There are also several important similarities across inferred and measured genotype studies of alcohol-related GxE. Consistent with GxE findings for antisocial behaviors (Moffitt, 2005), it is noteworthy that both the aggregate genotype and molecular genetic studies we reviewed frequently found positive GxE findings in the absence of main effects. There are a number of potential explanations for the lack of main effects. First, greater heritability or genotypic effects on drinking problems are not necessarily equivalent to higher mean levels of drinking problems, only more variability: certain environments may provide more diverse options and opportunities for individuals to show their genetic predispositions. For example, some adolescents may respond to the greater diversity of social opportunities in urban settings by consuming less alcohol. Second, the estimated main effect of genes on drinking behaviors may be weak if the genetic effect is manifest only among individuals exposed to a particular type of environmental risk. Similarly, environmental influences may not be detected if the environmental factor only confers risk among individuals with genetic liability. Third, because there is often greater power to detect GxE
