Likewise, the variance in adolescent externalizing behavior was parsed into the influence of adolescents’ genes (A1’, parent and adolescent shared genes, A2: unique genetic influences on externalizing), shared (C2) and nonshared (E2) environments of adolescents on their own externalizing behavior, along the bottom of Figure 1. 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, .5 for DZ twins and full siblings, .25 for half siblings, and 0 for genetically unrelated step siblings. The correlation between C2 for adolescent sibling 1 and adolescent sibling 2 was set to 1, and the correlation for E2 for adolescent sibling 1 and adolescent sibling 2 was fixed at 0, based on the definitions of shared and nonshared environmental influences provided above.
