A twin study of EEG spectral characteristics (Lykken et al., 1974; Stassen et al., 1988b) found a striking similarity of EEG power spectra in MZ twins and much lower resemblance in DZ twin pairs. Intrapair MZ correlations were high and significant, with their magnitude approaching twin correlations for morphological characteristics such as height and weight (r>.8). Corroborating earlier observations, Lykken et al. demonstrated that across different quantitative characteristics of the resting EEG, the magnitude of MZ intrapair correlations is of the same order as the corresponding test-retest correlations within the same individual. Furthermore, this study employed a unique sample of MZ twins reared apart. Twin correlations for EEG characteristics in this sample were virtually at the same level as for MZ twins reared together (Lykken et al., 1974; Stassen et al., 1988a), suggesting that twin resemblance results from shared genetic, rather than shared environmental, factors. Because MZ twins reared apart share their genotypes but not environment, their phenotypic resemblance is due solely to their genetic commonality. Consequently, correlation between MZ twins reared apart can serve as a direct estimate of
