Five twin studies have examined whether parental and peer factors moderate genetic risk for drinking. Miles, Silberg, Pickens, and Eaves (2005) found that genetic influences accounted for more of the standardized variance in the initiation of alcohol consumption among adolescent female twins from Virginia raised in families categorized into lower versus higher levels of parental closeness. However, the difference between groups was relatively small (heritability of 96% vs. 74%) and tests of alcohol-related GxE were not significant for four other measures of family environment (Table 1). Using data from Finnish adolescent same-sex twin pairs (Finn Twin study) assessed at ages 14 and 17, Dick et al. (2007a) reported drinking frequency heritability was significantly higher among teens with the greatest number of alcohol-using peers (60%) compared to those with the fewest alcohol-using peers (22%). In contrast, parental monitoring and time spent with parents did not moderate genetic influences on adolescent drinking at ages 14 or 17, though there were significant moderation effects observed for adolescent smoking, with genetic influences assuming greater importance with reduced parental monitoring.
