We conducted within-family association analyses on a sample of 22,135 sibling pairs from STR-Twingene, STR-SALTY, UKB, and WLS. For each cohort, we standardized EduYears within the cohort and then residualized this variable using the same controls as in the GWAS. We then regressed the sibling difference in the residuals on the sibling difference in genotype. We restricted analyses to SNPs with minor allele frequency above 5% in each of the sibling cohorts and meta-analyzed the cohort-level results using inverse-variance weighting.
