The first DSM had many important strengths, but I would argue that part of what went wrong with it was a fairly arbitrary decision: the promulgation of a large number of disorders, despite the early state of the science, and the conceptualization of each disorder as a distinct category. That decision eschewed the possibility that some diagnoses are better represented in terms of quantifiable dimensions, much like the diagnoses of hypertension and diabetes, which are based on measurements on numerical scales.
These fundamental missteps would not have proven so problematic but for the human tendency to treat anything with a name as if it is real. Thus, a scientifically pioneering diagnostic system that should have been treated as a set of testable hypotheses was instead virtually set in stone. DSM categories play a controlling role in clinical communication, insurance reimbursement, regulatory approval of new treatments, grant reviews, and editorial policies of journals. As I have argued elsewhere, the excessive reliance on DSM categories, which are poor mirrors of nature, has limited the scope and thus the utility of scientific questions that could be asked. We now face a knotty problem: how to facilitate science so that DSM-6 does not emerge a decade or two from now a trivially revised descendant of DSM-III, but without disrupting the substantial clinical and administrative uses to which the DSM system is put.
I believe that the most plausible mechanism for repairing this plane while it is still flying is to give new attention to overarching families of disorders, sometimes called meta-structure. In previous editions of the DSM, the chapters were almost an afterthought compared with the individual disorders. It should be possible, without changing the criteria for specific diagnoses, to create chapters of disorders that co-occur at very high rates and that appear to share genetic risk factors based on family, twin, and molecular genetic studies.
This will not be possible for the entire DSM-5, but it would be possible for certain neurodevelopmental disorders, anxiety disorders, the obsessive-compulsive disorder spectrum, so-called externalizing or disruptive disorders (such as antisocial personality disorder and substance use disorders), and others. Scientists could then be invited by funding agencies and journals to be agnostic to the internal divisions within each large cluster, to ignore the over-narrow diagnostic categories. The resulting data could then yield a very different classification by the time the DSM-6 arrives.
Psychiatry has been overly optimistic about progress before, but I would predict that neurobiologically based biomarkers and other objective tests will emerge from current research, along with a greater appreciation of the role of neural circuits in the origins of mental disorders. I would also predict that discrete categories will give way, where appropriate, to quantifiable dimensions. At the very least, the science of mental disorders should be freed from the unintended cognitive shackles bequeathed by the DSM-III experiment.