Prevalence of antisocial child outcome as a function of familial liability and official-report maltreatment history are depicted in Figure 1, panels A and B. In the epidemiologic twin sample (panel A), we observed a steady increase in the proportion of children with clinical-level elevation in CBCL externalizing score at increasing levels of familial liability (ascertained on the basis of co-twin status). In general, across all levels of familial liability, maltreatment was associated with a 10–25% increase in prevalence of child antisocial outcome, consistent with additive effects (not interactive effects) of inherited liability and maltreatment on antisocial outcome. To isolate the effects of inherited influence we examined discrepancies in externalizing outcome between MZ and DZ twins within each category subsumed by maltreatment and co-twin affectation status. Based on the close correspondence of the tracings for URM and SRM/FC in Figure 1A, we optimized statistical power for these analyses by collapsing these report types into a single maltreatment variable. Rao-Scott Chi-square = 2.79, p=.09 for maltreated identical versus non-identical co-twin affected; 6.45, p=.01 for maltreated, identical versus non-identical co-twin unaffected; 8.30, p=.004 for non-maltreated, identical versus non-identical co-twin affected; 12.16, p<.0005 for non-maltreated, identical versus non-identical co-twin unaffected.
