Introduction 13
hance understanding of disease origins and pathophysiological commonalities between
disorders and provide a base for future replication wherein data can be reanalyzed over
time to continually assess validity. Ongoing revisions of DSM-5 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 diso rders according to what has been termed internal-
izing and externalizing  factors represents an empirically  supported framework. Within both
the internalizing group (representing disorder s with prominent anxiety, depressive, and
somatic symptoms) and the externalizing group (representing disorders with prominent
impulsive, disruptive conduct, and substance use symptoms), the sh aring of genetic and
environmental risk factor s, as shown by twin studies, like ly explains much of the system-
atic comorbidities seen in both clinical an d community samples. The adjacent placement of
“internalizing disorders,” characterized by de pressed mood, anxiety, and related physio-
logical and cognitive symptoms, should aid in developing new diagnostic approaches, in-
cluding dimensional approaches, while facilitati ng the identification of biological markers.
Similarly, adjacencies of the “externalizing group,” including disord ers exhibiting antiso-
cial behaviors, conduct disturbances, addictio ns, and impulse-control disorders, should en-
courage advances in identifying diagnoses, markers, and underlying mechanisms.
Despite the problem posed by categorical diagnoses, the DSM-5 Task Force recognized
that it is premature scientifically to propose alternative definitions fo r most disorders. The
organizational structure is meant to serve as a bridge to new diagnostic approaches with-
out disrupting current clinical practice or  research. With support from DSM-associated