To our knowledge, this is the first report of an interaction between MAOA and environmental stress that studied a continuous measure of physical environmental risk and the resulting influence on problematic behavioral outcomes. We assessed whether externalizing behavior increases linearly as a function of physical discipline exposure. That is, do we observe increases in externalizing only at high levels of discipline, or does the behavioral outcome increase gradually alongside increases in physical discipline? Our results indicate that the relation between these variables is indeed linear; individuals experiencing high levels of discipline are more likely to exhibit problematic levels of externalizing, but there does not appear to be a “threshold” of discipline experience beyond which the pattern of externalizing changes. However, we did conduct preliminary analyses using a binary measure of maltreatment, which was defined as an intentional strike that left physical marks for at least 24 hours. The statistical power of these analyses was limited, and no significant GXE effects were detected, although interaction effects were in the expected direction.
