The amount of heritable variance in P3 amplitude and the P3 common factor was estimated using standard biometric approaches to modeling twin-family data (Neale, Boker, Xie, & Maes, 2003) and conducted using the OpenMx package for R (Boker et al., 2011). Our approach and the logic of biometric model fitting are described in Iacono et al. (2014). We fit models to twin data as well as data from the entire family. For both endophenotypes, models allowed for three latent factors, which influence (“cause”) the endophenotype: additive genetic influences (A), common environmental influences (C), and unique, or unshared, environmental influences (E). Parameter estimates from the common pathway model, which is described in more detail in the supporting information, were used to derive genetic factor scores. Because genetic factor scores and genotype are identical for MZ twins, only one twin from each MZ pair was used in analysis of these scores. The correlation between the two (covariate-adjusted) P3 endophenotypes was .925.
