social, @mallarchak.bsky.social, @coevolvinglab.bsky.social + others not on BlueSky. Preprint: doi.org/10.1101/2025.09.10.675377

Also…

6. We complemented these analyses with clinical cohorts featuring altered sex chromosome and gonadal biology (N=313). Sex effects on cortical anatomy are congruent with effects of X-chr dosage and testosterone production. This informs hypotheses for the causal drivers of sex-biased cortical anatomy.

5. In line with spatial dissociability of sex effects on SA and CT and their regionally varying contributions to CV sex-biases, SA and CT diverge in enrichments for functional and molecular signatures. Taken together, they may have distinct mechanistic roles for sex-biased cortical organization.

4. Sex differences in CV, SA and CT observed in the HCP sample also show high out-of-sample reproducibility in an independent dataset gathered using different MRI scanners in a different country (UK Biobank, N=669, the 5-year age band of 44-50 years closest to HCP).

3. These dissociable facets of sex-biased cortical anatomy are highly reproducible within the HCP dataset in a sample size dependent manner (R=0.67, 0.62 and 0.75 for CV, SA and CT at split-half sex-balanced sample size of 450).

2. Across the cortex, sex effects on SA and CT are uncoupled, and those on CV are more driven by SA than CT.

However, region-by-region analysis show many instances of sex-biased CV that appear to be primarily driven by CT. Moreover, 42% of sex-unbiased CV are sex-biased in SA and/or CT.

1. Typically developing young adults from the HCP dataset (N=1,085) show significant sex effects on regional cortical volume (CV), surface area (SA) and cortical thickness (CT) in 30% of regions (fdr<0.05) with small-to-large effects (0.13≤|d|≤0.82).