The Finnish HM2 imputed data were analyzed separately by cohort and followed identical protocols. A separate analysis was performed for each scale. Data for HA and RD were also analyzed separately by sex, as previous work in twins indicated sex differences in the source of genetic variance for these scales.28 An additive model was assumed for SNP genotype/dose, and principal components (from a PCA analysis of the genotype IBS matrix between persons) that were significantly related to the phenotype were included as covariates (as per the method of Price et al.29) to guard against possible stratification. The first two PCs were always included as covariates, as previous work showed they correlated strongly with geographic birthplace and ancestry.30 The HM2 imputed dosage data were analyzed using probABEL.31
