My occupation is a statistician. I tell people it is like "CSI without dead bodies" because analyzing a set of data that has been collected is like doing an autopsy on a deceased person in the sense that I'm trying to learn what I can from what statistics and information are available. Except in this case the information does not involve gross things. For me the research process can be humorous, scary, but always captivating.
Since 1952, the American Psychiatric Association has published the Diagnostic and Statistical Manual of Mental Disorders (DSM for short) which has spelled out the criteria for diagnosing all of the known psychiatric disorders. It has undergone four editions and two revisions with the fifth one due out this year. In the past there were controversies such as whether or not homosexuality should be included in the manual and it was dropped from the DSM in the 1970s. New changes are made to the manual as new information is brought to light and cultural views of what is and what is not a disorder change.
There have been advances in neuroscience and genetics which shed light on many of these disorders and made many pharmacological treatments possible but the main reliance for diagnosis is still on behavioral symptoms. The NIMH is creating a Research Domain Criteria for Diagnoses (RDoC) based on biological criteria which it believes are more objective. The behavioral symptoms are often subject to interpretation and often still not enough is known about the brain and genetics to differentiate between pathologies. Consider the figure at left. Is this a rabbit with its head held high or a duck? This image is subject to interpretation just as all behaviors and incomplete scientific data are. Science is fundamentally a human endeavor where politics often plays a role.