Before assessing the effect of ETLE exposure and MAOA genotype on risk for increased physical aggression, we first tested whether subgroups in our sample differed significantly for the distribution of psychiatric diagnosis, ETLE exposure, and MAOA genotype. Such deviations might confound the interpretation of subsequent findings and reveal the existence of gene-by-environment correlation effects. Men and women did not differ in the distribution of psychiatric diagnosis (χ2 = 0.92, df = 1, P = 0.34) and exposure to ETLE (χ2 = 0.03, df = 1, P = 0.87). Compared to healthy controls, exposure to ETLE between 0 and 15 years of age was more frequent among psychiatric patients (χ2 = 3.67, df = 1, P = 0.05). Importantly, the ETLE groups did not differ significantly in MAOA genotype distribution, arguing against the possibility that genotype influenced exposure to traumatic events (χ2 = 0.21, df = 1, P = 0.64). Finally, the distribution of MAOA genotype did not differ between the healthy and psychiatric patient groups (χ2 = 0.35, df = 1, P = 0.56).
