A hierarchical biometric model is presented of the origins of comorbidity among substance dependence, antisocial behavior, and a disinhibited personality style. The model posits a spectrum of personality and psychopathology, united by an externalizing factor linked to each phenotype within the spectrum, as well as specific factors that account for distinctions among phenotypes within the spectrum. This model fit self-report and mother-report data from 1,048 male and female 17-year-old twins. The variance of the externalizing factor was mostly genetic, but both genetic and environmental factors accounted for distinctions among phenotypes within the spectrum. These results reconcile evidence for general and specific causal factors within the externalizing spectrum and offer the externalizing factor as a novel target for future research. Common mental disorders are often correlated with each other, co-occurring at greater than chance rates in both clinical and epidemiological samples (Clark, Watson, & Reynolds, 1995; Lil- ienfeld, Waldman, & Israel, 1994; Sher & Trull, 1996; Widiger & Sankis, 2000). What is the meaning of this "comorbidity" phe- nomenon? Krueger and colleagues (Krueger, 1999b, 2002; Krueger, Caspi, Moffitt, & Silva, 1998; Krueger, McGue, & Ia- cono, 2001) have proposed that this phenomenon may result from common mental disorders acting as reliable indicators of latent factors, or hypothetical core psychopathological processes, that underlie putatively separate disorders. To date, this hypothesis has been supported by data gathered from unrelated persons. Such data have allowed for multivariate analyses of observed, phenotypic correlations among mental dis- orders. These analyses have revealed a broad, latent factor linking substance dependence and antisocial behavior disorders in late adolescence and adulthood. Following the lead provided by mul- tivariate analyses of emotional and behavioral problems in chil- dren (Achenbach & Edelbrock, 1978, 1984), this factor has been labeled externalizing (cf. Kendler, Davis, & Kessler, 1997). In the analyses presented herein, we extended this line of re- search by addressing three specific questions in a genetically informative sample. First, what is the etiologic basis for the phe- notypic externalizing factor? Second, are there etiologic factors that distinguish among specific externalizing disorders? Third, are disinhibitory personality traits part of the externalizing spectrum?