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).