The second sample was a subset of the first sample, limited to groups of European ancestry biological full siblings (confirmed by genotyping) nested within the larger COGA sample. This process identified 4733 individuals nested within 1655 sibling groups (2–12 siblings per group). As detailed below, the n = 4733 sibling GWAS samples were used to calculate the educational attainment polygenic scores used for the sibling comparison analyses. The sample was subsequently filtered by age at phenotypic assessment for the linear mixed model analyses. In total, the sibling comparison analyses included 3098 individuals [1616 (52%) female] who were 25 years of age or older (Mage = 37.89 years) from 1226 sibling groups nested within 773 extended families.
