In line with earlier G × E studies (Caspi et al, 2002; Ducci et al, 2008; Foley et al, 2004; Kim-Cohen et al, 2006; Widom and Brzustowicz, 2006) we found that in both sexes, the low activity MAOA-LPR allele imparted risk for hyperactivity in the context of early life stress. Moreover, our results indicate a crossover of risk whereby in the 25% of girls who had been exposed to the greatest stress (≥ 9 life events from 6 months to 3.5 years), the low activity allele was associated with increased hyperactivity. In contrast, in the 75% of girls who had been exposed to less or no stress, the low activity allele was associated with lower hyperactivity. Based on our results it is tempting to speculate that the MAOA-LPR low activity allele confers risk for behavioral inhibition in the absence of stress and behavioral disinhibition in the presence of significant stress whereas modest early life stress exposure is beneficial to all genotypes (the crossover point of graphs in Figure 1, B1 and B2). Childhood inhibited and disinhibited behaviors both predict psychopathology
