It is important to note that this emphasis on the unique contributions of internalizing processes to SUDs may appear to be in contrast to existing studies in which internalizing symptoms contribute little to the prediction of substance use in adolescence once risk associated with externalizing symptoms is considered (e.g., Hussong et al., 1998). As previously noted, this finding has led some to question the practical importance of internalizing symptoms as an indicator of risk for substance involvement. However, the unique contributions of internalizing and externalizing symptoms may be difficult to detect in adolescence (Serrano, Bauer, Curran, & Hussong, 2008). First, we have found that the unique effects of internalizing symptoms are only apparent at more severe levels of use or for more severe drugs of abuse (Hussong, Curran, Lee et al., 2008; Hussong, Bauer, Serrano, et al., 2008). Second, comorbidity rates of internalizing and externalizing symptoms peak in adolescence as compared with childhood and adulthood, and such high rates of co-occurrence may obscure the unique effects of internalizing symptoms during adolescence. This makes detection of internalizing processes as defined solely
