Our primary dependent measure was P3 amplitude at the midline parietal location (Pz). Generation (twin or parent), gender, chronological age, a dummy variable representing recording system (BioSemi or Grass) in order to account for possible mean differences between them, and the 10 PCs from EIGENSTRAT served as covariates in subsequent analyses. Additive SNP effects were modeled, with each SNP represented as a count of the number of minor alleles.
