Studies of developmental endophenotypes such as P3AR can help inform our understanding of behavior and clinical disorders. Yoon et al. (2006) successfully demonstrated an application of this approach when they showed that adolescents, particularly boys, showed P3AR if they were misusing substances (e.g., binge drinking; frequently smoking; using cannabis, amphetamines, or psychedelics), thus suggesting that these substance use behaviors are themselves manifestations of the underlying genetic risk. P3AR can also help shed light on which disorders are part of the externalizing spectrum. For instance, Yoon and colleagues (2008) found that ADHD alone in 11-year-old subjects was not associated with P3AR, whereas ADHD combined with conduct disorder was. Studies like this can thus illuminate the relationship of ADHD to the externalizing spectrum, as well as help to corroborate systems like ICD-10, which treat the combination of conduct disorder and ADHD as an important and distinct diagnostic entity.
