towards and relative availability of substances across ethnic groups, and within the EA twins, to differences in the legal status of the drugs (Boardman et al., 2010; Shanahan and Hofer 2005). For instance, while cannabis is more socially accessible in AA populations (Wallace and Muroff 2002), AA girls (but not boys) are less likely than their EA counterparts to report lifetime and recent cannabis use (Schepis et al., 2011). Another possible contributor to reduced familial and increased E variance in AA twins and siblings may be exposure to an authoritarian form of parental monitoring (Tamis-LeMonda et al., 2008), which has been shown to be more associated with reduction in substance involvement in AA than in EA youth. Interestingly, studies have shown that the relative importance of familial sources of variance is attenuated in the presence of increased parental supervision (Dick et al., 2007). An alternate explanation is reduced power associated with the notably smaller number of AA pairs (e.g., for MZ: 111 vs. 853, Table 2). Despite this limitation, confidence limits on the correlations suggest that the reduced AA rMZ and rDZ are meaningful.
