+ C + E; MZ covariance, A + C; and DZ covariance, 0.5A + C. Estimates of these VCs may be obtained by using structural equation modeling (SEM) software, typically through maximum likelihood estimation.4,5 Differences in the relative impact of these VCs predict different patterns of the MZ and DZ correlations. Accordingly, examination of MZ and DZ twin correlations can be used to infer the relative magnitude of the VCs. Thus, MZ and DZ correlations of zero would imply no influence of A, C, or D. If the ratio of MZ to DZ twin correlations is between 1 and 2, A and C are implicated, and ratios greater than this suggest D. Variance due to sibling interaction, where the behavior of one twin affects the behavior or the rating of the other twin, is implicated if MZ and DZ variances for a trait differ.6,7
