Mood disorders have the highest prevalence among all psychiatric illness. In the United States major depressive disorder has a lifetime prevalence around 16%. Depression is often associated with concurrent anxiety disorders or substance use. The specific diagnostic criteria for each disorder can be found in the Diagnostic and Statistical Manual of Mental Disorders (DSM). The most current edition for reference purposes is the DSM-IV-TR.
Acknowledgements: Alexandra Stefan MD (Author), Paula Cleiman MD (Peer Reviewer)