Four Key Issues in Considering Classifications of Mental Disorder

We turn now to discussion of the four key issues we described briefly in the beginning of this article. For each issue, we start with a general introduction and then discuss how each of the three systems—ICD-11, DSM-5, and RDoC—addresses the topic.

As awareness has grown that virtually all mental disorders result from many different factors, ICD and DSM have both acknowledged this complexity and moved to incorporate etiological factors in the relatively few instances in which they are known. In contrast, RDoC was developed in large part to support research into the etiologies of mental disorder. Its ambitious goal is to understand how functional deviations in various brain and behavioral response systems interact to result in mental disorder, while emphasizing that these processes are developmental rather than static and that they occur in the context of individuals’ interpersonal, social, and cultural environments.

Although these reports focused on schizophrenia, a recent groundbreaking study suggests a broader and more nuanced picture. The Bipolar and Schizophrenia Network on Intermediate Phenotypes is investigating patients diagnosed with schizophrenia, psychotic bipolar disorder, or schizoaffective disorder, an intermediate category between the other two disorders ( Clementz et al., 2016 ). Rather than comparing the three disorders against one another, the investigators set aside the patients’ different diagnoses and sought other measures that could sort them in novel ways. A clustering analysis returned two major factors of “cognitive control” (cognitive and self-regulatory functioning) and “sensorimotor reactivity” (brain activity in response to simple stimuli such as tones and lights). Various combinations of these two factors resulted in three groups, labeled “biotypes,” each of which included patients from all three diagnostic groups. The amount of gray-matter loss differed systematically across the three biotypes, but not as a function of their DSM diagnostic categories, and one biotype was associated with increased use of marijuana. Of course, no one study is conclusive, but these data illustrate the potential for biological and behavioral measures to identify intermediate phenotypes more directly related to interacting neurodevelopmental and environmental factors relevant for etiology.

Recent studies on psychosis provide an example of an RDoC-themed approach to identifying and understanding etiological factors in a particular disorder spectrum. Schizophrenia has for some time been recognized as a neurodevelopmental disorder, with the overt symptoms of psychosis being the end state of an extended process ( Rapoport, Giedd, & Gogtay, 2012 ). Various lines of investigation have explored possible avenues to understanding the aberrant development that leads to psychosis. One long-standing hypothesis is that synaptic pruning—the reduction in cortical synapses that occurs as a part of normal development across adolescence (e.g., Feinberg, 1982 )—is excessive in schizophrenia, such that there is an aberrant reduction in these synapses. Support for this hypothesis is accumulating: Cannon et al. (2015) found accelerated loss of gray matter in the years leading up to an overt episode of psychosis (i.e., the schizophrenia prodrome) was a critical factor in disease onset, and a genetics study has provided evidence for one mechanism by which this excessive pruning may occur ( Sekar et al., 2016 ).

RDoC is based in etiological thinking. Indeed, a major emphasis of the project is to learn more about the causes and mechanisms of mental illness, ultimately integrating knowledge relating to all four of its components (i.e., functional domains and units of analysis in the contexts of neurodevelopment and all that “the environment” encompasses).

In sum, as knowledge on the causes of mental disorders has advanced, DSM-5 has been able to progress from the atheoretical stance of DSM-III. To the extent allowed by current knowledge, it describes biological (e.g., genetics, neurodevelopment) and both general (e.g., culture) and specific (e.g., traumatic events) environmental factors that play a role in the onset and prognosis of mental illness. It also acknowledges the complexity of these causal factors, stating “the range of genetic/environmental interactions over the course of human development affecting cognitive, emotional and behavioral function is virtually limitless” ( APA, 2013 , p. 19) and cautions that “a diagnosis does not carry any necessary implications regarding the etiology or causes of the individual’s mental disorder” ( APA, 2013 , p. 25).

Beginning with DSM-IV , etiological considerations were introduced into descriptive text sections devoted to risk and prognostic factors and culture- and gender-related issues. The sections on risk and prognostic factors include explicit references to environmental causation when appropriate (e.g., the relevant section on PTSD references the severity of the trauma, personal injury, etc.) but typically indicate more general risk factors (e.g., season of birth and urban rearing in schizophrenia). The culture section includes discussion of the causal contribution of cultural systems. For example, the relevant PTSD section states the following:

By relying on a mix of etiological views that incorporate both biological and psychodynamic factors, pre– DSM-II U.S. classification explicitly acknowledged multiple causal factors in the development of mental disorders. A major emphasis in developing DSM-III was to be atheoretical with regard to etiology or pathophysiological process unless one or both of these were well established (e.g., in the “Organic Mental Disorders” section for adjustment disorder, which stated “the disturbance is a reaction to psychosocial stress”; APA, 1980 , pp. 6–7). This stance was taken partly to shed earlier references to psychodynamic causation, as well as to acknowledge that the cause of most mental disorders was unknown. Moreover, the possibility of multicausality was acknowledged: “Undoubtedly, with time, some of the disorders of unknown etiology will be found to have specific biological etiologies, others to have specific psychological causes, and still others to result mainly from a particular interplay of psychological, social and biological factors” ( APA, 1980 , p. 7). Identifying either specific biological causes or a particular interplay of psychological, sociocultural, and biological factors has proved to be difficult, although there are a few disorders in DSM-5 that have subtypes with specific, identifiable causes (e.g., narcolepsy, major and mild neurocognitive disorders).

In sum, WHO classifications offer a relatively comprehensive framework for identifying factors that may contribute to the etiology and expression of mental disorders and other health conditions. These categories could be used as a framework for additional epidemiological and clinical research and further refined on this basis, but national health-data systems and reimbursement policies generally do not facilitate systematic collection and reporting of this type of information (e.g., by reimbursing health professionals to record it as a part of standard health-encounter documentation), so the availability of these data for analysis on a global level is extremely limited. In general, WHO classification systems do not restrict the range of causes and contributory factors that may be considered in conceptualizing mental disorder. Rather, a specific subset of factors is prioritized in allocating resources for public-health data collection, research, and health-service reimbursement. These decisions are most frequently made at the level of national governments.

Further, as a broad classification of health conditions, ICD encompasses a variety of ways in which causal influences in mental disorders can be recorded. For example, one may note toxic environmental factors using categories from the chapter on “Injury, Poisoning and Certain Other Consequences of External Causes.” The chapter “Factors Influencing Health Status and Contact With Health Services” contains a wide range of categories for documenting contributory factors to an individual’s illness, including potential health hazards related to socioeconomic and psychosocial circumstances such as education and literacy, unemployment, problems related to the physical environment (e.g., occupational exposure, noise, pollution), the social environment (e.g., acculturation difficulty, social exclusion), housing, and negative events in childhood ( WHO, 2016b ). Further expansion of these categories has been proposed for ICD-11 (WHO, 2017). Moreover, ICD ’s sibling classification, the International Classification of Functioning, Disability and Health ( WHO, 2001 ), contains a comprehensive classification of environmental factors that may affect functioning and disability in the context of a given health condition, such as human-made environmental changes, supportive or nonsupportive relationships, and services, systems, and policies. The categories of the two classifications were designed to be used together to provide a more comprehensive picture of individuals’ health status and functioning ( Reed, Spaulding, & Bufka, 2009 ).

In some areas, the classification proposed for ICD-11 goes considerably further than that of ICD-10 in incorporating etiology. For example, the ICD-10 classification of sexual dysfunctions relies on an artificial dichotomy between “organic” sexual dysfunctions, classified mostly in the chapter “Diseases of the Genitourinary System,” and “nonorganic sexual dysfunctions,” classified under mental and behavioral disorders. This mind-body split is not consistent with either current research or best practices, which are based on a view of sexual response as a complex interaction of psychological, interpersonal, social, cultural, physiological, and gender-influenced processes, any or all of which may contribute to the development of sexual dysfunctions. For ICD-11 , an integrated classification of sexual dysfunctions has been proposed and a system of etiological qualifiers provided because of their relevance to treatment selection ( Reed et al., 2016 ).

Like ICD-10, ICD-11 will incorporate the classification of specific causal factors when they are clearly relevant to treatment strategies. For example, separate categories are provided for mental and behavioral syndromes that are symptomatically similar but caused by substances (illicit or prescribed) or an underlying medical condition (e.g., a brain tumor). Delirium is classified according to its etiology because the particular cause of a patient’s delirium is a critical factor in the immediately necessary treatment response; likewise with dementia and other neurocognitive disorders, in that etiology guides the prediction of a case’s course and outcome and the selection of management strategies.

Further, the “Mental and Behavioural Disorders” chapter in ICD-10 is one of those with multiple organization schemes. Whereas some groupings are based on causation (e.g., mental and behavioral disorders due to psychoactive substance use), most are based on similarity of symptoms and evidence of shared validators such as familiality (the tendency for mental disorders to run in families) and temperamental antecedents. Thus, mood disorders form one grouping, whereas schizophrenia, schizotypal, and delusional disorders constitute another. Still others are now seen as unhelpful conglomerations of entities based on outdated theoretical perspectives and will be reorganized in ICD-11 . For example, ICD-10 ’s grouping of neurotic, stress-related, and somatoform disorders is proposed in ICD-11 to be reorganized into several narrower groupings, none of which is referred to as “neurotic.”

Many diseases and health conditions in ICD are characterized, like mental and behavioral disorders, by multiple, interacting causes (e.g., acute myocardial infarction, type 2 diabetes mellitus). Where they are placed in ICD may reflect only one of those causal factors or another organizing principle that is considered to be clinically important. For example, diabetic retinopathy is classified with other forms of retinopathy under diseases of the visual system, even though it is known to be a consequence of diabetes mellitus, which is classified under endocrine, nutritional, and metabolic diseases. In fact, type 2 diabetes itself, even though insulin resistance is essential in its etiology, has treatment ramifications that resemble those of cardiovascular disease. Given our considerable knowledge of the mechanisms or pathophysiology of many of these disorders, however, their placement causes little to no difficulty in ICD. In contrast, even though we understand that mental processes and mental events all have substrates in the brain, the chapters on “Mental, Behavioural, and Neurodevelopmental Disorders” and “Diseases of the Nervous System” are separate in ICD, likely for two main reasons: First, brain substrates are only one aspect of the etiology and phenomenology of these disorders, which primarily involve impairments in the higher order functions of cognition, emotion, and behavior and are influenced by interpersonal, social, and cultural factors; and second, our understanding of their causal mechanisms is still rudimentary.

For the most part, the organization of ICD across all health conditions is not based on etiology. Rather, most of its chapters are organized according to organ systems (e.g., diseases of the circulatory system, diseases of the respiratory system) or their most characteristic symptoms (e.g., sleep-wake disorders, mental and behavioral disorders). Some chapters have multiple organization schemes. For example, the “Infectious Diseases” chapter contains groupings of disease categories based on the types of organisms that cause them (e.g., bacteria, viruses, fungi), their mode of transmission (e.g., predominantly sexually transmitted infections), the organ system they primarily affect (e.g., viral infections of the central nervous system), or their presenting symptoms (e.g., viral infections characterized by skin and mucous-membrane lesions).

Another proposed revision for ICD-11 is elimination of the problematic organic/psychogenic dichotomy for sexual dysfunctions, which partly involved moving these to a separate chapter on “Conditions Related to Sexual Health.” DSM-5 made similar changes within the limits of the fact that it remains a classification of mental disorders. Each classification implemented an approach that recognizes the potential role of many factors in contributing to the development and maintenance of sexual dysfunctions. Likewise, ICD-11 and DSM-5 share an emphasis on the importance of illicit substances, prescribed medications, and general medical conditions in the causation of mental disorders, and practitioners should consider these factors as causes before making a definitive diagnosis and commencing treatment, because substance intoxication and withdrawal, adverse reactions to medication, and general medical conditions can result in symptoms indistinguishable from those of “primary” mental disorders.

In addition, in recent versions of ICD and DSM-III to - IV , environmental causation in the form of exposure to one or more stressful life circumstances or traumatic events is a required part of several diagnoses (e.g., PTSD, acute stress disorder). Both ICD-11 and DSM-5 have taken this criterion a step further and include a new section devoted to disorders specifically related to stress (trauma- and stressor-related disorders), which also includes adjustment disorder, reactive attachment disorder, and disinhibited social engagement disorder and, in ICD-11 , complex PTSD and prolonged grief disorder. Exposure to a traumatic event or a stressor is a diagnostic requirement for these disorders and therefore a necessary element in their etiology. However, how the stress or trauma fits into a larger etiological framework that includes both pathophysiological processes and cultural factors requires further investigation, as do relations among the various disorders specifically associated with stress, and even certain disorders for which stress is not a diagnostic requirement but that have overlapping phenomenology (e.g., mood and anxiety disorders). In contrast to the relatively few diagnoses that include explicit psychological or environmental etiologies, ICD-11 and DSM-5 contain multiple examples of etiological thinking based on biological causation. The specific ways in which these are organized are somewhat different in the two manuals, so we discuss them separately below.

Both the third and fourth editions of the DSM and the eighth through tenth editions of the ICD included a few disorders for which evidence of a psychological causation was required. For example, conversion disorder was based on the psychoanalytic concept of defense mechanisms and involved the expression of unconscious psychological conflicts as somatic symptoms (e.g., paralysis). The DSM-IV required that “psychological factors are judged to be associated with the symptom or deficit because the initiation or exacerbation of the symptom or deficit is preceded by conflicts or other stressors” ( APA, 2000 , p. 492), whereas ICD-10 was even more direct, requiring for a definitive diagnosis of dissociative (conversion) disorders “evidence for psychological causation, in the form of clear association in time with stressful events and problems or disturbed relationships (even if denied by the individual)” ( WHO, 1992b , p. 123). This criterion for conversion disorder was rewritten in DSM-5 to eliminate one of the last remaining vestiges of “purely” psychologically defined etiology, now requiring simply that “clinical findings provide evidence of incompatibility between the symptom and recognized neurological or medical conditions” ( APA, 2013 , p. 318). Similar changes are proposed for ICD-11 .

Thus, the significance of the multicausality of mental disorders is not that it marks a point of difference between mental disorders and many other health conditions. Rather, it is important to raise issues of multicausality because of continuing concerns that biological causes and treatments for mental disorders receive disproportionate attention and resources, whereas psychological, social, and cultural factors are relatively unaddressed, despite compelling evidence for their importance. Therefore, the multicausality issue in relation to mental-disorder classification might be reframed as being about the ways in which these classifications offer systematic opportunities to note and record the influences of psycho-socio-cultural factors, thereby providing a basis for more research into them and for the development of additional assessment and intervention strategies. The three institutions discussed in this article tackle the problem of etiology in different ways; however, the efforts to harmonize DSM and ICD that have occurred since DSM-III have resulted in quite similar approaches to etiology, so we first discuss their shared aspects.

For example, intellectual disability can be caused by a wide variety of infections, chromosomal abnormalities, environmental insults, metabolic diseases, nutritional deficiencies, toxins, or traumatic injuries. In most cases, the specific cause is never clearly identified. Prevention or reversal of these cases would obviously require their specific identification, but the fact that we cannot currently offer cures for them does not suggest that etiological research on intellectual ability is unimportant. Of equal importance, the fact that the cause of a particular case of intellectual disability is unknown does not mean that the disability itself cannot be validly and reliably assessed or that there are not effective and cost-effective interventions that could enhance the individual’s functioning, autonomy, and quality of life. These assessment and intervention strategies are generally unrelated to the cause of the disability. Likewise, the fact that cognitive-behavioral therapy is an effective treatment for a variety of mental and behavioral disorders does not mean that the maladaptive thought patterns targeted by such therapy constitute the cause of these disorders; for example, contrary to the cognitive model of depression ( D. A. Clark, Beck, & Alford, 1999 ), which posits that depressive affect arises, at least in part, from maladaptive cognitions, there is evidence that cognitive change does not mediate symptom change in major depression ( Vittengl, Clark, Thase, & Jarrett, 2014 ).

The fact that we do not fully understand the causes of most mental disorders is sometimes used to question the entire diagnostic enterprise, even though mental disorders are not different from many medical conditions (e.g., hypertension, migraine, myocardial infarction) whose risk factors are well established but whose cause at the individual level can rarely be determined with certainty. Moreover, currently available treatment strategies for mental disorders are often connected only loosely to their proposed causes.

For many years, a diathesis–stress framework was the dominant model for understanding the interplay between individuals’ genetic/biological factors and other, primarily environmental, forces that resulted in psychopathology (e.g., Ingram & Luxton, 2005 ; Rende & Plomin, 1992 ). According to this model, the onset of psychopathology was related to the interaction between a person’s underlying vulnerability (diathesis) and the degree of disruption produced by a disturbing event or condition (stress). The vulnerability factor could be a specific gene or set of genes, a temperamental variable, or even an event or set of events (e.g., certain early-life experiences, such as the death of a parent, appear to increase vulnerability for later depression). However, recent developmental psychopathology research has shown that many genes previously thought to be risk factors are better conceptualized as “plasticity genes” ( Belsky, Jonassaint, Pluess, Brummett, & Williams, 2009 ). That is, certain variants of these genes are more reactive than other variants, not only to adverse environmental effects but also to supportive or even simply benign environments. As a result, depending on whether their experiences are adverse or are supportive or benign, individuals with such gene variants have either worse or actually better outcomes, respectively, than those who have less reactive variants of the same genes. This new framework is known as the differential-susceptibility hypothesis . Much more knowledge than we currently have is needed to determine the relative contributions of each type of factor (i.e., genetics, individual life history, social structure, and cultural systems) to the onset and form of mental illness and, more importantly, how these factors interact to result in psychopathology.

The multicausality of mental illness means that disorders do not have a single origin. As a general principle, for example, genetics has no more causal primacy than people’s experiences or the totality of their environments. Rather, those who suffer from mental illness are at the nexus of multiple forces—contributions not only from their biology and personal life history but also from factors that transcend them as individuals, such as social structures and cultural systems. The influence of these various forces on individuals’ health and well-being is constantly in flux, given that individuals, families, and societies not only “inherit” them, both biologically and socioculturally, but also reconfigure and recreate them in different ways throughout their lives.

Individuals’ social position often plays a substantial role by aiding or hindering their access to key goods and services, including health-promoting resources, or, conversely, channeling individuals toward illness-producing life circumstances ( Metzl & Hansen, 2014 ). MDD, for example, is much more common among people of lower socioeconomic status ( Lorant et al., 2003 ), and changes in socioeconomic status can prompt changes in depression ( Lorant et al., 2007 ). Moreover, changes in social position can have serious consequences related to mental disorder. For example, although overall mortality rates in affluent countries worldwide fell from 2000 to 2015, those of less-educated, middle-aged non-Hispanic Whites in the United States increased, primarily because of increases in drug- and alcohol-related problems and in suicides ( Case & Deaton, 2015 ). The authors hypothesized that the observed increases were related to economic insecurity among members of this demographic group in the context of rising income inequality in the United States. Finally, the meaning of events and expectations of mental illness within a culture also influence the risk and form of psychopathology ( Kleinman, 1977 ). Among Tibetan refugees, for example, having experienced torture was no more likely to be associated with mental illness than witnessing the intentional destruction of religious symbols ( Sachs, Rosenfeld, Lhewa, Rasmussen, & Keller, 2008 ).

In contrast, the RDoC system represents a fundamentally different approach to mental illness, with the goal of identifying the basic brain and behavioral processes that, together with sociocultural forces, give rise to multiple dimensions that may become dysfunctional and constitute psychopathology. It seeks deeper understanding of the scientific basis of psychopathology through the integration of biological and behavioral measurements, while also recognizing that these processes are developmental and embedded in interpersonal, social, and cultural contexts. NIMH’s overarching goal is to serve the public’s mental health care needs; with the RDoC system, it is pursuing this goal with a longer time horizon. Given the complexity of mental illness, the RDoC initiative sets forth a research program that may require many years before its findings materially affect current diagnostic and classification systems.

Both ICD and DSM, each in its own way, have made modifications to acknowledge the existence of dimensional features that are relevant to mental disorder and to incorporate dimensional features into their diagnostic and classification systems. For example, DSM-5 incorporated the concept of severity consistently across its classification system and added a set of cross-cutting symptom dimensions that can be used to provide a more complete clinical picture without using additional diagnoses that increase spurious comorbidity. Likewise, the developers of ICD-11 have taken steps toward abandoning artificial subtypes by, for example, proposing a severity dimension as the primary basis for classifying PD, with trait dimensions as specifiers. They have also proposed to implement a more dimensional system of symptom expression in schizophrenia and other primary psychotic disorders. And yet both systems remain fundamentally categorical for reasons that are germane to the various purposes for which they were developed and still are primarily used: compilation of health statistics, allocation of mental-health resources, clinical communication, and decision making in regulatory, legal, and health-insurance systems, all of which ultimately serve public mental health care needs. Providing a basis for decisions about what constitutes a case of mental disorder is a fundamental requirement of these systems, and such decisions are inevitably categorical.

An early example of the potential for this approach to mental disorders with respect to cognitive domains is provided by data from the Pennsylvania Neurodevelopmental Cohort study, in which children admitted to a hospital for a variety of reasons were given a large battery of neuroimaging and cognitive tasks and followed for several years. A retrospective analysis of the youths who later developed psychotic symptoms showed that their cognitive functioning fell behind that of typically developing children at about 9 years of age and stayed about 1 year behind normal cognitive development for the rest of the study ( Gur et al., 2014 ). Studies aimed at understanding the nature of such developmental delays, and more precise means of identifying individuals at particular risk for psychosis, could eventually lead to targeted prevention interventions.

Second, RDoC’s focus on dimensions of mental and behavioral functioning provides a more quantitative basis for prevention research. Scales that have been validated for various mental functions and behaviors can help identify individuals who are beginning to trend toward dysfunction that, if the trend were to continue, could eventuate in disorder. This approach is directly comparable to approaches in other areas of medicine, such as the measurement of blood sugar, in which progressive divergence from normal levels prompts increasingly aggressive treatment options—from lifestyle changes to medication or other, more intensive interventions. Thus, as with the rest of medicine, assessment of symptom levels across the full normal-to-abnormal spectrum is fundamental to the development of preventive interventions.

For instance, large community studies have shown that hallucinations and delusions are distributed continuously in the population ( van Os, Linscott, Myin-Germeys, Delespaul, & Krabbendam, 2009 ) and that the need for services is more correlated with the extent of these phenomena than with their simple presence versus absence ( Kaymaz & van Os, 2010 ). Research also suggests that genetic loads may be continuous across clinical and nonclinical populations. For instance, parents with a child diagnosed with ASD who had one or more unaffected siblings were asked to report on the psychosocial functioning of all their children, and they reported considerable overlap in the functioning levels of those with and without the disorder. Within the range of overlap, the relation between genetic loading and functioning level was virtually identical in both groups. These data suggest that the same genetic factors are operative in both typically developing and diagnosed children ( Robinson et al., 2016 ).

There are multiple reasons for taking this approach. First, given RDoC’s emphasis on etiology, its near-term goal is not to improve current diagnoses or develop an alternative clinical nosology but rather to direct the research community toward a more comprehensive understanding of how a variety of factors intersect over time and across different contexts to yield various types and degrees of psychopathology. It is becoming increasingly clear that most mental and behavioral functions in psychopathology are on continuous dimensions with functioning in the general population rather than qualitatively distinct. The pathological trait criteria of PD provide a clear elaboration of this continuity, as do some criteria of MDD (e.g., diminished interest or pleasure in activities) and GAD (e.g., excessive anxiety and worry; L. A. Clark, 2005 ) that are related to RDoC constructs. However, similar patterns hold for other mental functions that initially might seem to be more distinct from normality.

The RDoC research framework embodies a fully dimensional approach to mental disorder. This approach does not merely entail assessing severity dimensions of currently recognized disorders (e.g., a dimension of mild to moderate to severe MDD) but rather reflects the view that psychopathology should be studied with respect to the full range of operation of its various constructs, from healthy through severely pathological range.

Finally, as mentioned previously, the proposed ICD-11 system for PD is conceptually quite similar to the DSM-5 AMPD, but there are three notable differences between the ICD and DSM models. First, the DSM-5 model has greater specification for both functional impairment and for traits, but this has resulted in a model that WHO considered to be too complex for implementation except in research or in the most specialized settings in high-resource countries. Second, the models each have five broad trait domains but share only four: negative affectivity, detachment, disinhibition, and a domain that is called antagonism in DSM-5 and dissociality in ICD-11 . Then, each has a fifth trait domain that the other does not: anankastia (a focus on the control and regulation of one’s own and others’ behavior to ensure conformity to one’s high standards) in ICD-11 and psychoticism (eccentric perceptions, cognitions, beliefs, experiences, and behaviors) in DSM-5 . This domain is not included in the ICD-11 proposal because schizotypal disorder is classified as part of the schizophrenia spectrum in ICD, whereas in DSM-5 schizotypal PD’s primary placement is in the PD chapter, and trait psychoticism is needed to characterize this PD. Third, the proposed ICD-11 model includes a borderline qualifier, in contrast to the six combinations of traits forming specific PDs in DSM-5 ’s AMPD. This is not intended as a claim that the borderline pattern has unique ontological status. Rather, because this has been the most frequently diagnosed ICD-10 PD, its inclusion is specifically intended to facilitate the transition from a categorical to a dimensional PD model, and to give clinicians tools for documenting the variability in presentation of people who were previously considered to have BPD by also noting the particular trait domain(s) that characterize individuals’ PD presentation.

The cross-cutting and severity measures, which were mostly self-administered by patients, were intended for use at initial evaluation and at follow-up visits to help both clinicians and patients make treatment decisions and track treatment outcomes, consistent with the contemporary U.S. emphasis on measurement-based care and patient-reported outcome measurement. However, the APA Board of Trustees decided that there was not yet sufficient evidence that use of most of the diagnostic-severity and cross-cutting symptom measures would improve clinical care and patient outcomes to warrant approval for general clinical use. Therefore, the Board of Trustees approved only a few simple scales for assessing severity—such as BMI for anorexia nervosa, symptom counts for substance use disorders, and “setting counts” (indicating whether symptoms were seen in only one, only two, or three or more settings) for oppositional defiant disorder—for inclusion in the main Section II of DSM-5 . A representative sample of other measures is included in DSM-5 ’s Section III as “emerging measures” needing further testing, and all measures are available online ( https://www.psychiatry.org/psychiatrists/practice/dsm/educational-resources/assessment-measures ). Clinicians are encouraged to use the measures and report their experiences to the APA to provide evidence of the measures’ utility for clinical practice.

The cross-cutting symptom measures were tested in the DSM-5 field trials and had generally good to excellent test-retest reliabilities in all three age groups ( Narrow et al., 2013 ). Importantly, the diagnostic approach that was tested in the field trials—namely, using dimensional assessments in addition to categorical diagnoses—was well accepted by clinicians. Patients and their parents or guardians found the self-rated dimensional assessments useful for describing their symptoms and helping their clinicians understand their experiences.

For ease of administration, the Diagnostic Assessment Instruments Study Group recommended a two-stage process using patient-rated—or parent-rated, for children—measures. The first stage, Level 1, includes a few screening questions for each symptom domain. For several domains, for example, a score of at least 2 on one 5-point rating scale (i.e., corresponding to at least “mild” severity) indicates the need for a Level 2 assessment. The Level 2 assessments are more complete symptom assessments, which allows them to be used easily to assess outcomes, including change over multiple time points. Table 1 lists the Level 1 cross-cutting symptom domains and the corresponding Level 2 measures for adults, adolescents (ages 11–17), and children (aged 6–17, assessed through parent or guardian reports). Some Level 1 domains do not have associated Level 2 assessments. For these domains, scores above a predetermined threshold are “flags” for the clinician to conduct further individualized follow-up assessments. It is noteworthy that many of the Level 2 measures are relatively pure assessments of clinically important symptoms rather than of diagnostic syndromes, which often include heterogeneous groups of symptoms. As such, they may have use in bridging clinical practice and RDoC research on basic neuroscience and behavioral domains.

The DSM-5 work groups developed measures of many of these nondiagnostic, cross-cutting symptoms for such symptom domains as depression, somatic complaints, and substance use, and they serve two purposes. First, they provide documentation of the presence and severity of extra-diagnostic symptoms to guide clinicians’ treatment decisions and allow them to follow patients’ outcomes. Second, when administered before a clinical visit (e.g., via patient self-ratings), the measures draw attention to symptoms that might otherwise be missed because they are outside patients’ formal diagnoses. The DSM-5 work groups and study groups provided input as to which cross-cutting symptom domains to include.

Soon after the publication of DSM-III , it was recognized that rigid diagnostic categories often do not correspond to clinical reality. Rather, patient presentations often include any number of clinically significant symptoms that are not among the criteria of a particular diagnosis. Some of these symptoms (e.g., sleep problems and anxiety) are seen frequently across a wide range of mental disorders. Others (e.g., suicidal ideation, illicit drug use) are less frequent but are of high clinical significance when they do occur. Increasingly, these nondiagnostic, co-occurring symptoms (e.g., presence of anxiety symptoms in MDD or depressive symptoms in schizophrenia) are viewed as important predictors of patients’ treatment response and prognosis ( Conley, Ascher-Svanum, Zhu, Faries, & Kinon, 2007 ; Fava et al., 2008 ).

To reduce clinicians’ burden, measures were to have a patient-administered format, although it was recognized that clinician-completed measures might be necessary for some disorders and symptoms. To be included, severity measures were to be freely available (i.e., not proprietary), short enough for use in busy clinical settings (containing approximately 10 items at most), and, for clinician-rated measures, able to be used without formal training in their administration. When severity instruments that met these requirements were not already available for a particular diagnosis, a DSM-5 work group could develop one. Most of the severity measures recommended by the work groups were based on symptom frequency, intensity, or duration; some (e.g., the Patient Health Questionnaire–9; Kroenke, Spitzer, & Williams, 2001 ) were based directly on the diagnostic criteria for the disorder. Other instruments measured severity as manifested through specific aspects of a disorder, such as BMI for anorexia nervosa. As proposed for ICD-11 , severity of schizophrenia in DSM-5 is assessed through several of its associated symptoms, some of which (e.g., impaired cognition, depression, and mania) are not included in the disorder’s diagnostic criteria. Finally, because individuals can have clinically significant symptoms that do not meet full diagnostic criteria, the instructions for the diagnosis-specific severity measures indicate that they may be used to assess individuals with such symptoms regardless of whether their symptoms are above threshold for diagnosis.

Under the guidance of the Diagnostic Assessment Instruments Study Group, the DSM-5 work groups were tasked with identifying or developing measures to assess severity of a wider range of diagnoses. In conjunction with the Impairment and Disability Study Group, the members of the former Study Group proposed that these be based on the symptoms of the disorder and not on the extent of disability resulting from those symptoms. Disability—the effect of individuals’ symptoms on their ability for self-care and engagement in social and other life activities—was recognized as an important consequence of disorder severity, but a domain to be measured separately. Likewise, the level of distress caused by symptoms was felt to be a consequence of symptom severity and disability, and thus not a suitable indicator of diagnosis-specific severity. The study groups proposed that measurements of severity generally should either take the form of a symptom count, as did the guidelines for assessing severity in DSM-IV , or be based on a more fine-grained assessment of symptom frequency, intensity, and/or duration.

The DSM-IV contained a brief section defining diagnostic severity in its introduction and, like ICD-10 , provided specific guidance for conduct disorder, MDD, and “mental retardation.” The importance of being able to note the severity of individuals’ symptoms when making diagnoses is particularly well exemplified by the latter two disorders. Individuals whose MDD is of mild severity compared with those whose symptoms are moderate or severe have been shown to respond differentially to certain treatments ( Hollon & Ponniah, 2010 ). Further, treatment research has long used change in severity (e.g., as measured by the Hamilton Depression Rating Scale; Hamilton, 1960 ), in addition to the less informative categorical presence or absence of the disorder, to assess outcomes. The severity levels of “mental retardation” have been used widely to gauge the level of support needed for individuals with this diagnosis. Not surprisingly, the service needs of an individual with profound or severe intellectual deficits are considerably more intensive than those of an individual with mild deficits. Thus, diagnostic severity has implications for research, provision of treatment and rehabilitation services, service planning, and resource allocation.

The limitations of the categorical diagnostic system were clearly recognized by the developers of DSM-5 . Early in the planning process, it was decided that the categorical system, despite its flaws, had an intrinsic appeal to clinicians—especially psychiatrists, DSM’s key target audience—who are trained to determine patients’ diagnoses. Nonetheless, the loss of information inherent in categorical diagnoses was seen as a deficit worth ameliorating with supplemental dimensional approaches. Two key candidates for incorporating dimensions into a categorical diagnostic system were diagnostic severity and cross-cutting symptoms.

At the same time, these proposals for dimensional classification are in some ways more complex than the purely categorical approach they are intended to replace, and they may impose increased clinical and administrative burdens on their users, at least initially, when the new system is unfamiliar. After a period of adjustment, however, clinicians may find that, overall, the new system is actually simpler than the one it is replacing. For example, ICD-10 defines 10 specific personality disorders, whereas the ICD-11 proposal has only three levels of severity and six optional specifiers, requiring a maximum of nine. Nonetheless, these new proposals have little hope of being adopted and widely implemented in clinical practice unless they provide useful information at the level of clinical encounters that justifies the time and effort that learning the new systems will involve. Moreover, they will be of little use for health statistics or other policy applications based on aggregated patient-encounter data if clinicians cannot apply them consistently ( Reed et al., 2013 ), Whether clinicians can apply these dimensional assessments appropriately and consistently and whether they find that doing so yields clinically important information is currently being tested in field studies that will influence the final form of the ICD-11 diagnostic guidelines ( Keeley et al., 2016 ).

These proposals to incorporate more sophisticated dimensional elements in ICD-11 address a series of specific problems. For disorders of intellectual development, for example, the proposal addresses an inadequate measurement model that takes insufficient account of how people actually function in daily life ( Tassé, Luckasson, & Nygren, 2013 ). For paraphilic disorders, the previous formulation led to overpathologizing private behaviors that lacked clinical relevance or public-health importance ( Reed et al., 2016 ). The classification of specific PDs and subtypes of schizophrenia in ICD-10 (and similar elements in DSM-IV ) had produced reified categories commonly seen as unchanging, lifetime diagnoses that identified specific types of people. However, these ideal types were insufficiently informative for effective management of actual patients with personality pathology or schizophrenia, and the proposals for ICD-11 better represent current scientific evidence regarding the nature of these disorders ( Gaebel, 2012 ; Tyrer et al., 2015 ).

Likewise, the ICD-10 subtypes of schizophrenia (e.g., paranoid, hebephrenic, catatonic) have been proposed for elimination in ICD-11 because of their lack of validity. They are to be replaced by a set of symptom ratings that may be applied not only to individuals with schizophrenia but also those with other primary psychotic disorders ( Gaebel, 2012 ). The rated dimensions include positive symptoms (delusions, hallucinations, disorganized thinking and behavior; experiences of passivity and control); negative symptoms (constricted, blunted, or flat affect; alogia, or paucity of speech; avolition; anhedonia), depressive mood symptoms, manic mood symptoms, psychomotor symptoms, and cognitive symptoms.

PDs may then be described further through the use of six qualifiers. Five of these are trait-domain qualifiers, which are a set of dimensions that correspond to the underlying structure of the full range of adaptive-to-maladaptive personality traits: negative affectivity (the tendency to experience, and to have difficulty regulating, a wide range of distressing emotions and related cognitions), detachment (the tendency to maintain emotional and interpersonal distance), dissociality (the tendency to disregard social obligations and conventions and the rights and feelings of others), disinhibition (the tendency to act impulsively in response to immediate external or internal stimuli without consideration of longer term consequences), and anankastia (the tendency to maintain a narrow focus on controlling one’s own and others’ behavior and situations to ensure conformity to one’s own “correct” standards). As many of these trait domains may be noted as are judged to be prominent and contributing to the PD and its severity. A borderline qualifier (the last of the six) is also included and may be used if a certain characteristic pattern of maladaptive functioning is evident.

More fully dimensional characterizations of disorder entities proposed for ICD-11 have been made possible by specific structural innovations. (For a description of structural changes from ICD-10 to ICD-11 , see First et al., 2015 .) One example is the proposal to eliminate specific PDs in ICD-11 because of well-established problems with their validity and application in clinical systems and to replace these with a dimensional classification ( Tyrer et al., 2015 ). Specifically, the proposed ICD-11 model contains, first, a set of essential features (i.e., features that must be present to make a diagnosis) that are centered on self- and interpersonal dysfunction. If the essential features are met, then the clinician makes a determination regarding how severe the disturbance is and assigns a diagnosis of mild, moderate, or severe PD. (A subclinical level—personality difficulty—is also included.)

The proposed changes in the ICD-11 diagnostic guidelines for paraphilic disorders, called disorders of sexual preference in ICD-10 , provide another example of categories that incorporate multiple dimensional judgments. In ICD-10 , the diagnostic guidelines for these disorders often merely described the behaviors they involved. In ICD-11 , in keeping with the ICD’s central function as a global public-health tool that provides the framework for international public-health surveillance and reporting, a distinction has been made between conditions that are relevant to public health and indicate the need for health services and those that involve private behaviors without any appreciable public-health impact and for which treatment is neither indicated nor sought ( Krueger et al., 2017 ). The core proposed diagnostic requirements for a paraphilic disorder in ICD-11 are, first, a sustained, focused, and intense pattern of sexual arousal—as manifested by persistent sexual thoughts, fantasies, urges, or behaviors—that involves others whose age or status renders them unwilling or unable to consent, and, second, the individual’s having acted on these thoughts, fantasies, or urges or having been markedly distressed by them. However, a paraphilic disorder diagnosis may also be assigned when the pattern of sexual arousal is associated with marked distress or significant risk of injury or death, even if it does not focus on nonconsenting individuals. This formulation thus implicitly requires that clinicians who are applying the guidelines assess several components of the diagnosis, each of which can be conceptualized along a dimension—degrees of arousal, consent, action, distress, and harm—to determine whether a diagnosis of a paraphilic disorder is warranted.

Likewise, subcategories of “mental retardation” in ICD-10 were based on the severity of intellectual impairment: mild, moderate, severe, and profound. Determination of these levels was based primarily on standardized tests of intellectual functioning (i.e., IQ), using cutoffs that were relatively well accepted when ICD-10 was developed in the late 1980s. The equivalent diagnostic categories in ICD-11 , now called disorders of intellectual development, provides an example of a second strategy for incorporating dimensional information in a categorical system: being more explicit about the dimensions that clinicians must consider to arrive at a particular diagnostic determination. Specifically, to derive the appropriate severity-based subcategory for a disorder of intellectual development in ICD-11 , the clinician must make judgments on multiple dimensions, considering both intellectual functioning and adaptive behavior across the domains of conceptual, social, and practical skills.

One way to integrate dimensional constructs into a categorical system is to divide a given dimension into ordinal subcategories. For instance, ICD-10 included subcategories corresponding to the severity of a current depressive episode in depressive and bipolar disorders, which have been retained in ICD-11 with some refinements. Other ICD-11 diagnoses have subcategories created by imposing clinically important cutoffs on dimensional phenomena, such as dangerously low body weight in anorexia nervosa and extent of functional language impairment in ASD.

The ICD-11 classification of mental and behavioral disorders is required to follow the same set of structural and taxonomic rules as those used in the rest of ICD’s classification system. This requirement imposes different and much stricter restrictions on its classification model for mental and behavioral disorders than is the case for RDoC or, theoretically, even for the DSM, the taxonomic focus of which is limited to mental and behavioral disorders. At the same time, WHO’s IAG (2011) has pointed out that the inclusion of mental disorders in ICD facilitates coordination with classification of other disorders, including neurological and other medical conditions that are frequently comorbid with mental and behavioral disorders, and facilitates the search for related mechanisms of etiology, pathophysiology, and comorbidity of disease processes. The representation of mental and behavioral disorders alongside other health conditions in the ICD-11 also provides a solid basis for the parity of the mental health field with the rest of medicine for clinical, administrative, and financial functions in health care.

The dimensional nature of many, perhaps most, phenomena underlying mental disorders has long been clear to careful readers of the scientific literature and observers of clinical phenomenology. However, ICD remains structured as a categorical taxonomic system because this format is necessary for its application as the classification system for global health statistics and, to a large extent, for its use in clinical systems (e.g., in treatment selection and the determination of eligibility for health care services). For this reason, ICD follows particular rules and conventions that have deep historical roots and are well accepted as the basis for classification in other areas of medicine, even though one can point to many aspects of health conditions across diverse areas of medicine that are more accurately and precisely conceptualized as dimensions (e.g., blood pressure).

We said earlier that “current classification systems only partially reflect the empirical relational patterns of symptom dimensions.” However, they do reflect them to some extent, and they currently represent our best option for clinical use. The three institutions that are our article’s focus all recognize and acknowledge the multidimensional nature of mental disorder, but because they are responding to different constituencies with diverse needs and requirements, they take different approaches to address the dimensional aspects of mental disorder.

Neither the validity nor the clinical utility of such a wholly symptom-dimension-based approach to understanding psychopathology has been studied as extensively and systematically as those of the current diagnostic systems, but evidence is accruing that the approach has considerable value (e.g., Allardyce, McCreadie, Morrison, & van Os, 2007 ; Villalta-Gil et al., 2006 ). For example, a large-scale study in the United Kingdom ( Brittain et al., 2013 ) compared the predictive power of symptom dimensions versus diagnoses for 14 clinical outcomes (e.g., aggressive behavior, relationship problems, self-injury). There was no difference for eight outcomes, and symptom dimensions outpredicted diagnoses for five outcomes (aggressive behavior, nonsuicidal self-injury, problems due to hallucinations or delusions, depressed mood, and activities of daily living). For example, the number of negative symptoms (e.g., restricted or blunted affect, poverty of thought) and disorganization symptoms (e.g., incoherent speech, bizarre behavior) better predicted problems with activities of daily living (e.g., bathing and dressing oneself) than did having a diagnosis of schizophrenia. Diagnoses outpredicted symptoms only for duration of inpatient stay, which may have been due to diagnosis-based hospital administrative processes (i.e., certain diagnoses were allowed longer hospitalization periods than others).

Out of 20 symptom dimensions, Markon (2010) found that four broad higher order factors across a wide range of clinical syndromes and PDs captured the covariation among them: an internalizing dimension (e.g., subjective, distressing experiences, such as feelings of depression and somatic symptoms), an externalizing dimension (e.g., observable behaviors that often directly affect others, such as those related to substance use, attention seeking, and aggression), a dimension of thought disorder or cognitive disturbance (e.g., eccentric thought processes, paranoia, cognitive rigidity), and a pathological-introversion dimension composed of such symptom dimensions as social anxiety and unassertiveness/dependency. Moreover, each of the 20 symptom dimensions could be subdivided into component dimensions—smaller clusters within a symptom dimension that are even more highly related. For example, the dimensions of worry, apprehension, and irritability formed the anxiety dimension. Together, the lower order, mid-level, and higher order factors constitute a multilevel dimensional hierarchy (see also Boschloo et al., 2015 ; Fullana et al., 2010 ; Watson et al., 2012 ).

Fortunately, science is self-correcting in the long run, partly because it typically involves looking at things in more than one way, which is one reason that the hegemony of DSM eventually became problematic. Research into patterns of comorbidity among categorical diagnoses eventually began to suggest the utility of studying more directly the interrelations among symptom dimensions that make up commonly comorbid diagnoses. There has been an upsurge of such research since the turn of this century. Results indicate that current classification systems only partially reflect the empirical relational patterns of symptom dimensions ( Markon, 2010 ), raising the possibility of a more valid approach. Importantly, both diagnostic comorbidity and symptom dimensions have been shown to have a hierarchical structure, with certain more specific diagnoses or symptom dimensions being related systematically, such that they combine into broader diagnostic categories or symptom dimensions, respectively, similarly to how certain related biological species form broader genuses, related genuses form still broader families, and so on.

In some contexts, the overall degree of severity is the most significant dimension in that it indicates which individuals are in the greatest or most immediate need of treatment. However, knowing only the severity of an individual’s mental illness may not be particularly helpful in determining the best type of treatment. In most cases, more specific information is needed, so clinicians typically assess the severity of various symptom dimensions. Given the total number of symptom dimensions, there are too many possible combinations for the human mind to process them all simultaneously. A primary function of any classification system is to aid understanding of complexity by organizing important recurrent patterns into categories. In the case of mental disorder, diagnostic categories are intended to reflect meaningful, recurrent symptom patterns. To be sure, using diagnoses to describe the symptom profiles of individuals with mental illness facilitates assessment and conveys a considerable amount of information succinctly. However, because simplifying complex symptom profiles into diagnoses does not perfectly reflect reality, the reification of diagnostic categories eventually impedes a deeper understanding of them ( Hyman, 2010 ). With the widespread acceptance of DSM-III , professionals and lay people alike came to consider and treat mental disorder diagnoses as “true objects in nature” rather than convenient groupings of symptom dimensions.

Most manifestations of mental disorder can be described along a number of these symptom dimensions. For example, panic disorder involves dimensions of emotional symptoms (i.e., fear) cognitive symptoms (i.e., derealization/depersonalization, fear of losing control or dying); behavioral dimensions (i.e., behavioral change designed to avoid having panic attacks, such as avoidance of unfamiliar situations); and physical symptoms (e.g., palpitations; sweating, trembling, or shaking; chest pain or discomfort; gastrointestinal distress).

emotions, many of which have an optimal middle range, such as mood, for which healthy levels generally lie between depressed and elated extremes, and anxiety, which has what is known an “inverse U” relation to performance, such that performance is lower at both the low and the high extremes, whereas moderate levels of anxiety are associated with maximal performance (Yerkes-Dodson law; e.g., Keeley, Zayac, & Correia, 2008 );

Mental disorders are not all-or-none phenomena. First, the overall degree of severity of a person’s mental illness is one of its most critical aspects. In fact, recent evidence suggests the existence of a broad, general-psychopathology dimension (e.g., Caspi et al., 2014 ; Kotov et al., 2017 ; Laceulle, Vollebergh, & Ormel, 2015 ) that encompasses a wide range of—or even all—variations of psychopathology, perhaps in a very fundamental way, much as general intelligence is a broad dimension that has multiple interrelated components. (We discuss this further in the section “Comorbidity.”) Second, many symptoms of mental disorder overlap with psychological states that are common in the general population (e.g., depressed mood) and range in severity, from relatively rare and circumstantial symptoms in generally healthy individuals to mild, transient disturbance to moderate symptoms that are components or reflections of mental disorder to severe and prolonged distress.

Once again, we see that the public-health and clinical-use purposes shared by ICD and DSM have led them to adopt similar—although not identical—approaches to defining and using thresholds in mental-disorder diagnosis, whereas RDoC can take a more flexible approach. That is, RDoC-based research can either set one or more thresholds if doing so is important for a particular research purpose, or not address threshold issues if they are not relevant for research purposes or applications in clinical settings or health policy. A long-term RDoC goal is to provide information that will facilitate setting thresholds in diagnostic systems of mental disorder. This goal includes providing for the possibility that various thresholds will be needed for different purposes. To analogize from general medicine, research has informed the setting of various thresholds for the treatment of blood pressure and obesity (i.e., at lower levels, diet and exercise may be sufficient, whereas higher levels may require more “aggressive” treatment). However, at present, reasoned clinical judgment is still required in most circumstances.

This aim of RDoC has two implications for research. First, the emphasis upon particular functional constructs (e.g., fear, cognitive functioning) that can be measured by various means across different RDoC units (e.g., physiological responses, observed behaviors, subjective reports) is intended to lay the groundwork for a strongly quantitative, psychometrically sound approach to assessment. Second, taking a dimensional approach will often necessitate research designs that involve an examination of psychopathology based upon continuous rather than categorical variables. For instance, rather than a design that involves patients with two subcategories of major depression (e.g., mild and severe) and controls, an RDoC design might include research participants with a range of mood or anxiety symptoms (including those with minimal or no symptoms) to explore how changes in neural reward-system activity relate to reward-related behavior in a laboratory setting, or to the correlation between reward-related behavior and cognitive performance or clinical mood states. Thus, at the current time, RDoC does not have a strong position about thresholds as they apply to contemporary clinical practice. However, a major aim of RDoC is to promote a research literature that will help clinicians in the future provide more sensitive assessments that, in turn, will lead to improved ways to determine thresholds (or ranges) for empirically based diagnosis, prevention, and treatment interventions.

The RDoC approach to thresholds follows directly from a consideration of their consequences. As noted, the necessity of organizing research designs around DSM or ICD categories has constrained the kinds of research that can be conducted, particularly with respect to individuals with symptoms that fall below current thresholds for diagnosis. There certainly is clinical utility in setting at least some thresholds, but the problem for researchers is to determine the particular kinds of criteria to use for setting thresholds (e.g., various symptoms or types of functioning) and where to set thresholds to facilitate empirically based decisions (e.g., whether to treat, hospitalize, or prescribe medication). These types of research questions constitute a critical part of the RDoC framework. An important aim is to support research that will provide systematic information about the range of mental functioning and distress from typical levels through various levels of impairment, and about asymptomatic risk states indexed by biomarkers that may precede symptoms, such as cortical thinning (e.g., Cannon et al., 2015 ). Relevant findings could be used to inform future revisions of the DSM and ICD on how and where to set treatment thresholds, including whether to establish more than one treatment range (e.g., mild, moderate, severe) and offer different treatment recommendations for each range.

With both the research and clinical communities using the DSM system and its specific thresholds, the lay public has also widely embraced it, in large part because it provides reassuring clarity that the problems with which they or their loved ones suffer are “real” disorders and not “all in their heads,” and because researched treatments are available for many disorders. The media have also increasingly featured articles and news pieces about specific DSM diagnoses, further cementing the erroneous view that they are discrete, natural entities with nonarbitrary boundaries that are the same all over the world. Such articles ignore the fact that the vast majority of DSM-5 –based research has been conducted in English-speaking countries or Western Europe. In contrast, we believe that advancing the more nuanced view that DSM-5 ’s diagnostic thresholds are semiarbitrary will foster public understanding of mental disorders and help reduce the stigma that follows upon the false belief that there is a clear line between those with mental disorders and “the rest of us.” This perspective is conceptually consistent with that taken by ICD-11 .

In addition, the DSM is used in psychopathology training of mental-health professionals across many disciplines, most research establishing empirically based treatments is conducted using specific DSM diagnoses, and the manual is used widely in administrative and billing systems in the United States and some other countries (e.g., The Netherlands). As a result, DSM thresholds have had a strong influence on clinicians’ approach to patients and their treatment, although many clinicians find the highly specified thresholds of DSM diagnoses clinically limiting and have developed ways to use the manual flexibly—for example, they may use diagnoses for administrative purposes but implement treatment on the basis of individual patients’ symptom profiles. Nonetheless, there still is widespread acceptance among clinicians of DSM disorders as nonarbitrary (i.e., real or valid), at least quasi-discrete natural entities.

The evolution of more reliable diagnostic systems via specific-criterion approaches was the basis for an explosion of knowledge about psychopathology in the years following DSM-III ’s publication. Nonetheless, the near-ubiquitous use of the DSM’s specific criteria and thresholds limited the types of research questions asked. Of course, researchers could measure and analyze additional relevant dimensions, but as long as there was an expectation that DSM diagnoses would be used in NIMH-funded research, alternative approaches were constrained. Thus, the use not only of the DSM’s specific criteria but also its semiarbitrary thresholds had a huge influence on psychopathology research—for both good and ill.

However, because of the complexity of implementation, the reification of existing thresholds by U.S. regulatory and research-funding agencies, and concerns about a breakdown in the “common language” that the DSM has promoted for decades, implementing different thresholds did not gain much traction and was not seriously considered in DSM-5 ’s development. This marks a contrast between that manual and the ICD , in which thresholds differ among the CDDG , the primary-care version, and the Diagnostic Criteria for Research. As with previous editions of the DSM, the DSM-5 work groups instead implemented a single cut point for each diagnosis based on their evaluation of the available evidence.

It is widely acknowledged that specific DSM-5 thresholds are often somewhat arbitrary and should be viewed with some flexibility in their application. For example, an individual whose symptoms do not fully meet the criteria for a diagnosis at a given time may nonetheless have a need for treatment, either to prevent the development of a more severe condition or to address urgent symptoms such as suicidal thinking. Of importance to research on the causes of mental disorders, individuals who are “subthreshold” for a disorder may possess genetic and neurophysiological characteristics similar to those of individuals (e.g., family members) whose presentations are at or above a diagnostic threshold. The DSM has long attempted to accommodate subthreshold presentations through the use of NOS diagnoses in DSM-III, DSM-III-R , and DSM-IV , which became “other specified” and “unspecified” diagnoses in DSM-5 . It also is widely acknowledged, albeit tacitly, that higher or lower diagnostic thresholds are needed for specific purposes (e.g., administrative, forensic, treatment related). Moreover, the scientific argument that different diagnostic thresholds may need to be specified based on the setting in which they are used has some empirical grounding ( Finn, 1982 ).

Partly in response to this problem, ICD-11 has proposed a narrower operationalization of PTSD characterized by the required presence of three core symptoms ( Maercker et al., 2013 ), which is being tested in a variety of studies (e.g., Danzi & La Greca, 2016 ; Hansen, Hyland, Armour, Shevlin, & Elklit, 2015 ). Whether or not the ICD-11 proposal will ultimately help to reduce diagnostic heterogeneity for PTSD, within-category heterogeneity that results from disorders’ multidimensionality remains a significant challenge for classification systems to address.

This level of diagnostic flexibility has both benefits and costs: It accommodates a wide variety of symptom presentations, but it does so at the associated cost of increasing within-diagnosis heterogeneity and both frequency and variation in overlap with other diagnoses. Again using PTSD as an example, Galatzer-Levy and Bryant (2013) calculated that there were 636,120 ways to meet DSM-5 criteria for PTSD. Moreover, Gallagher and Brown (2015) found that depending on which particular PTSD criteria individuals met, they were more likely also to meet criteria for a depressive disorder, an anxiety disorder, or both. In sum, as a result of the multidimensionality of PTSD and the use of specific thresholds for each of its dimensions, patients diagnosed with PTSD can present with a great variety of symptoms and patterns of comorbidity.

The multidimensionality of mental disorder is reflected in some DSM-5 diagnoses’ use of subcriteria. For example, the multidimensionality of ADHD is apparent in its very name, which conveys its two primary criteria, each of which has nine subcriteria, with a diagnostic threshold of six items (five for those over the age of 17). Thus, a child or adolescent might receive an ADHD diagnosis by having six or more inattentive symptoms (but fewer than six hyperactive/impulsive symptoms), six or more hyperactive/impulsive symptoms (but fewer than six inattentive symptoms), or six or more symptoms of each type. It also is important to note that the nine criteria on each list are not fully independent from one another but are intended to represent possible behavioral manifestations of an underlying dimension. The two types of symptoms commonly co-occur and may shift over time within individuals; thus, including them in a single diagnostic criterion set is intended to avoid comorbid diagnoses and unhelpful diagnostic changes over time for patients.

Ultimately, DSM-5 implemented a compromise. The definition of mental disorder used in DSM-III through DSM-IV-TR was modified to emphasize that disorders reflect dysfunctional mental processes and are “ usually associated with significant distress or disability in social, occupational, or other important activities” ( APA, 2013 , p. 20; emphasis added), substituting “are usually associated with” for DSM-IV ’s “causes.” Effectively, this brings the DSM-5 ’s definition more in line with the definition used in ICD-10 and the forthcoming ICD-11 . Moreover, individual work groups were allowed to decide whether to retain or remove the clinical significance criterion. Whereas some did opt to eliminate it, most disorders in DSM-5 retain the criterion. Therefore, ICD-11 and DSM-5 have approached this same issue from opposite directions, with ICD-11 including an impairment requirement only when it is deemed necessary and DSM-5 eliminating it when possible. As a result, the two systems have become more similar, although not identical, in the way that they describe distress and disability as required features for particular diagnoses.

Use of the clinical significance criterion means that a patient’s report of significant distress or significantly affected daily functioning becomes the de facto threshold of many, if not most, mental disorders. Nonetheless, there was considerable resistance from the DSM-5 work groups to remove the criterion. A frequent concern was that without it, there would be inadequate thresholds between mild forms of the disorder and nondisorder, leading to overdiagnosis of mental disorders and, consequently, increased public perception that the DSM-5 pathologizes the emotional ups and downs of everyday life (e.g., Horwitz & Wakefield, 2007 ). In part, the clinical significance criterion was deemed important because of the DSM developers’ commitment to a criterion-based diagnostic approach that has been regarded as useful in increasing reliability. Reliability, in turn, was considered important for not only clinical but also practical reasons, such as those relating to forensic settings, reimbursement, and research. The ICD CDDG’s use of more prototypic conceptualizations to define disorders may allow clinicians greater flexibility in determining whether the disorder is present without an explicit clinical significance criterion. Thus, the uses of the classification systems also have a role in shaping them.

We currently have little understanding of how disability and distress arise in individuals with mental disorder, including the roles of individual symptoms, environmental factors, and other intrinsic factors not directly related to specific symptom criteria. Further, the measurement or specification of the core symptoms of many disorders is lacking. For example, many of the criteria for ADHD reflect consequences of inattention, hyperactivity, and impulsivity (e.g., “makes careless mistakes”) rather than direct measures of these domains because either there currently is no reliable and valid means of directly assessing the core symptoms or the assessments are not feasible in routine clinical practice. As another example, the frequency, intensity, and duration of schizophrenia symptoms (delusions, hallucinations, disorganized speech, grossly disorganized or catatonic behavior, negative symptoms) have not been precisely specified; DSM-5 also requires a marked decline in “level of functioning in one or more major areas, such as work, interpersonal relations, or self-care” ( APA, 2013 , p. 99) for diagnosis.

In part because of the goal of “harmonizing” the ICD and DSM systems, it was widely debated whether the clinical significance criterion should be eliminated in DSM-5 . Wakefield (2007) continued to advocate for the importance of retaining it, whereas Üstün and Kennedy (2009) argued that DSM-5 should adopt WHO’s position that disorders (defined by symptoms and signs) and their resulting disability should be assessed separately, although they should be considered together to determine caseness ( WHO, 2001 ). Hyman (2010) expressed the concern that if the definition of mental disorder includes the criterion that the symptoms must cause clinically significant impairment or distress, “it denies an appropriate clinical status to early or milder symptom presentations . . . [and] illogically confounds a severity measure with a symptom list” (pp. 164–165).

First, what constituted clinically significant distress or impairment was not defined, and DSM-IV simply acknowledged that “assessing whether the criterion is met . . . is an inherently difficult clinical judgment” ( APA, 2000 , p. 8). A second problem was that the scale in DSM-IV for assessing functioning, the Global Assessment of Functioning, intermingled symptom severity, social functioning, and assessments of dangerousness rather than considering these elements separately. Moreover, many “symptoms” in the diagnostic criteria themselves refer to psychosocial impairment (e.g., “often has difficulty organizing tasks and activities” is listed as a symptom of ADHD rather than a consequence of the disorder).

Diagnostic criteria for many disorders in DSM-III and DSM-III-R made implicit or explicit reference to clinical significance, but the assumption was that “careful specification of symptom criteria for each disorder would suffice in establishing a disorder threshold” ( Narrow, Kuhl, & Regier, 2009 , p. 88). However, unexpectedly high rates of mental disorders were found in general population surveys. Therefore, to reduce the concern that the manual overdiagnosed mental disorders, DSM-IV introduced the clinical significance criterion, which was typically worded “. . . causes clinically significant distress or impairment in social, occupational, or other important areas of functioning.” The stated rationale for its inclusion was that it “helps establish the threshold for the diagnosis of a disorder in those situations in which the symptomatic presentation by itself (particularly in its milder forms) is not inherently pathological and may be encountered in individuals for whom a diagnosis of ‘mental disorder’ would be inappropriate” ( APA, 2000 , p. 8). The criterion’s addition did reduce the rates of mental disorder in community surveys ( Narrow, Rae, Robins, & Regier, 2002 ), but a number of problems remained ( Narrow et al., 2009 ).

Beginning with DSM-III , the threshold between normality and disorder in the DSM has been based largely on specific symptoms, requiring either all or a subset of symptoms for diagnosis. In DSM-III and DSM-III-R , the specific number of symptoms required (e.g., five of nine for MDD; four of 13 for panic disorder) was typically based on clinical heuristics given the absence of clear research evidence. More recently, some diagnostic criteria have been established using a more substantial research base (e.g., for alcohol and other substance use disorders; Hasin et al., 2013 ; Kerridge, Saha, Gmel, & Rehm, 2013 ). However, WHO has questioned the clinical and public-health utility of this approach ( Poznyak et al., 2011 ), and some research findings challenge whether the current thresholds (e.g., two of 11 symptoms for alcohol use disorder) are optimal or too low (e.g., Mewton, Slade, McBride, Grove, & Teesson, 2011 ).

The ICD-11 approach reflects the perspective that if a disorder can be described adequately on the basis of its symptoms alone, without reference to distress or impairment, then it is more parsimonious to consider distress or impairment as consequences of the disorder, and, accordingly, these elements need not—indeed, should not—be included in its diagnostic criteria. The question in ICD is not whether disorders are associated with distress or impairment, because typically they are, but rather whether including distress or impairment as a diagnostic criterion alters what constitutes a “case” of a particular disorder. The development of assessments that conceptualize distress and functioning as outcomes rather than as inherent features of disorder is an area for future work ( Robles et al., 2016 ).

Üstün and Kennedy (2009) took an extreme position on this, insisting on a complete separation of functioning and disability from diagnostic thresholds or ratings of disorder severity. However, IAG (2011) noted that this ideal would be impossible to implement in ICD-11 given the current state of science and clinical practice, because of the lack of direct, objective disease indicators for a wide range of mental disorders, as well as their continuity with normal variations in behavior. The advisory group recommended that the ICD-11 avoid incorporating functional impairment as a part of diagnostic guidelines whenever possible and, for categories in which inclusion of functional impairment in the diagnostic guidelines is needed, set a clearly identifiable threshold between disorder and nondisorder.

The issues related to functional impairment are even more complex. Recall that the definition of mental disorder in the ICD-10 CDDG , proposed for retention in the ICD-11 , refers to functional impairment but does not require it. In fact, the ICD-10 CDDG states as a general principle that interference with the performance of social roles (e.g., at home or at work) should not be used as a diagnostic guideline. The major problem with functional impairment as a diagnostic requirement is that it is more properly conceptualized as an outcome of a mental disorder or other health condition, and conflating the disorder itself with its consequences creates a variety of problems—for example, in evaluating the effectiveness of treatments.

In practice, the distress component of the clinical significance criterion is typically easy to satisfy, given that various forms of psychological distress (e.g., anxiety) are themselves symptoms of many mental disorders and that individuals’ requests for mental-health services are generally taken as direct evidence of their distress or their concern about the effects of their current mental health on their functioning. There even are a number of categories in which distress is a central component of the clinical phenomenology of the disorder. For example, most of the criteria for PTSD involve distress or avoidance of things that cause distress. Therefore, the distress criterion has practical implications that are especially relevant in epidemiological research among individuals who are not seeking mental health services, in that it provides a way to avoid assigning a diagnosis to individuals in the community whose symptoms are similar to those of individuals presenting in clinical settings but who deny being distressed by those symptoms. Whether one considers such cases to be false negatives (i.e., they actually should be assigned the diagnosis) or true negatives (i.e., not assigning the diagnosis is the correct decision) depends upon the purpose and goals of the assessment.

Compared with ICD-10, DSM-IV invoked the clinical significance criterion as a basis for distinguishing disorder from nondisorder in mental-disorder classification much more often. In fact, the criterion represented the most important source of differences between the two manuals ( First, 2009 ). Per the clinical significance criterion, a person whose symptoms cause either distress or difficulty with functioning socially or in some other important way could be judged to have a mental disorder, whereas another person with the same symptoms who is not bothered or functionally impaired by them would not receive a diagnosis. The more frequent use of an explicit distress- and impairment-based criterion in DSM compared with ICD may be due to the DSM’s emphasis on specified criteria to set thresholds for disorder; ICD’s more general descriptions of disorder may allow clinicians greater flexibility in determining whether a disorder is actually present.

Likewise, the CDDG is not intended for application to the general population in nonclinical settings by lay interviewers as a part of epidemiological studies. This is not only because its administration requires the exercise of clinical judgment, but also because it implicitly uses clinical populations in mental-health settings as its standard of comparison. In contrast, epidemiological studies may require the use of stricter or otherwise modified thresholds based on the psychometrics of particular cutoffs in relation to the community-level prevalence of particular symptoms ( Finn, 1982 ; Kendler, Gallagher, Abelson, & Kessler, 1996 ; Vilagut, Forero, Barbaglia, & Aloso, 2016 ).

The approach to thresholds taken in ICD-11’s CDDG and its primary-care version is consistent with WHO’s goal of improving the identification of people with mental-health needs who currently, at a global level, are unlikely to receive appropriate care and even less likely to see a psychiatrist in their lives. This approach also is consistent with WHO’s goal of reducing global disease burden, but it will not work for all purposes. For example, the CDDG could not be used as a basis for defining patient groups for research purposes that are homogeneous with respect to highly specified operational criteria, specifically because of its flexibility and the need for clinical judgment in its application. For studies in which this is important, a more fully operationalized adaptation of the guidelines would be needed, as had been provided for ICD-10 by the Diagnostic Criteria for Research ( WHO, 1993 ). To create an analogous version for the ICD-11 , decisions would need to be made for every disorder about what specific number of which symptoms must be present over a specified period of time to warrant an individual’s inclusion in an experimental group for the purpose of a particular study. These requirements will likely vary across studies (a point that echoes the RDoC rationale). However, to the extent that they may have some uniformity, a structured interview for identifying more specifically defined research groups for a range of diagnoses is currently being developed.

This also is the goal of the proposed primary-care version of the ICD-11 , which consists of 27 mental disorders judged to be clinically important in primary-care settings—both those that are commonly seen in such settings and less common but more severe mental disorders that are important to recognize in these settings. In this version of the classification, disorders are described in a way that reflects primary-care presentations to facilitate their identification by primary-care professionals. For most disorders, they identify a subset of the cases that would be identified by the application of the complete CDDG ( Goldberg et al., 2016 ).

Thus, rather than attempting to establish discrete, specific cutoffs through the use of criteria, the CDDG describes the essential features of each disorder, providing explicit guidance about the symptoms and/or characteristics that clinicians can reasonably expect to find in all cases of the disorder ( First et al., 2015 ). This diagnostic approach is intended to enable more flexible application of clinical judgment and allow for cultural variation in symptom presentation. Although the ICD-11 ’s lists of essential features superficially resemble diagnostic criteria in their overall format, they generally do not contain the precise symptom counts, duration thresholds, or polythetic sets of items (stipulating that, e.g., a patient must have three of a list of four symptoms) that characterize the diagnostic criterion sets in DSM-5 ( First et al., 2015 ). Whereas DSM diagnoses generally attempt to set a precise threshold for every disorder to be applied across all settings, the ICD-11 CDDG uses more flexible language in an effort to conform to the way clinicians typically make psychiatric diagnosis—that is, by exercising clinical judgment regarding the context and consequences of the specific decision that is being made. The aim of the ICD-11 CDDG is to help clinicians identify the diagnostic formulation that is most likely to be useful in making treatment and management decisions.

The ICD-11 ’s CDDG for mental and behavioral disorders is intended primarily for use by mental-health professionals in a wide range of settings around the world. WHO’s IAG (2011) explicitly noted that diagnostic classification is only a part of patient assessment, stating that “the focus of the ICD is on the classification of disorders and not the assessment and treatment of people , who are frequently characterized by multiple disorders and diverse needs” (p. 91). Various additional factors must be considered in making decisions about patient care, such as associated disability, severity, risk of harm to self or others, exacerbating psychosocial factors, level of social support, and cultural factors, as well as the relative effectiveness of locally available treatments. Information about risk factors and protective factors may also be important in formulating population-based strategies.

This ICD-10 and ICD-11 definition is conceptually similar to the one subsequently adopted for DSM-5 ( APA, 2013 ), but the DSM-5 definition is more elaborately worded and mentions exemptions for culturally approved responses to a common stressor or loss (e.g., bereavement) and social deviation. In the ICD-11 , these exemptions are made clear in the context of diagnostic guidelines for specific relevant disorders (e.g., bereavement reactions should not be mistaken for depression; sexual behaviors should not be diagnosed as paraphilic disorders solely because they are socially stigmatized) but are not mentioned in the overall definition of mental disorder. The difficulty inherent in distinguishing mental disorder from normal variation on the basis of symptoms and behaviors alone was described in the previous section, “Categories and Dimensions.” With most conditions, there simply is no clear line that separates the two, so any threshold is to some extent arbitrary. Moreover, different thresholds may be appropriate for different purposes or in different settings. For example, in primary-care settings in developing countries, the diagnosis of depression may focus on identification of those cases with the most severe symptoms and greatest functional impairment ( WHO, 2016c ).

The CDDG for ICD-10 mental and behavioral disorders define a mental disorder as “a clinically recognizable set of symptoms or behaviors associated in most cases with distress and with interference with personal functions” ( WHO, 1992b , p. 11). “Clinically recognizable” is a critical phase in this definition because it is not difficult to think of sets of symptoms or behaviors that typically are associated with distress or interfere with personal functioning but are not considered mental disorders (e.g., bereavement following the death of a loved one or anxiety following a job loss). After considerable discussion, the IAG (2011) recommended retaining this definition for ICD-11 , favoring its simplicity over the more complex definition used in DSM-IV (discussed in the Threshold Issues in DSM-5 section), which was generally intended to encompass the same range of conditions.

Health professionals use either explicit or implicit thresholds to decide whether to assign a particular diagnosis or apply a particular treatment. Some treatments are relatively benign and unlikely to have negative side effects, such as cognitive-behavior therapy for anxiety disorders. Other treatments may have potentially dangerous side effects, necessitating the patient’s full understanding of their risks and benefits, such as second-generation antipsychotic medications administered to adults with schizophrenia. The consequences of false positives (diagnoses assigned when a mental disorder is not actually present, which increase when lower thresholds are used) and false negatives (diagnoses not assigned when a disorder actually is present, which increase when higher thresholds are used) vary widely according to the specific circumstances surrounding particular diagnostic decisions.

Third, diagnostic thresholds may also be highly consequential in forensic settings—for example, insofar as they may be used to establish “diminished capacity,” which affects culpability for crimes; ineligibility for the death penalty as a result of intellectual disability; or need for civil commitment because of the high risk of reoffending associated with certain forms of mental disorder. Diagnostic thresholds also are a foundational aspect of epidemiology, with estimates of the incidence and prevalence of specific mental disorders used as a basis for policy and planning. Thus, from a public-health perspective, major changes in thresholds (e.g., via revision of a diagnostic manual) have considerable implications for policy and resource allocation because they affect prevalence estimates. For example, diagnostic thresholds are used to determine eligibility and reimbursement for health services, as well as for social and educational programs. If changing the diagnostic threshold for a particular disorder were to raise prevalence estimates from 5% to 25%, either considerably more resources would need to be directed toward its treatment or the level of severity required for service eligibility would need to be raised. Public-health agencies are also concerned about the continuity of data. Continuous use of the same thresholds facilitates interpretation of changes in prevalence estimates over time—for instance, assessments of the effectiveness of public-health campaigns to reduce teenage drug use that are based on changes in the prevalence of substance-use disorder among young adults.

Second, threshold placement is highly consequential because of the many social ramifications of a mental-disorder diagnosis. Diagnoses in general have consequences that are both positive (e.g., access to and payment for treatment; the right to “reasonable accommodations” under the Americans with Disabilities Act) and negative (e.g., social stigma; loss of ability to perform certain occupations). For example, children and adolescents with ASD are eligible to receive certain educational accommodations in the United States ( Carter, Skimkets, & Bornemann, 2014 ; Corrigan, Druss, & Perlick, 2014 ), but many individuals who had been diagnosed with Asperger’s disorder and their families were strongly opposed to the DSM-5 ’s inclusion of that disorder in the autism spectrum, at least partly because of the stigma associated with autism.

Important boundaries in everyday life are often somewhat arbitrary—for instance, the boundaries between states, countries, and time zones; between ages at which people are and are not of legal drinking age or eligible for Medicare; between tax brackets, and so forth. This arbitrariness (or semiarbitrariness) is widely recognized, and although we may not like a particular threshold and may even fight to change it, we generally accept the idea that thresholds are pragmatically necessary. In the case of mental disorder also, establishing thresholds between health and disorder is accepted as pragmatically necessary, particularly when a major reason for doing so is to identify those individuals who should receive mental-health services in the context of health systems with limited resources. However, regardless of their necessity, setting thresholds for mental disorder is highly consequential for multiple reasons. First, where thresholds are placed affects clinicians’ and researchers’ conceptualization of the relevant phenomena, clinical care, and knowledge generation. “Misplaced” boundaries may even interfere with optimal clinical care and research efforts.

There are a few mental disorders that essentially consist of a unidimensional symptom, such as trichotillomania, the primary symptom of which is recurrent pulling out of one’s hair, resulting in hair loss, despite repeated attempts to decrease or stop. However, as discussed, most mental disorders are multidimensional, composed of multiple emotional, cognitive, and behavioral dimensions, many of which are shared across disorders, although some are unique. Further, disorder severity is an overarching dimension common to all disorders, although how it is manifested depends on individuals’ particular problems. To diagnose a mental disorder, therefore, one must determine what kind, what combination(s), and “how much” of different aspects of a patient’s clinical presentation are needed to constitute a particular disorder. Because most individuals with mental illness show a mix of symptoms, multiple thresholds typically need to be considered in the diagnostic process. To diagnose individuals with both mood disturbance and psychotic cognitions, for example, it is necessary to determine the sequence, relative duration, and severity of these two types of symptoms before choosing a diagnosis from among schizophrenia, mood disorder with psychotic features, or schizoaffective disorder. Again, in their respective sections below, we discuss the somewhat (but not radically) different approaches of ICD-11 and DSM-5 to dealing with specific aspects of mental disorders’ multidimensionality.

As for RDoC, it is just as agnostic to defining mental disorder as it is to current diagnostic categories. By providing a framework for the exploration of functional processes that extend from the healthy to the pathological range, it aims to provide empirical data for establishing caseness in the long term rather than positing thresholds at this time.

Abstract debates about defining mental disorder aside, ICD-11 and DSM-5 still must meet their users’ need to decide such things as who should be counted as having a particular disorder in national health statistics and who should be eligible to receive and be covered financially for a particular type of heath service. In fact, determining “caseness”—whether or not a person has a particular condition—explicitly involves setting a threshold between disorder and nondisorder, and is perhaps the most fundamental requirement of these systems. Consistent with Wakefield’s view, classifications of mental disorder that are intended to guide determinations of eligibility for services or treatment decisions often have attempted to resolve the difficulties inherent in setting diagnostic thresholds by requiring that symptoms and signs be associated with clinically significant distress or impaired psychosocial functioning, a solution typically referred to, especially in relation to DSM, as the clinical significance criterion (note that this criterion implicitly acknowledges the presence of these symptoms and signs in the general population and their continuity with normal variation). We discuss the somewhat different stances that ICD-11 and DSM-5 have taken regarding this issue in their respective sections below.

The difficulty and inherent arbitrariness of assigning a diagnostic threshold for mental disorders are widely agreed upon and not conceptually controversial. Nonetheless, there is ongoing debate regarding the definition of mental disorder itself. Wakefield (e.g., 1992 , 2007) has been a primary proponent of the view that to be labeled a disorder, an abnormal health conditio