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In a conceptual consideration at diagnostic comorbidity, Klein and Riso (1993) revisited two fundamental issues: whether disorders are discrete and natural classes or artificial categories created by the establishment of arbitrary cutoffs on a continuum and whether categorical or dimensional models of psychopathology better capture the essence of psychopathology. (We review research on the first of these issues in this section and research on the second later.) A phenomenon that is pervasive in clinical research is that patients and community participants rarely meet diagnostic criteria for just one mental disorder, contrary to the intentions of the authors of the diagnostic manual to develop criteria sets that lead to the identification of one single pathology. Klein and Riso listed six conceptual and statistical methods that could be used to demonstrate the existence of discrete boundaries between disorders and 11 possible explanations for diagnostic co-occurrences; they ultimately concluded that even these sophisticated methods may not properly account for all instances of comorbidity.

Evaluating the impact of high rates of comorbidity on clinical practice and research design in a large sample of young adults, Newman, Moffit, Caspi, and Silva (1998) concluded that groups that underrepresent comorbidity (e.g., student samples) probably also underestimate effect sizes for relations between a disorder and its correlates (e.g., physical health problems, interference with daily living, use of treatments, etc.), whereas groups that overrepresent comorbidity (e.g., clinical samples) overestimate effect sizes.

Concerns about the nature and extent of comorbidity led to the development of the National Comorbidity Survey (NCS), a nationwide stratified multistage survey of the U.S. population from 15 through 54 years of age. In an initial NCS report, Blazer, Kessler, McGonagle, and Swartz (1994) reported higher 30-day and lifetime prevalence estimates of major depression than the estimates reported in the ECA study and confirmed the high rates of co-occurrence between major depression and a range of other mental disorders. Kessler et al. (1995) examined the prevalence and comorbidity of

DSM-III-R posttraumatic stress disorder (PTSD) in a second NCS article. PTSD was strongly comorbid with other lifetime DSM-III-R disorders in both men and women—especially the affective disorders, the anxiety disorders, and the substance use disorders. In another NCS report, Magee, Eaton, Wittchen, McGonagle, and Kessler (1996) reported that lifetime phobias are highly comorbid with each other, with other anxiety disorders, and with affective disorders; they were more weakly comorbid with alcohol and drug dependence. As with major depression, comorbid phobias are generally more severe than pure phobias.

Four additional comorbidity studies all investigated the frequent co-occurrence of substance-related and other psychiatric disorders. Two (Hudziak et al., 1996; Morgenstern, Langenbucher, Labouvie, & Miller, 1997) explored links between PDs and substance abuse; a third (Brown et al., 1995) traced the clinical course of depression in alcoholics; the fourth (Fletcher, Page, Francis, Copeland, & Naus, 1996) investigated cognitive deficits associated with long-term cannabis use. All confirmed that substance abuse and the PDs—especially borderline and antisocial PD—co-occur at high frequency, as does substance abuse and mood disorder as well as long-term substance abuse and cognitive dysfunction.

Reflecting recent clinical interest in comorbid mental and physical disorders, Sherbourne, Wells, Meredith, Jackson, and Camp (1996) reported that patients with anxiety disorder in treatment for chronic medical conditions like hypertension, diabetes, or heart disease functioned at levels lower than those of medical patients without comorbid anxiety. These differences were most pronounced in mental-health-related quality-of-life measures and when anxiety rather than depression was comor-bid with the chronic medical conditions. A study with related aims (Johnson, Spitzer, Williams, Kroenke, & Linzer, 1995) reported that many of the primary care medical patients they studied also suffered from alcohol abuse or dependence; nearly half also had other co-occurring mental disorders. Although the substance abusers reported poorer health and greater functional impairment than did primary care patients without any mental disorders, they were less impaired than were patients who were diagnosed with mood, anxiety, eating, or somatoform disorders.

O'Connor, McGuire, Reiss, Hethering, and Plomin (1998) attempted to fit adolescent and parent reports and observational measures of depressive symptoms and antisocial behavior from a national sample of 720 same-sex adolescents to behavioral genetic models to determine the respective genetic and environmental influences on individual differences in and co-occurrence of the two psychopathological behaviors. Approximately half the variability in the depressive symptoms and antisocial behaviors could be attributed to genetic factors, although shared and nonshared environmental influences were also significant.

Reflecting another major societal concern, a 1996 issue of Archives of General Psychiatry featured five reports on the co-occurrence of violence and mental illness (Eronen, Hakola, & Tiihonen, 1996; Hodgins, Mednick, Brennan, Schulsinger, & Engberg, 1996; Jordan, Schlenger, Fairbank, & Caddell, 1996; Teplin, Abram, & McClelland, 1996; Virkkunen, Eggert, Rawlings, & Linnoila, 1996). Summarizing the principal findings from these studies, Marzuk (1996) observed that this relationship "appears strongest for the severe mental illnesses, particularly those involving psychosis, and it is increased by the use of alcohol and other psycho-active substances" (pp. 484-485). Results from a 26-year prospective study of a 1966 Finnish birth cohort (Tiihonen, Isohanni, Rasanen, Koiranen, & Moring, 1997) supported the same conclusions: Risk for criminal behavior was significantly higher among persons with psychotic disorders, especially persons suffering from alcohol-induced psychoses or schizophrenia and coexisting substance abuse.

Overall, the extensive research on comorbidity to date has confirmed both the identity of the disorders most commonly comorbid (e.g., substance-related disorders, personality disorders, depression, anxiety) and comorbidity's substantial adverse social, physical, psychological, and psychiatric consequences.

Diagnostic Bias

Diagnostic biases based on race and gender have recently been confirmed. Garb (1997) concluded that African American and Hispanic patients are less likely than Caucasians are to be diagnosed with psychotic mood disorder and more likely to be diagnosed with schizophrenia despite comparable symptoms. Becker and Lamb (1994) reported that females are more likely to be diagnosed with histrionic personality disorder than are males, whereas males are more likely than females are to be diagnosed with antisocial personality disorder despite equivalent symptoms. Depression is also diagnosed more often in women than it is in men, even when symptoms of mood disorder are comparable across the genders (Potts, Burnam, & Wells, 1991). Gender also influences the differential diagnosis of major depression and organic mental disorder (Wrobel, 1993). Both male and female clinicians show these gender-based diagnostic biases (Adler, Drake, & Teague, 1990).

An important question for future research is the source and nature of these apparent biases. Biases can be inherent to a diagnostic nomenclature (e.g., the presence of a particular diagnosis could reflect cultural biases within the organization that constructed the nomenclature), or they could be confined to the diagnostic criteria (i.e., the disorder does in fact exist but the criteria set is biased against a particular ethnic or gender group), clinicians' applications of the criteria set, or instruments of assessment. Biases in assessment are discussed in more detail within volume 10 (devoted entirely to the topic of assessment) of this series.

The Categorical-Dimensional Debate

Categorical versus dimensional classification first became a matter of concern when DSM-III more than doubled the number of diagnoses included in its predecessors. As diagnoses proliferated with DSM-III and DSM-IV, the frequency of comorbidity substantially increased, causing clinicians to ask whether the comorbidity that resulted represented the cooccurrence of two or more separate mental disorders or a single disorder that had simply been labeled in different ways. As a consequence, the advantages and disadvantages of dimensional and categorical approaches to personality and diagnosis has begun to be debated and explored extensively (e.g., Clark, Livesley, & Morey, 1997; Clark, Watson, & Reynolds, 1995; Klein & Riso, 1993; Widiger, 1997b). The focus of these efforts has been primarily on the personality disorders, in which symptom overlap is greatest, but the issues and proposals apply to other sections of the manual as well. For example, Watson and Clark (1995), Watson, Clark, et al. (1995), and Watson, Weber, et al. (1995) have explored dimensions that underlie depression and anxiety. Some of that research has already been considered previously, in our review of efforts to validate DSM-III and DSM-III-R diagnoses. The research has also been reviewed extensively by Mineka, Watson, and Clark (1998), as well as by Clark (2000).

According to Clark (1999), dimensional approaches to personality disorder (a) are theoretically consistent with the complexity of symptom patterns that is observed clinically, (b) increase reliability, (c) are theoretically consistent with the observed lack of discrete boundaries between different types of psychopathology and between normality and psy-chopathology, and (d) provide a basis for understanding symptom heterogeneity within diagnoses by retaining information about component trait levels.

Clark (1999) distinguished between two different dimensional approaches to the personality disorders. The first, rooted in the traditional categorical system, conceptualizes each separate disorder as a continuum, so that in principle any patient could exhibit different levels of traits of several personality disorders. The alternative is the trait dimensional approach, in which assessment aims to create a profile of the personality traits that underlie the disorder. Although several instru ments that reflect the higher order factors describing normal personality have proven useful for studying relations between personality and personality disorders (e.g., Widiger, 1993), only recently have instruments been developed specifically for the purpose of tapping into the lower order traits relevant to personality disorders. These include the 15-dimension Schedule for Nonadaptive and Adaptive Personality (SNAP; Clark, 1993) and the 18-dimension Dimensional Assessment of Personality Pathology-Basic Questionnaire (DAPP-BQ; Livesley, 1990).

In unpublished research by Clark and her colleagues relating diagnostic and trait dimensional approaches to personality disorder (Clark, 1999), two patient samples were interviewed with the Structured Interview for DSM-III-R Personality Disorders-Revised (SIDP-R; Pfohl, Blum, Zimmerman, & Stangl, 1989) and completed the SNAP. Multiple correlations between SNAP scales and diagnostic interview scores revealed a great deal of common variance: The information in a SNAP profile enabled prediction of between one quarter and three quarters of the variance in the interview-based diagnostic ratings, suggesting that the trait dimensions assessed by the SNAP underlie clinical ratings of personality pathology. These findings are especially impressive in view of data reviewed by Clark et al. (1997) to the effect that obtaining convergent validity for measures of PD assessment has been extremely difficult.

It is widely believed that categorical and dimensional models are inherently incompatible, and that one must choose between them. In actuality, however, it is more accurate to describe these models as existing in a hierarchical relation to one another, with dimensions being the blocks from which categories may be built. (Clark etal., 1997, p. 206)

O'Connor and Dyce (1998) recently reviewed the clinical data supporting the several models of PD configuration. They found moderate support for the DSM-IV dimensions and Cloninger's (1987) tridimensional theory, and they found stronger support for the five-factor model (Widiger, Trull, Clarkin, Sanderson, & Costa, 1994) and Cloninger and Svrakic's (1994) empirically derived seven-factor model. On balance, they concluded that four of the five factors within the five-factor model explain the bulk of the variance associated with PD. Unfortunately, these authors failed to include in their comparisons either the tripartite model or the trait dimensional approaches characterized by the SNAP and the DAPP-BQ. The integration of the SNAP and DAPP-BQ models with the five-factor model has been demonstrated in studies by Clark and Reynolds (2001) and Clark and Livesley (1994). However, although impressive progress has been made in recent years in amassing conceptual and historical support for dimensional versus categorical approaches to the personality disorders and other overlapping psychopatholog-ical entities, the ultimate utility of the trait dimensional approach will not be known until substantially more research data have been gathered that demonstrate empirically the advantages of this approach to these disorders.

New Definitions of Mental Disorder

As previously indicated, a continuing criticism of DSM-III and DSM-IV has been their definition of mental disorders, which critics have seen as so broad and all-encompassing as to include many nonpathological behaviors within its purview. As a result, alternative definitions have been proposed. Two of the most widely discussed of these are briefly reviewed here.

Wakefield's Harmful Dysfunction

Wakefield first defined mental disorder as harmful dysfunction in 1992 and in subsequent publications (e.g., 1997, 1999a, 1999b) defended and clarified the definition. To Wakefield, whether a condition is harmful requires a value judgment as to its desirability or undesirability, and dysfunction refers to a system's failure to function as shaped by processes of natural selection.

A condition is a mental disorder if and only if (a) the condition causes some harm or deprivation of benefit to the person as judged by the standards of the person's culture (the value criterion), and (b) the condition results from the inability of some mental mechanism to perform its natural function, wherein a natural function is an effect that is part of the evolutionary explanation of the existence and structure of the mental mechanism (the explanatory criterion). (Wakefield, 1992, p. 385)

Bergner (1997), however, has argued that Wakefield's harmful dysfunction conceptualization requires clinicians to make judgments about patients' mental mechanisms and that many such judgments cannot reliably be made. Lilienfeld and Marino (1995, 1999) also take issue with Wakefield's definition. They argue that many mental functions are not direct evolutionary adaptations but are instead neutral by-products of adaptations. They also note that the concept of the evolution-arily designed response neglects the fact that natural selection often produces extreme behavioral variability across individuals and that many disorders that have achieved consensus are best portrayed as evolutionarily adaptive responses to danger, threat, or loss.

Disagreeing, Spitzer (1997) calls Wakefield's construct a "brilliant breakthrough" (p. 259) because it emphasizes that what is not working in the organism is the function that we expect to be present and in operation by virtue of evolution and selection. Richters and Hinshaw (1997) also laud Wakefield's construct, even though they acknowledge that it requires a thorough knowledge of internal, neurobiological operations as well as value judgments about external, social data—both requirements that are difficult to satisfy.

Bergner and Ossorio's Significant Restriction

Claiming that consensus on a definition of psychopathology has not been achieved despite years of trying, Bergner (1997) concluded that this situation has seriously affected efforts to study psychopathology, to treat it, and to deal with its social consequences. He endorsed a definition of psychopathology previously proposed by Ossorio (1985): Psychopathology is best defined as "significant restriction in the ability of an individual to engage in deliberate action and, equivalently, to participate in available social practices" (Bergner, 1997, p. 246). This definition "meets the intellectual criteria that an adequate definition represent a non-empirical articulation of the necessary and sufficient conditions for correct application of a concept, and that it successfully discriminate instances of a concept from non-instances."

Comparing Bergner's definition to his own (Wakefield, 1992), Wakefield (1997) concluded that it is neither necessary nor sufficient to define a disorder. Its most serious problem is its overinclusiveness: Many restrictions on deliberate action are imposed in normal mental functioning. By contrast, Wakefield understandably affirmed that his own harmful dysfunction analysis, criticized by Bergner (1997), adequately discriminates between disorder and nondisorder. Spitzer (1997), whose own attempt to define mental disorder is represented by DSM-III (APA, 1980) and its successors, admitted to fatigue at efforts to define psychopathology and expressed uncertainty over the value of a consensus definition. He wrote that the Bergner-Ossorio definition simply "muddles the issues," whereas he lauded Wakefield's harmful dysfunction conceptualization as a "brilliant breakthrough" because it clarifies important underlying issues (p. 259). Although Widiger (1997a) was pleased that Bergner addressed the fundamental issues and principal difficulties in defining mental illness, he agreed with others that the Bergner-Ossorio definition of mental disorder ultimately will not be more successful than earlier efforts were. A major reason is the absence of distinct boundaries between either physical disorders or normality for the construct proposed— an attraction for a scientist like Widiger who has espoused dimensional approaches to some forms of psychopathology. Finally, Nathan (1997) took issue with Bergner's statement that a consensus on a definition of mental disorder does not exist, in view of the widespread acceptance of the value of DSM-IV and its predecessors by mental health professionals. Moreover, Nathan (1997) noted, however attractive Bergner's construct may be, in the final analysis, data on utility—absent to this time—will be the ultimate arbiter of the construct's worth.

In a subsequent expansive articulation of his position, Bergner (in press) restated and defended his conception of pathology as behavioral disability or functional impairment, concluded that it unifies theoretically divergent explanations of psychopathology, offered a consequent model of integra-tive psychotherapy, and weighed the considerable scientific and clinical implications of this integrative framework.

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