Our literature review and analysis of data support a dimensional-spectrum conceptualization of externalizing psychopathology in adulthood. A review of the pertinent literature, as well as our data analysis, shows that externalizing phenomena are well conceived in terms of a broad but coherent group of disorders that vary continuously both within and among syndromes. A number of specific recommendations for placing DSM–V on solid empirical footing follow from these observations.
First, the coherence of the externalizing spectrum should be explicitly recognized in DSM–V
. A logical way to recognize this coherence is to organize externalizing syndromes into a specific chapter of the manual. In addition, the organization of these syndromes calls for a reevaluation of the putative distinction between Axis I (syndrome disorders) and Axis II (personality disorders). This distinction is not well supported by data on the externalizing spectrum, which transcends Axes I and II (cf. Krueger & Tackett, 2003
Along these lines, the hierarchical–dimensional externalizing spectrum model should frame the relevant section of DSM–V, because the fit of the model to the data leads to improved understanding and practical application in comparison with the existing prototype categorization model. For example, from a research standpoint, the current perspective is more efficient in terms of exploring etiology. A coordinated theory of several disorders can be formulated, rather than separate theories for each disorder, and research efforts coordinated accordingly. From a treatment standpoint, the model leads to a focus on generalized interventions for disorders within the spectrum, as well as variegated strategies for treatment of specific syndromes within the spectrum (e.g., specific forms of substance dependence) on the basis of the relative role of the broad externalizing vulnerability in different cases.
This latter point is of particular importance. Although the evidence reviewed herein clearly points to the coherence of the externalizing spectrum, the literature also points to the importance of specific genetic and environmental factors in shaping the broad genetic liability linking all externalizing syndromes. To date, such evidence mostly has pertained to extant DSM
syndromes. The possibility of further subdivisions within the DSM
syndromes has received less attention. Nevertheless, this is an important area for future research because emerging evidence points toward meaningful variants within some syndromes. For example, as we described earlier, conduct disorder consists of at least two distinguishable subdimensions pertaining to more aggressive and more rule-breaking behaviors (Tackett et al., 2003
). The development of an enriched hierarchical model that encompasses levels ranging from these very fine-grained distinctions up to the level of the overarching externalizing dimension is an important goal for future research.
Second, research should continue to develop our understanding of the breadth of the externalizing spectrum. A number of disorders described in DSM–IV
may be good candidates for inclusion within the spectrum. For example, antisocial personality disorder shares significant genetic variance with pathological gambling, suggesting that this disorder might also be conceptualized as an externalizing spectrum disorder (Slutske et al., 2001
). Similarly, disorders that have received less research attention (e.g., intermittent explosive disorder; McElroy, 1999
) might be reconceptualized as configurations of fundamental elements within the externalizing spectrum, such as aggression and impulsivity (Coccaro, Kavoussi, Berman, & Lish, 1998
). In addition, the methods and conceptual approach developed here also could be applied to disorders that may transcend spectra (e.g., borderline personality disorder, which includes features that appear to be both internalizing and externalizing in nature; Sanislow et al., 2002
) as well as to putatively distinct psychopathology spectra (e.g., the schizophrenia spectrum; Wolf et al., 1988
Third, given that the current evidence supports a dimensional conceptualization, DSM–V should explicitly recognize that externalizing syndromes are dimensional in nature. Along these lines, the inclusion of specific symptoms as indicators of specific externalizing syndromes should be based on empirical models of the relevant data. That is, symptoms should be selected because research shows them to be informative with reference to specific syndromes. In particular, the approach we have outlined in this article, which involves explicit quantitative modeling of psychopathological data, should prove useful in identifying optimal conceptualizations and informative symptoms. In theory, such research might also identify syndromes in which a categorical conceptualization provides a better account of the data. Although this seems unlikely with reference to the externalizing syndromes reviewed herein (given the current state of the evidence), the cardinal point is that the process of identifying the optimal conceptualization of psychopathology must be based on data and close collaboration between methodological and substantive experts, not on a priori preferences.
In addition, it is also important to note that models of psychopathological phenomena can accommodate data from any modality. In this sense, external correlates also can enter into the process of adjudicating among various models. For example, reduced amplitude of the P3 event-related potential appears to be a promising indicator of genetic risk for externalizing spectrum disorders (Iacono, Malone, & McGue, 2003
). This putative endophenotype has been associated with familial risk for a variety of externalizing disorders, including substance use and antisocial behavior disorders (Iacono, Carlson, Malone, & McGue, 2002
). Moreover, Iacono et al. (2002)
showed that mentally healthy adolescents at low familial risk for developing externalizing psychopathology who nevertheless did so by young adulthood had reduced P3 amplitudes as adolescents, thus illustrating that this endophenotype can identify high-risk individuals in the absence of other common externalizing risk indicators. Other promising biological markers of risk for psychopathology (e.g., specific genotypes that appear to confer risk for externalizing psychopathology, such as monoamine oxidase A; Caspi et al., 2002
) can and should be modeled along with data pertinent to specific syndromes in constructing the diagnostic system.
Finally, we note that conceptualizing externalizing syndromes as continuous in nature does not in any way mean that thresholds cannot or should not be identified for clinical and practical purposes (cf. Widiger & Clark, 2000
). The key is to understand the fundamental distinction between psychopathology in and of itself and the implications of psychopathology for society and the individual. That is, even if externalizing phenomena vary continuously in the population, beyond a certain level they become a problem for the person and/or for other people. What it means for something to “become a problem” is partly a value judgment, not something that can be decided solely with reference to data (cf. Kendler, 1990
). Nevertheless, empirical research on how the probability of adverse consequences is linked to variation within the externalizing spectrum can help professionals decide where to place cutoffs relevant to specific clinical decisions. This situation is exactly akin to other situations in clinical medicine, for example, the need to place cutoffs on continuous variables such as cholesterol count that correspond to unacceptable risk for adverse consequences. In preparation for DSM–V
, therefore, research should be pursued that examines and models the nature of the relationship between externalizing phenomena and social consequences such as economic impact (e.g., days lost at work, missed educational opportunities), personal impact (e.g., family strife, child abuse and neglect), and impact on society (e.g., involvement with the criminal justice system). The results of such research should inform professional decisions about how DSM–V
will demarcate the distinction between societally sanctioned and societally problematic externalizing behavior.