The Diagnostic and Statistical Manual of Mental Disorders (DSM) is the authoritative guide for identifying abnormal mental states. Complete with descriptions, symptoms and other criteria, the handbook provides a common language in an area that tends to be quite subjective.
It is periodically updated by the American Psychological Association to ensure that health care professionals are using information that reflects the findings of current research. When historians and psychologists are looking to offer diagnoses for people who are long dead, their tool belt should include the DSM, personal correspondence, diary entries and firsthand accounts of the individual in question.