We used GCTA (Yang et al., 2011) to estimate the proportion of variance in P3 amplitude accounted for by the combined additive effect of all SNPs on the Illumina genotyping array (or in linkage disequilibrium [LD] with them). In a sample of genetically unrelated individuals, the degree to which any two are phenotypically similar must be due to the specific genetic variants they share. GCTA estimates genotypic similarity in the form of a genetic relatedness matrix (GRM), somewhat akin to a correlation matrix representing pairwise genetic similarity. In samples comprising families, Yang and colleagues (Yang, Lee, Goddard, & Visscher, 2013) have recommended filtering the sample on the basis of genetic relatedness, using several thresholds and looking for consistency across the resulting estimates. We used thresholds of .025, .05, and .10, which remove all but distant relatives. The same covariates were used as in all other analyses (age, gender, generation, recording system, and the 10 PCs from EIGENSTRAT). Because LD can bias SNP heritability estimates upward (Speed, Hemani, Johnson, & Balding, 2012), we repeated these analyses after weighting SNPs by local
