Subsidiary analyses were conducted to investigate the possibility of confounds arising from HCs having a different racial composition to COS and SIB groups. First, because in this sample, and in prior studies of racially mixed samples (Palmatier et al., 1999), the Val allele is over represented in non-white groups (Val dose in HCs of European vs. non-European descent, Chi-squared = 13, p = 0.002), race rather than genotype could potentially explain differences in cortical anatomy between genotype groups. To examine this possibility, we ran the HC-only analysis after excluding all HCs of non-European descent and found that our result were unchanged (Supplementary Figure 1). Second, because clinical groups in this study varied in their racial composition, observed differences between HCs and COS probands, or between HCs and SIBs could reflect race rather than disease status. We tested for and found no evidence of such a confound, as there were no statistically significant group-by-race-by-genotype interactions for the rate of CT change.
