We focused on males in this report because there is uncertainty regarding the analysis of female data due to questions surrounding how X-inactivation affects MAOA in females. However, we did conduct secondary analyses on females wherein heterozygotes were included and coded as a separate genotype class, as has been done previously (Prom-Wormley et al., 2009). We did not detect main effects of genotype, and main effects of physical discipline were not consistently statistically significant. Furthermore, we did not detect significant GxE effects. However, trends in the direction of effect were opposite that observed in males: that is, high-activity alleles conferred risk in conjunction with increasing levels of physical discipline, while females homozygous for low-activity alleles exhibited lower levels of externalizing behavior with increasing environmental risk. These results are consistent with previous studies in females (Nilsson, Wargelius, Sjoberg, Leppert, & Oreland, 2008; Prom-Wormley et al., 2008; Wakschlag et al., 2009). Larger, more highly powered studies, focused on females, along with enhanced biological understanding of how X-inactivation affects MAOA, will be necessary to better understand the meaning of these findings.
