The purpose of these analyses was to explicitly compare continuous and categorical models of externalizing liability in a large, representative epidemiological sample. In the entire sample, as well as in females and males separately, the best-fitting model was a continuous normal trait model. Moreover, in the entire sample as well as in each sex group, the next best-fitting models were also trait models, indicating that continuous conceptions of externalizing liability provided substantial gains in fit over categorical conceptions of externalizing liability, as represented by latent class models. Overall, the modeling results presented here suggest that externalizing liability is a continuum that is normal in distribution.
The current results replicate previous results from a smaller sample15
in indicating that externalizing liability is best modeled as continuous normal in distribution. In that sample as in this one, the best-fitting models were trait models and the best-fitting model was a continuous normal model. The current results extend results of the previous study by using a much larger sample that was designed to be nationally representative. The current results also extend results of the previous study to DSM-IV
criteria and additional externalizing spectrum disorders. Finally, the current results extend those of the previous study in characterizing the nature of sex differences in the relationship between externalizing liability and disorder; although males and females exhibit the same general relationship between liability and disorder, the overall likelihood of externalizing disorders is elevated in males.
Although the findings presented here replicate findings in another, complimentary study,15
limitations of the current work should be noted. First, it should be noted that interviews were conducted by nonclinicians without access to independent sources of information. The distribution of externalizing liability given other sources of information remains an important topic for future research.
Second, it is important to note that the interview data was collected at a single point in time. Longitudinal data providing detailed information about stability and changes in symptomatology over time have proved useful in understanding liability to other forms of psychopathology.36,37
Similar longitudinal data on externalizing disorders would help refine our understanding of externalizing etiology and liability. Combined with a population genetic design (eg, twin, family design), longitudinal data would be especially useful in delineating the genetic and environmental developmental dynamics of externalizing liability. Such a design would help delineate, for example, whether the liability continuum acts early in development to initiate a chain reaction of disorders, or acts relatively constantly throughout development as a predisposition toward externalizing disorders. These two different accounts of externalizing psychopathology cannot be distinguished in the current data set, which contains only cross-sectional information about unrelated individuals.
The results of the current study may differ somewhat from those of the previous study15
in suggesting slightly different interpretations of how externalizing spectrum disorders reflect an underlying continuum of liability. In particular, in the previous study, adult antisocial behavior was the most informative disorder regarding externalizing liability, whereas in the current study, antisocial personality disorder was the second least informative about externalizing liability.
These interpretive differences may be because of differences between the 2 studies in the treatment of child conduct disorder criteria. In the current study, antisocial personality disorder was diagnosed according to DSM-IV
criteria, which included child conduct disorder criteria. In the previous study, adult antisocial behavior was diagnosed separately from child conduct disorder, which was the least informative disorder with regard to underlying externalizing liability. Combining the 2 criteria sets in the current study may have decreased the overall informativeness of antisocial personality disorder with regard to externalizing liability. If this is the case, it suggests that the child conduct disorder criteria and adult antisocial behavior criteria are best treated as separate—albeit related—disorders, at least insofar as they reflect an externalizing spectrum. The results of the 2 studies suggest that conduct disorder does not necessarily presage adult antisocial behavior and support recent suggestions38
that conduct disorder not be treated as a prerequisite for diagnosis of adult antisocial disorder.
Despite these limitations and interpretive considerations, the current results greatly clarify the distributional qualities of externalizing liability and provide a framework for further research on externalizing phenomena. The current results also underscore an important observation about liability to mental disorders more generally—that it is possible that underlying liability to discretely assessed disorders may itself be continuous in distribution. It is notable in this regard that the dichotomous externalizing disorders are best modeled as having a continuous liability distribution—a finding which emphasizes the distinction between assessed behavior and underlying liability.
Current results indicate that general externalizing liability is best conceptualized as a continuum, with current disorders representing extremes of this continuum. The conclusion that externalizing disorders represent extremes of a continuum is not necessarily inconsistent with current diagnostic practice, as it is possible to conceive of current diagnoses as being thresholds along continua of severity. However, the existence of a unitary externalizing liability continuum indicates that the externalizing continuum itself is an important target for research and treatment, in addition to its manifestations in particular disorders. Moreover, to the extent that externalizing disorders represent extremes of a continuum, assessing less severe forms of externalizing phenomena along a graded range is likely to provide important information about pathology and functioning. This additional information might be lost if pathology were only assessed at the level represented by current diagnostic criteria. It is important that diagnostic systems recognize a common, superordinate externalizing liability, and that this liability is best assessed along a graded range of severity.
The current results provide a framework for examining the etiology of externalizing disorders. Recent studies have documented the importance of studying general externalizing liability in addition to its specific manifestations. These studies have indicated that the general liability toward externalizing spectrum disorders is highly heritable,11,39
and that this liability therefore represents an important focus for molecular genetic and neurobiological research on the etiology of externalizing spectrum disorders. Although our results do not directly address externalizing etiology, they do provide phenotypic clues for possible targets of etiologic research. For example, our results reinforce the importance of neurobiological factors affecting multiple externalizing disorders simultaneously, including substance use and antisocial behavior disorders. The ventromedial prefrontal cortex is an example of a neural region that might be targeted for further research on general externalizing liability, as it has been implicated in both substance use40
and antisocial behavior disorders.41,42
Our results indicate that molecular genetic, neurobiological, and other etiologic studies focusing on general externalizing liability are likely to be most informative if they target underlying liability as a continuum, rather than a set of discrete groups of risk. Our results also help define degrees of severity along the externalizing spectrum, which may be useful for extreme-group designs sometimes used in expensive molecular genetic or neurobiological studies.43,44
In particular, our results document the differential informativeness of various disorders along different ranges of the externalizing continuum, and suggest that different disorders may be differentially useful for examining different points on the liability distribution.
It is important to note that the continuous nature of externalizing liability does not preclude establishment of discrete categories for clinical treatment decisions. In many fields, thresholds of risk along a continuum are established according to levels of increased risk for adverse outcomes (eg, blood pressure ranges). It is possible to establish similar thresholds of externalizing liability according to levels of increased risk for adverse outcomes. These thresholds of outcome risk might then be used to target individual treatments according to level of externalizing liability.
It is also important to emphasize that the existence of an externalizing liability continuum should not be construed to suggest that individual disorders do not possess meaningful information in themselves. Although externalizing disorders are influenced by a common liability continuum, they also contain important unique variance. This conclusion is supported by various population and molecular genetic studies. Population genetic analyses of externalizing disorders demonstrate the presence of genetic influences that act on specific externalizing disorders, in addition to genetic influences that act on general externalizing liability.10,11,45
Molecular genetic studies also support this conclusion, demonstrating that in addition to genetic influences on general externalizing liability, substance use disorders are influenced by genes regulating the metabolism of specific substances.46–50
These converging lines of evidence suggest that research into the etiology of externalizing disorder is likely to be most successful when general externalizing liability is studied jointly with unique liabilities to specific externalizing disorders.
It is noteworthy in this regard that the models used here to delineate relationships between externalizing disorders and general externalizing liability could also be used to delineate relationships between symptoms of each externalizing disorder and its unique liability. Such an approach would aid in understanding what symptoms are most strongly related to general externalizing liability, and what symptoms characterize unique liabilities to specific forms of externalizing psychopathology. Characterizing manifestations of general externalizing liability, unique liabilities to specific forms of externalizing, and relationships between the two, will help better define phenotypic targets for etiologic and treatment research.