Beginning with DSM-III, these systems dramatically increased the number of diagnostic categories and reduced the number of exclusion criteria so that many people who would have received only a single diagnosis in previous systems now receive multiple diagnoses. The intention was to retain potentially important differentiating information that could be useful in refining understanding of etiology, course, and likely treatment response (First, Spitzer, & Williams, 1990). However, it can also be argued that this had the unintended negative consequence of artificially inflating estimates of comorbidity. This uncertainty might be resolved in future attempts to determine the validity of diagnostic distinctions, based on the new National Institute of Mental Health (NIMH) Research Domain Criteria (RDoC) initiative (www.nimh.nih.gov/ research-priorities/ rdoc/ index.shtml). The RDoC initiative will break down DSM diagnoses, which are currently based only on observable symptoms, into their underlying neural circuit-based domains and constructs. Identifying the common domains and constructs in which dysfunction is occurring may ultimately help to explain the high diagnostic overlap and comorbidity between depression and other DSM disorders. Until that time, though, we are left with a situation in which MDD appears to be highly comorbid with a number of other disorders. (Gotlib, Ian H.; Hammen, Constance L. (Eds.) (2014): Handbook of depression. Third edition. New York: The Guilford Press, p.27)