E component with percentage of older adolescents for both behavior problems and alcohol use, with unique environmental influences decreasing in importance as there were more older adolescents in the neighborhood, contrary to expected. It is not immediately obvious why this would be so; the interpretation of the E component is complicated by the fact that it reflects not only the influence of environmental factors unique to the individual twins, but also the error component. The opposite direction of effect observed for the E component with older adolescents may also reflect the inverse correlation (albeit very low) between percentage of older adolescents and urban/rural residency (which also showed moderation of E). However, it is notable that both urban/rural residency and percentage of older adolescents show moderating effects on genetic influences, despite low correlation between the variables, suggesting that the effect of the composition of older adolescents is not merely a reflection of urban/rural status.
