Other factors reducing research interests in the role of internalizing, as opposed to externalizing, mechanisms in substance use include challenges associated with measurement. Measures of internalizing symptoms are typically less reliable (particularly at younger ages) and show lower rates of inter-reporter agreement than those targeting externalizing symptoms (De Los Reyes & Kazdin, 2008). In addition, heterogeneity within measures of internalizing symptoms that reflect different forms of affective symptoms may have greater negative repercussions for the study of SUDs than heterogeneity within measures of externalizing symptoms. In general, various forms of externalizing symptoms all serve to increase risk for SUDs (Iacono, Malone, & McGue, 2008b). However, some forms of internalizing symptoms may actually reduce this risk (e.g., separation anxiety in late childhood; Kaplow, Curran, Angold, & Costello, 2001) whereas others may increase it (e.g., depression in adolescence) but only at certain ages (e.g., anxiety in later adolescence, Sung, Erkanli, Angold, & Costello, 2004).
