Unique environmental influences on the covariation between these phenotypes are complicated. These causal pathways from PEER to SELF potentially represent social influence: one’s peer’s drinking behaviors impact one’s own consumption. This effect is specific to non-shared (unique, or E) environmental factors contributing to affiliation with drinking peers. Specific examples of such factors were not measured or included in the current models, but could include exposure to different peers in school, or perhaps stressful life events experienced by only one member of a twin pair. The causal pathways from SELF to PEER suggest a different relationship: rather than an individual being influenced by his peers, in this case the individual is the source of influence on his peers, or his alcohol consumption influences what kind of peers he affiliates with. These paths can be conceptualized as a form of social selection. A critical implication of these causal paths is that changes in non-shared environmental influences on peer alcohol use would necessarily result in corresponding changes in one’s own use, and vice versa.
