Journal of Abnormal Psychology 2005 Vol 114 No 4 551 556 Copyright 2005 by the American Psychological Association 0021 843X 05 12 00 DOI 10 1037 0021 843X 114 4 551 Dimensional Versus Categorical Classification of Mental Disorders in the Fifth Edition of the Diagnostic and Statistical Manual of Mental Disorders and Beyond Comment on the Special Section Timothy A Brown and David H Barlow Boston University The value of including dimensional elements in the Diagnostic and Statistical Manual of Mental Disorders DSM has been recognized for decades Nevertheless no proposals have been made for introducing dimensional classification in the diagnostic system in a valid and feasible manner As an initial step in this endeavor the authors suggest introducing dimensional severity ratings to the extant diagnostic categories and criteria sets Although not without difficulties this would begin to determine the feasibility of dimensional classification and would address some limitations of the purely categorical approach e g failure to capture individual differences in disorder severity and clinically significant features subsumed by other disorders or falling below conventional DSM thresholds The utility of incorporating broader dimensions of temperament and personality in diagnostic systems beyond the fifth edition of the DSM is also discussed Keywords diagnostic classification of mental disorders dimensional versus categorical assessment of psychopathology risk factors for mental disorders comorbidity of mental disorders Diagnostic and Statistical Manual of Mental Disorders groundwork is needed to understand how dimensional classification can be validly and practically realized in the DSM Core dimensions must be identified measured and validated As most researchers would concur that the current categorical model of classification should not be abandoned entirely e g Brown Barlow 2002 Krueger Markon Patrick Iacono 2005 Widiger Samuel 2005 it must also be determined at what level dimensional elements are best incorporated into the diagnostic system e g dimensional severity ratings for the existing DSM disorder constructs dimensional assessment of higher order constructs not currently recognized by the DSM that reflect putative risk factors for families of disorders and account for their high rate of comorbidity cf externalization antisocial behavior and substance use disorders Krueger et al 2005 trait negative affect neuroticism anxiety and mood disorders Brown Chorpita Barlow 1998 Clark 2005 Watson 2005 An equally daunting challenge is the development of a dimensional assessment system that is widely agreed upon by DSM investigators and that can be practically and reliably implemented by both clinicians and researchers The limitations of a purely categorical approach to diagnostic classification are widely documented For example in our work with the anxiety and unipolar mood disorders we have encountered many problems with DSM s categorical diagnostic system see Brown Barlow 2002 for a review A diagnostic reliability study of the DSM IV anxiety and mood disorders Brown Di Nardo Lehman Campbell 2001 found that for many categories e g social phobia obsessive compulsive disorder OCD diagnostic disagreements less often involved boundary issues with other formal disorders but were primarily due to problems in defining and applying a categorical threshold on the number severity or duration of symptoms This threshold problem is manifested in various ways It can be seen in diagnostic disagree The purpose of this special section is to promote the development of dimensional models of diagnostic classification and approaches to research on psychopathology Although each article in this series highlights the importance of incorporating dimensional elements in the formal nosology the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders DSM V and beyond little was said about how this might be accomplished in future editions of the DSM but see Widiger Costa McCrae 2002 cited in Widiger Samuel 2005 This reflects a longstanding predicament For nearly 30 years researchers have acknowledged the potential utility of such systems e g Barlow 1988 Kendell 1975 Maser Cloninger 1990 Widiger 1992 Over this considerable time span however no strong proposals have emerged with regard to exactly how dimensional classification could be introduced in the DSM Investigators involved in the preparation of DSM IV considered and rejected the adoption of a dimensional classification in part because there is yet no agreement on the choice of the optimal dimensions to be used for classification purposes American Psychiatric Association APA 1994 p xxii With the possible exception of the Axis II disorders Widiger Samuel 2005 DSM V may be bound for the same destiny As was the case over a decade ago when the DSM IV was published a tremendous amount of empirical and conceptual Timothy A Brown and David H Barlow Center for Anxiety and Related Disorders Boston University Correspondence concerning this article should be addressed to Timothy A Brown Center for Anxiety and Related Disorders Boston University 648 Beacon Street 6th Floor Boston MA 02215 2013 E mail tabrown bu edu 551 552 BROWN AND BARLOW ments where both raters concur that the key features of a disorder are present but disagree as to whether these features cause sufficient interference or distress to satisfy the DSM IV threshold for a clinical disorder common with social phobia and specific phobia The problem is also evident in the high rates of disagreements involving not otherwise specified NOS diagnoses both raters agree on the presence of clinically significant features of the disorder but one rater does not assign a formal anxiety or mood disorder diagnosis because of subthreshold patient report of the number or duration of symptoms common with generalized anxiety disorder GAD and major depressive disorder MDD A similar problem is at the root of diagnostic disagreements involving MDD versus dysthymia core features of clinically significant depression are observed by both raters but disagreement occurs with regard to the severity or duration of these symptoms Finally whereas dimensional ratings of the severity of MDD symptoms are reliable r 74 the DSM IV categorical severity specifiers of this disorder are not e g 30 Brown Di Nardo et al 2001 The same pattern of results was obtained for the severity specifiers of panic and agoraphobia in DSM III R APA