The interaction between ADH1B-rs1229984 and childhood adversity was assessed by including an interaction term (ADH1B*childhood adversity) in separate linear-Poisson regression models (Spiegelman, Hertzmark, & Wand, 2007) for each alcohol phenotype, performed using Proc GENMOD in SAS 9.3. We assessed this interaction on the additive scale, as additive interaction corresponds more closely than multiplicative interaction with the current conceptual understanding of biological interaction and causal synergism (“Modern Epidemiologic Approaches to Interaction: Applications to the Study of Genetic Interactions,” 2006; Spiegelman et al., 2007; Spiegelman & Hertzmark, 2005). The estimate for the interaction term represents the interaction contrast (IC), a “difference in differences” effect. This “difference in differences” is evaluated as follows. First, among those who experienced childhood adversity, the mean difference in the alcohol phenotypes between those with the GG genotype and those with the AA/AG genotypes is evaluated. Next, among those without childhood adversity the mean difference in the alcohol phenotypes between those with the GG genotype and those with the AA/AG genotype is evaluated. Last, the IC, i.e., the difference in these two mean differences is evaluated, to determine
