The failure of psychiatry to validate its diagnostic constructs is often attributed to the prioritizing of reliability over validity in the structure and content of the Diagnostic and Statistical Manual of Mental Disorders. Here I argue that in fact what has retarded biomedical approaches to psychopathology is unwarranted optimism about diagnostic discrimination: the assumption that our diagnostic tests group patients together in ways that allow for relevant facts about mental disorder to be discovered. I consider the Research Domain Criteria framework (...) as a new paradigm for classifying objects of psychiatric research that solves some of the challenges brought on by this assumption. (shrink)