Physical discipline was a significant and positive predictor of nearly every measure of externalizing behavior. Previous studies using the entire CDP population have reporter higher levels of externalizing (Keiley, Howe, Dodge, Bates, & Pettit, 2001; Lansford et al., 2007), internalizing (Keiley et al., 2001), and a variety of other problematic behavioral outcomes (Lansford et al., 2007) in children who were physically maltreated before the age of 5 than in those who were not maltreated. While our results are in agreement with previous findings that early maltreatment can lead to later problematic behavior, they also indicate that levels of physical discipline that might be considered mild or moderate can actually create an environmental risk. Clearly, exposure to an adverse environment such as harsh physical discipline is a powerful risk factor for problematic behavior throughout development. Such environmental risk is exacerbated by the presence of the biological vulnerability conferred by low-activity alleles at this locus. The neurobiological ramifications of low levels of MAOA transcript are still unknown, and much work remains to be done to elucidate the mechanisms by which transcript variation influences behavior.
