Structural problems rooted in the basic de sign 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 come s from diverse sources, including studies of
comorbidity and the substantia l 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. Stud ies 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 sepa rate 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 comorb idity. Earlier editions of DSM focused on ex-
cluding false-positive results fr om diagnoses; thus, its categories were overly narrow, as is
apparent from the widespread need to use NO S diagnoses. Indeed, the once plausible goal
of identifying homogeneous populations for trea tment and research resu lted in narrow di-
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 homogene ity by progressive subt yping within disorder
categories no longer is sensible; like most  common human ills, mental disorders are het-
erogeneous at many levels, ranging fr om genetic risk factors to symptoms. 
Related to recommendations about alteration s in the chapter structure of DSM-5, mem-
bers of the diagnostic spectra study group examined whether scientific validators could
inform possible new groupings of related diso rders within the existing categorical frame-
work. Eleven such indicators were recommended for this purpose: shared neural sub-