In standard biometric models based on twin data, the genetic similarity between members of monozygotic (MZ; 100%) and dizygotic (DZ; 50%) twin pairs can be used to decompose the variance in a trait into three sources: additive genetic effects (that is, the combined effects of individual genes summed over loci, A); shared environmental effects (or the extent to which twins are similar regardless of zygosity because the environment has made them more alike, C); and non-shared environmental effects (or the extent to which members of twin pairs differ, presumably because of differing environmental influences, E; measurement error is also contained in E). In the standard univariate “ACE” model, all of the variance in a single trait (Vt) is decomposed into the A, C, and E components. A meta-analysis of P300 amplitude heritability studies (van Beijsterveldt & van Baal, 2002) concluded that the best fitting ACE model indicated that the non-shared environment could be dropped from the model without sacrificing model fit, leaving genes accounting for about 60% of the variability in P300 amplitude and the non-shared environment accounting for the
