Benjamin B. Lahey1 and Irwin D. Waldman2
Background: A better understanding of the nature and etiology of conduct disorder (CD) can inform nosology and vice versa. We posit that any prevalent form of psychopathology, including CD, can be best understood if it is studied in the context of other correlated forms of child and adolescent psychopa- thology using formal models to guide inquiry. Methods: Review of both cross-sectional and longitudi- nal studies of the place of CD in the phenotypic and causal structure of prevalent psychopathology, with an emphasis on similarities and differences between CD and oppositional deﬁant disorder (ODD). Papers were located using Web of Science by topic searches with no restriction on year of publica- tion. Results: Although some important nosologic questions remain unanswered, the dimensional phenotype of CD is well deﬁned. CD differs from other disorders in its correlates, associated impair- ment, and course. Nonetheless, it is robustly correlated with many other prevalent dimensions of psychopathology both concurrently and predictively, including both other ‘externalizing’ disorders and some ‘internalizing’ disorders. Based on emerging evidence, we hypothesize that these concurrent and predictive correlations result primarily from widespread genetic pleiotropy, with some genetic factors nonspeciﬁcally inﬂuencing risk for multiple correlated dimensions of psychopathology. In contrast, environmental inﬂuences mostly act to differentiate dimensions of psychopathology from one another both concurrently and over time. CD and ODD share half of their genetic inﬂuences, but their genetic etiologies are distinct in other ways. Unlike most other dimensions of psychopathology, half of the genetic inﬂuences on CD appear to be unique to CD. In contrast, ODD broadly shares nearly all of its genetic inﬂuences with other disorders and has little unique genetic variance. Conclusions: Conduct disorder is a relatively distinct syndrome at both phenotypic and etiologic levels, but much is revealed by studying CD in the context of its causal and phenotypic associations with other disorders over time. Advancing and reﬁning formal causal models that specify the common and unique causes and biological mechanisms underlying each correlated dimension of psychopathology should facilitate research on the fundamental nature and nosology of CD.
Keywords: Conduct disorder, oppositional deﬁant disorder, taxonomy, etiology, homotypic and heterotypic continuity.