This study is the first published GWAS of amplitude of the P3 wave, the most widely studied ERP measure and a strong candidate endophenotype for disinhibitory behavior and psychopathology in particular (Iacono & Malone, 2011; Porjesz et al., 2005). We conducted biometric analyses in our sample of MZ and DZ twin families to determine the extent to which variation in P3 amplitude reflects heritable individual differences. Estimates of heritability from ACE models ranged from .50 to .66 for the two phenotypes, which is consistent with results of a meta-analysis that estimated P3 heritability as approximately .60 (van Beijsterveldt & van Baal, 2002). GCTA analyses provide estimates of SNP heritability, or phenotypic variance due to the measured genetic variants on our genotyping array (or variants in LD with them). Using several thresholds of pairwise genetic relatedness to select unrelated subjects from our family sample based on a weighted and unweighted GRM, we obtained median estimates of SNP heritability of .29 for P3 amplitude and .27 for genetic factor scores. This represents approximately 40% to 50% of the heritable variance in each
