We performed within-family analyses among full siblings in Add Health, COGA, and the UKB siblings hold-out cohort (Supplementary Information section 5.2.5). We analyzed 492 families in Add Health (Nsiblings = 994), 621 families in COGA (Nsiblings = 1,353), and 19,252 families in the UKB (Nsiblings = 39,640). In Add Health and COGA, we applied least squares regression on a single outcome: the factor scores of the phenotypic externalizing factor (a continuous variable), while adjusting for family-specific dummy variables (Supplementary Table 33), and calculated the standardized difference (i.e., a Z-statistic) between the within-family coefficient (β^WF) to the coefficient from a model without family dummies (β^) (Supplementary Information section 5.2.6). In the UKB siblings hold-out cohort, we performed an analogous analysis of exploratory phenotypes (Supplementary Table 34). We analyzed heteroskedasticity-consistent and cluster-robust standard errors, clustered at the family level.
