The American Psychiatric Association has approved its new manual of diagnoses — Diagnostic and Statistical Manual of Mental Disorders — after five years of controversy [nytimes].

For example, it has removed Aspberger’s as a diagnosis, lumping it in with autism, but it has split out hoarding from the more general category of obsessive-compulsive disorder. Lumping and splitting are the two most basic activities of cataloguers and indexers. There are theoretical and practical reasons for sometimes lumping things together and sometimes splitting them, but they also characterize personalities. Some of us are lumpers, and some of us are splitters. And all of us are a bit of each at various times.

The DSM runs into the problems faced by all attempts to classify a field. Attempts to come up with a single classification for a complex domain try to impose an impossible order:

First, there is rarely (ever?) universal agreement about how to divvy up a domain. There are genuine disagreements about which principles of organization ought to be used, and how they apply. Then there are the Lumper vs. the Splitter personalities.

Second, there are political and economic motivations for dividing up the world in particular ways.

Third, taxonomies are tools. There is no one right way to divide up the world, just as there is no one way to cut a piece of plywood and no one right thing to say about the world. It depends what you’re trying to do. DSM has conflicting purposes. For one thing, it affects treatment. For example, the NY Times article notes that the change in the classification of bipolar disease “could ‘medicalize’ frequent temper tantrums,” and during the many years in which the DSM classified homosexuality as a syndrome, therapists were encouraged to treat it as a disease. But that’s not all the DSM is for. It also guides insurance payments, and it affects research.

Given this, do we need the DSM? Maybe for insurance purposes. But not as a statement of where nature’s joints are. In fact, it’s not clear to me that we even need it as a single source to define terms for common reference. After all, biologists don’t agree about how to classify species, but that science seems to be doing just fine. The Encyclopedia of Life takes a really useful approach: each species gets a page, but the site provides multiple taxonomies so that biologists don’t have to agree on how to lump and split all the forms of life on the planet.

If we do need a single diagnostic taxonomy, DSM is making progress in its methodology. It has more publicly entered the fray of argument, it has tried to respond to current thinking, and it is now going to be updated continuously, rather than every 5 years. All to the good.

But the rest of its problems are intrinsic to its very existence. We may need it for some purposes, but it is never going to be fully right…because tools are useful, not true.