for moderator M. The parameters β 0 and β 1 are assumed to be equal across twins within a pair, and across zygosity (Fig. 1b). With 8 parameters (6 to describe the variance part of the model: 3 related, and 3 unrelated to the moderator; and 2 parameters to describe the means model: the regression weight β 1 and the intercept β 0), this parameterization is more parsimonious than the bivariate moderation model, and often less susceptible to computational problems. In addition, low correlations between M and T, or different measurement levels of M and T, do not cause problems in this univariate moderation model.
