The co-occurrence of mental illnesses is well established. In 2005, a group of researchers at Harvard University reported that across 10,000 people, one in four met criteria for a single disorder, of these nearly half had two disorders, and over a quarter had more than three disorders. People with, say, PTSD, depression and a substance use disorder aren’t at all unusual.
That so many patients met criteria for multiple disorders astonished researchers (the paper has nearly 11,000 citations) and led to a large discussion over what we want a diagnosis to accomplish. Do we really want an infinite number of diagnoses that describe an infinite number of symptom combinations? If many diagnoses are required to describe your symptoms, they’re meaningless; if you have every disorder, you have no specific disorder.
Diagnosis is a messy business. In the 1940s, it became clear that psychiatric diagnoses were neither reliable nor valid. People received contradictory diagnoses by different clinicians—one study reported that clinicians only agreed on a diagnosis 20% of the time. The first effort to standardize diagnosis was by the military so it could reliably screen recruits. In fact, the first DSM was published in 1952 and was based on this military document; it included 106 diagnoses. In 1968, the APA released its second DSM, which included 182 diagnoses. But DSM-I and -II were largely ignored by the psychiatric community because the predominant (psychoanalytic) belief was that a person’s illness stemmed from his or her unique unconscious conflicts, which couldn’t be packaged into unwieldy diagnostic boxes.
– Scientific American