to Develop an Empirically Based Model Understanding Psychopathology : Melding Behavior Genetics, Personality, and Quantitative Psychology

Research on psychopathology is at a historical crossroads. New technologies offer the promise of lasting advances in our understanding of the causes of human psychological suffering. Making the best use of these technologies, however, requires an empirically accurate model of psychopathology. Much current research is framed by the model of psychopathology portrayed in current versions of the Diagnostic and Statistical Manual of Mental Disorders (DSM; AmericanPsychiatric Association, 2000). Although the modern DSMs have been fundamental in advancing psychopathology research, recent research also challenges some assumptions made in the DSM—for example, the assumption that all forms of psychopathology are well conceived of as discrete categories. Psychological science has a critical role to play in working through the implications of this research and the challenges it presents. In particular, behavior-genetic, personality, and quantitative-psychological research perspectives can be melded to inform the development of an empirically based model of psychopathology that would constitute an evolution of the DSM. KEYWORDS—classification; DSM; statistics; comorbidity; dimensions; categories Psychopathology research is at a historical crossroads. Powerful technologies, such as molecular genetics and sophisticated statistical models, now exist to aid us in our attempts to understand the origins of psychological suffering. To fully exploit these technologies, however, we need to know how to best conceptualize psychopathology. We need an empirically based model of psychopathology that can guide our inquiries into its origins. Most psychopathology research is currently framed by the system provided in the fourth edition (text revision) of the Diagnostic andStatisticalManual ofMentalDisorders (DSM-IV-TR; American Psychiatric Association, 2000). A number of specific assumptions underlie the classification of all disorders described in the DSM-IV-TR. A cardinal assumption is that mental disorders are categorical: The manual lists a large number of categories of mental disorder, and for each category, a series of criteria for category membership are listed. People are assumed to be either members of these categories or nonmembers; graded degrees of category membership are not permitted. Importantly, the DSM-IV-TR itself acknowledges potential limitations of this categorical approach to conceptualizing psychopathology, noting that ‘‘a categorical approach to classification works best when all members of a diagnostic class are homogenous, when there are clear boundaries between classes, and when the different classes are mutually exclusive’’ (p. xxxi). Each of these areas has proven problematic for DSM categories. Members of specific diagnostic classes tend to be heterogeneous, boundaries between classes are often unclear, and classes are rarely mutually exclusive. This is the sense in which psychopathology research is at a historical crossroads. DSMdefined categories are the most frequent targets of psychopathological inquiry, yet reliance on DSM-defined categories often results in significant problems in research design and interpretation. To pick a specific example for illustrative purposes, if one wants to understand depression, what should be done about the fact that the boundary between depression and other DSM categories is often unclear (e.g., depression overlaps with dysthymia; Klein & Santiago, 2003) and many people who meet criteria for depression meet criteria for other disorders as well (e.g., anxiety disorders; Kessler, DuPont, Berglund, &Wittchen, 1999)? Is it possible to develop an empirically based approach to psychopathology that could overcome these limitations? The development of such a system is a tractable goal, and the pursuit of this goal involves integrating a number of areas of inquiry that represent quintessential strengths of psychological science. Some broad outlines of such a system can be seen by tying together recent research findings from these areas: Specifically, research strategies, concepts, and findings from Address correspondence to Robert F. Krueger, Department of Psychology, University of Minnesota, Elliott Hall, 75 E. River Rd., Minneapolis, MN 55455; e-mail: krueg038@umn.edu. CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE Volume 15—Number 3 113 Copyright r 2006 Association for Psychological Science at UNIV OF COLORADO LIBRARIES on August 24, 2011 cdp.sagepub. com Downloaded from quantitative psychology, behavior genetics, and personality psychology provide the tools needed to develop an empirically based model of psychopathology. CONTRIBUTIONS OF QUANTITATIVE PSYCHOLOGY TO UNDERSTANDING PSYCHOPATHOLOGY One prominent movement in psychology during recent decades has been the use of explicit quantitative models to describe and predict psychological phenomena. Quantitative models are sets of mathematical and statistical equations describing and predicting psychological phenomena. Structural-equation models, item-response models, growth-curve models, and other latentvariable models have allowed tremendous increases in the sophistication of theories that can be tested and in the confidence of our conclusions about those theories. Thesemethods also hold promise for understanding psychopathology, because they allow empirical comparison of different classification paradigms. Such paradigms can be represented by different quantitative models, and can be rigorously compared by comparing the fit of those models to psychological data. Empirical comparisons between different factor models, for example, have indicated that common forms of psychopathology in adults can be understood in terms of a hierarchical factor model (Krueger & Markon, 2006; see Fig. 1) that bears a strong resemblance to influential factor models in child-psychopathology research (Achenbach & Edelbrock, 1984). At a high level of the hierarchy, psychopathological variation and covariation are organized by two broad, correlated dimensions, Internalizing and Externalizing. Internalizing psychopathology represents a spectrum of conditions characterized by negative emotion and includes phenomena such as depression, anxiety, and phobias. At a lower level of the hierarchy, the Internalizing spectrum splits into a Distress subspectrum and a Fear subspectrum; the former is characterized by ruminative disorders such as depression and generalized anxiety, the latter by paroxysmal disorders such as phobia and panic disorder. Externalizing psychopathology, in contrast, is characterized by disinhibition; this spectrum includes phenomena such as antisocial behavior and substance-use disorder. This hierarchical model provides a better account of patterns of psychopathology than do many competing factor models, including ones that contain fewer or more factors. Recent latent-variable-modeling studies have suggested that, indeed, these common forms of psychopathology are best thought of as continuous, rather than categorical, in nature. Generally speaking, continuous models classify people by locating them along graded dimensions, whereas categorical models classify people into distinct groups. Explicit comparisons of continuous and categorical models of the occurrence and co-occurrence of externalizing disorders indicate that this broad domain of psychopathology is continuous in nature, reflecting a liability or underlying level of risk for disorder that is graded in severity rather than discrete and categorical (Krueger, Markon, Patrick, & Iacono, 2005; Markon & Krueger, 2005). Although, to our knowledge, continuous and categorical models of the overarching internalizing domain have not been compared directly, relevant research does exist for specific internalizing syndromes. For example, continuous models of depression have increased validity over categorical models (Aggen, Neale, and Kendler, 2005), and depressive symptoms appear to be continuously distributed (Hankin, Fraley, Lahey, & Waldman, 2005). Finding that common forms of psychopathology are best conceptualized as continuous in nature calls into question the