Extended Children of Twins ModelThis is a representation of the ECOT model. The lower left-hand box represents the adolescent twin/sibling sample (NEAD). The correlation for parental knowledge for sibling 1 and sibling 2 was fixed to 1 because in NEAD the adolescent siblings share parents. The larger right-hand box represents the parent twin sample (TOSS). A1 = latent genetic influences of parents on their parental knowledge; E1 = latent nonshared environmental influences of parents on their parental knowledge; A2 = latent genetic influences of adolescents on their externalizing behavior; C2 represents latent shared environmental influences of adolescents on their externalizing behavior; E2 = latent nonshared environmental influences of adolescents on their externalizing behavior; A1’ = the effect of genes shared by parents and adolescents, that contribute to parental knowledge, on adolescents’ externalizing behavior. Path m = direct environmental effects of parenting on adolescents’ externalizing behavior; path n = child evocative effects of adolescents’ externalizing behavior on parenting; path s = the influence of shared genes of parents and adolescents; significant path s and m = passive rGE while significant path n and either A2 or s = evocative rGE. Measurement error is estimated as ε1 and ε2, and constrained to be equal during model fitting. 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. The influence of A1 (influence of parents’ genes on their own parenting) on A1’ (influence of that same set of genes on adolescent externalizing behavior) was set to .5 (because children inherit half of their genes from each parent). Based on the information provided above regarding average proportions of segregating genes shared by different sibling types, the correlation between A2 for adolescent sibling 1 and adolescent sibling 2 was set to 1 for MZ twins because MZ twins share 100% of their segregating genes, .5 for DZ twins and full siblings who share on average 50% of their segregating genes, .25 for half siblings and cousin pairs whose parents are MZ twins, who share on average 25% of their segregating genes, .125 for cousin pairs whose parents are DZ twins and share on average 12.5% of their segregating genes, and 0 for genetically unrelated step siblings who share no genes systematically. The correlation between C2 for adolescent sibling 1 and adolescent sibling 2 was set to 1, and the correlations for E1 for twin parent 1 and twin parent 2 and E2 for adolescent sibling 1 and adolescent sibling 2 were fixed at 0, based on the definitions of shared and nonshared environmental influences.
