Differences according to PER2 genotype and sex were examined using a two-way (PER2 × sex) ANOVA. To assess the effects of genotype, stressful life events and their interaction on alcohol use in regular drinkers, a linear regression model was performed. After adjusting for the main effects of PER2 and stressful life events, the gene x environment effect was tested as additional interaction term. Significant interactions were further investigated using simple main effects analyses. Sex was included as a covariate. For these analyses, PER2 rs56013859 was classified according to homozygosity for the major A allele (i.e., dominant model for the minor G allele). Stressful life events were entered as a continuous variable. The current sample of N = 131 regular drinkers has a power to detect interaction effects of small effect size according to Cohen (i.e., f2 = .02) with p<.05 of 36.2% and of medium effect size (i.e., f2 = .15) of 99.3% [33].
