Structural problems rooted in the basic design of the previous DSM Classification, con- structed of a large number of narrow diagnostic categories, have emerged in both clinical practice and research. Relevant evidence comes from diverse sources, including studies of comorbidity and the substantial need for not otherwise specified diagnoses, which repre- sent the majority of diagnoses in areas such as eating disorders, personality disorders, and autism spectrum disorder. Studies of both genetic and environmental risk factors, whether based on twin designs, familial transmission, or molecular analyses, also raise concerns about the categorical structure of the DSM system. Because the previous DSM approach considered each diagnosis as categorically separate from health and from other diagnoses, it did not capture the widespread sharing of symptoms and risk factors across many dis- orders that is apparent in studies of comorbidity. Earlier editions of DSM focused on ex- cluding false-positive results from diagnoses; thus, its categories were overly narrow, as is apparent from the widespread need to use NOS diagnoses. Indeed, the once plausible goal agnostic categories that did not capture clinical reality, symptom heterogeneity within dis- orders, and significant sharing of symptoms across multiple disorders. The historical aspiration of achieving diagnostic homogeneity by progressive subtyping within disorder categories no longer is sensible; like most common human ills, mental disorders are het- erogeneous at many levels, ranging from genetic risk factors to symptoms.
Related to recommendations about alterations in the chapter structure of DSM-S, mem- bers of the diagnostic spectra study group examined whether scientific validators could inform possible new groupings of related disorders within the existing categorical frame— work. Eleven such indicators were recommended for this purpose: shared neural sub- strates, family traits, genetic risk factors, specific environmental risk factors, biomarkers, temperamental antecedents, abnormalities of emotional or cognitive processing, symptom similarity, course of illness, high comorbidity, and shared treatment response. These indi- cators served as empirical guidelines to inform decision making by the work groups and the task force about how to cluster disorders to maximize their validity and clinical utility.
A series of papers was developed and published in a prominent international journal (Psychological Medicine, Vol. 39, 2009) as part of both the DSM-S and the ICD-ll develop- mental processes to document that such validators were most useful for suggesting large groupings of disorders rather than for ”validating” individual disorder diagnostic criteria.
The regrouping of mental disorders in DSM-S is intended to enable future research to en- hance understanding of disease origins and pathophysiological commonalities between time to continuale assess validity. Ongoing revisions of DSM-S will make it a “living doc- ument,” adaptable to future discoveries in neurobiology, genetics, and epidemiology.
On the basis of the published findings of this common DSM-5 and ICD-11 analysis, it was demonstrated that clustering of disorders according to what has been termed internal- izing and externalizing factors represents an empirically supported framework. Within both the internalizing group (representing disorders with prominent anxiety, depressive, and somatic symptoms) and the externalizing group (representing disorders with prominent impulsive, disruptive conduct, and substance use symptoms), the sharing of genetic and environmental risk factors, as shown by twin studies, likely explains much of the system— atic comorbidities seen in both clinical and community samples. The adjacent placement of ”intemalizing disorders,” characterized by depressed mood, anxiety, and related physio- logical and cognitive symptoms, should aid in developing new diagnostic approaches, in- cluding dimensional approaches, while facilitating the identification of biological markers.
Similarly, adjacencies of the “externalizing group,” including disorders exhibiting antiso- cial behaviors, conduct disturbances, addictions, and impulse-control disorders, should en- courage advances in identifying diagnoses, markers, and underlying mechanisms.