After imputation and quality control, the combined genotypic dataset consisted of 1,252,387 SNPs. The best guess genotype at each SNP was tested for association with the four TPQ scales using the family-based association test as implemented in Merlin (-fast-assoc, Chen & Abecasis, 2007), which accounts for family relationships including MZ twins. The additive genetic effect was calculated, in which the genotypic mean of the heterozygote (Aa) was modelled as the average of the two homozygotes (AA, aa). Because sex differences in the source of genetic variance have been found for Reward Dependence and Harm Avoidance, we also performed the association test for these variables for males and females separately. Association analyses of genotyped markers on the X-chromosome was conducted using Minx (as implemented in Merlin). Because the imputation software did not support sex chromosomes, SNPs at the X-chromosome are not imputed; the association analyses only included those SNPs that have been genotyped for at least 85% of the sample (N=7526). Association between a SNP and a phenotype is generally accepted to be genome-wide significant at α = 0.05 if the
