Group differences and sex modulation effects

Separate multiple linear regressions were run to examine group differences by diagnosis in centile scores for all global volumes as well as regional cortical volumes, thickness and surface area. The interaction between diagnosis and sex was examined, and given previous evidence of sex-specific neurobiological correlates in autism and ADHD ^{31,40,52,66,82,83,118,119}, a priori sex-stratified analyses examined diagnostic differences in males and females separately. We further assessed the similarity of diagnostic effect size maps between males and females by calculating the Spearman correlation between the two maps, and using spin permutation testing to assess significance with appropriate control of the potentially confounding effect of spatial autocorrelation of each map ¹²⁰.

All analyses included age and Euler number as a covariate (see QC section). Multiple comparisons were controlled for using the false discovery rate (FDR ¹²¹), for each analysis and cortical measure separately.

Multiple sensitivity analyses were conducted, including analyses controlling for global brain measures, using different QC methods, analysis of equal sex-matched subsamples, and Hartigan’s dip tests ¹²² for multi-modality in the distributions of each group (supplementary methods S2–5). We also investigated dimensional associations between cortical measures and autistic and ADHD traits, in subsets of the data with these measures available (supplementary methods S6).