Along the top of Figure 1 parents’ genetic and environmental influences are represented as A1 (genetic influences) and E1 (nonshared environmental influences). The influence of the shared environment on parental knowledge for twin parents was not included in the model because the intra-class correlations suggested that effects from the shared environment were negligible (see Table 1) and because the ECOT model performs better when shared environmental influences are estimated on only the child phenotype rather than on both the parent and child phenotype in samples of the current size (Narusyte et al., 2008). Based on the definition of nonshared environmental influences, the correlation for E1 for parent twin 1 and parent twin 2 was fixed at 0. Based on the information provided above regarding average proportions of segregating genes shared by different sibling types, the correlation between A1 for twin parent 1 and twin parent 2 was set to 1 for MZ twin parents and .5 for DZ twin parents. Genetic transmission from parents to adolescents was explicitly modeled using the latent factor A1’. The influence of A1 (influence of
