Belsky et al. (2009) have also pointed out that only with rare exceptions (Bakermans-Kranenburg & van IJzendoorn, 2006; Taylor et al., 2006) has the environment of a study sample included both positive and negative ends of the spectrum. Rather, the absence of environmental stressors has usually constituted the “low” end of the environment—for example, the absence of life stressors (Caspi et al., 2003) or the absence of maltreatment (Caspi et al., 2002). Parental monitoring, the environmental variable studied in the present work, reflects levels of parents’ knowledge about their children’s activities. Low levels of parental monitoring can be considered a negative, high-risk environment for externalizing behavior, whereas high levels of parental monitoring can be considered a positive, low-risk environment (Pettit, Laird, Dodge, Bates, & Criss, 2001). That said, although we studied parental monitoring because of the previous literature indicating the central importance of this construct for externalizing behavior and because of the availability of data on parental monitoring for both samples, we do not necessarily believe that the environmental moderation associated with CHRM2 is limited to parental monitoring. Rather, we
