The pattern of findings is consistent with the view that negative emotionality contributes to the co-occurrence of internalizing and externalizing symptoms (Khan et al., 2005). Anger (at T1 and T3 combined) and sadness at T1 were somewhat better predictors of co-occurring group status than pure internalizing or externalizing status. Moreover, deficits in EC but not impulsivity more frequently predicted co-occurring status than pure externalizing (to a modest degree) or pure internalizing (fairly markedly) status. The latter pattern of findings suggests that it is voluntary inhibition, not simply disinhibition, that plays a role in the co-occurrence of internalizing and externalizing spectrum disorders (Krueger & Markon, 2006). Given the apparent role of heredity and to a lesser degree, shared environment, in such comorbidity (e.g., O’Connor, McGuire, Reiss, Hetherington, & Plomin, 1998), it seems reasonable to examine genetic and parenting influences on temperamentally based EC and negative emotionality (see Rothbart & Bates, 2006) as factors that predict or foster comorbidity of externalizing and internalizing disorders.
