DSM is an acronym for Diagnostic and Statistical Manual (of Mental Disorders), put out by the APA. The methodology of this book stresses the fact that psychology is unlike other sciences, in that it is not a hypothesis-experiment system that tries to determine causality.

It is more like a system of observation and taxonomy. The categorization of mental illness is not derived from anything except the stats and the symptoms. If a certain new type of thinking and behaving differently from everyone else is found, a set of criteria a put forth. Since each criterion has to be satisfied for the diagnosis to be accurate, the DSM is under constant revision. New research data keeps redefining the exact criteria for different illnesses.

The DSM, in addition to being used for clinical, research and education purposes, also functions as a standardize the language used to describe psychopathology. The terms used in the DSM are used across several different organizations, and is recognized as the reference for all mental illness topics.

Version 4 is the latest release.