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What Kind of Kinds Are Psychiatric Diagnostic Categories? On Zachar and Kendler

In previous posts on personality disorders and autism, I have been dancing around a question that is central to philosophy of psychiatry: what kind of kinds are the categories found in diagnostic manuals, such as DSM-5 or ICD-11? I have touched on the debate, for example, between categorical and dimensional approaches to personality disorder classification, and, in the previous post, I mentioned Sam Fellowes’ musings about what might have happened if what Leo Kanner called “infantile autism” had not been separated out from “childhood schizophrenia” in the 1940s. These sorts of issues inevitably lead to deeper questions about the very status of psychiatric diagnostic categories.

 

To tackle this problem-cluster head on, I thought I would look at what seems (based on my subjective impressions of citation) to be a classic article in philosophy of psychiatry, Zachar and Kendler’s “Psychiatric disorders: A conceptual taxonomy” (2007). The authors propose six questions about diagnostic categories and sort responses around two poles for each. They then use their taxonomy to make sense of the similarities and differences between four different versions of what they call the “medical model” (a fraught and, in my mind, much-abused term—I may write about this again in a future post) and four alternatives. This article is not terribly deep, but it offers a wonderful point of orientation. I can easily see it being assigned in an intro-level philosophy of psychiatry course for people with very little experience in philosophy. I should add that Kendler has an MD and is one of the leading figures in psychiatric genetics, and Zachar has a PhD in psychology in addition to an undergraduate degree in philosophy.

 

The second of Zachar and Kendler’s binaries is the most interesting to me for present purposes: “essentialism versus nominalism” about psychiatric categories. They state the underlying question thus: “Are categories of psychiatric disorder defined by their underlying nature (essentialism), or are they practical categories identified by humans for particular uses (nominalism)” (558)? This kind of question is familiar from the history of philosophy outside of philosophy of psychiatry; indeed, it is a classic question. Yet actually getting clear on what is being asked can be tricky.

 

To motivate understanding of the debate, it is often easiest to start with imagery. To use the classic image from Plato, are we carving nature at the joints when classify? That is, do psychiatric categories map onto “natural kinds” (the term of art in contemporary analytic philosophy)? Do the categories we use when we speak and think correspond to real divisions in nature, external reality, the thing itself—or whatever you want to call it? Is classification ultimately a process of finding underlying structures? Or are we rather the ones imposing the structures, the divisions, the orderings? Are we making the categories rather than finding them?

 

A friend once told me a story about how the college she attended hired consultants to classify the student body. The consultants duly sorted the students into something like “stripes” and “polka dots.” The polka dots were, more or less, the hippies, queer kids, activists, and druggies. The stripes were the students who were more “normie,” more focused on their careers. I don’t think anyone would say that the categories stripes and polka dots carve the population of students at the joints. These categories stem from practical considerations, namely, the university’s attempt to change its public image, increase its rankings, attract more tuition dollars, and so on. (Following philosophical convention, I italicize to indicate that I am mentioning the category, rather than talking about what the category refers to in the world.)

 

In contrast, the distinction between elements in the periodic table is often invoked to illustrate properly natural kinds. The category gold would seem to fit the bill of a category defined by an “underlying nature.” And there are nice crisp lines between gold and, say, hydrogen.

 

So, in short, the question could be restated, a little crudely, as follows: is the category major depressive disorder more like gold or polka dots at the University of XYZ?

 

Now, it is easy to mistake this question for an empirical one, in which case the essentialist position would be obviously false. It is simply not the case right now that our existing psychiatric categories correspond to natural kinds. They are blunt instruments, heterogeneous groupings, based largely on phenomenological description rather than any kind of understanding of underlying structure. So, the real question is a normative one, that is, a question about the goals to which our categories can and ought to aspire. Is the telos, the proper aim, of scientific inquiry here to uncover essences, or is it to develop useful classifications that further the relevant interests and concerns?

 

Of course, this is possibly a false dichotomy; one could argue that we could do both, or that getting at essences is the best way to get at what is useful. And if we accept the second option at the expense of the first, is not “relevant interests and concerns” terribly vague? Exactly which interests and concerns matter for diagnostic categorization? What is useful to clinical practice may not be the same as what is useful for research, and both might get in the way of self-understanding, attempts to determine criminal responsibility in the criminal legal system, the project of improving underwriting practices for medical insurance, etc. Diagnostic categories are used in a lot of different ways.

 

The key problem for the essentialist is to show why we should expect there to be underlying structures when, thus far, everything appears so heterogeneous. Moreover, essentialism about anything living was dealt a terrible blow with Darwin, after whom it is hard to see biological categories as anything more than momentarily-stabilized groupings of dynamic variations. The key problem for the nominalist is to clarify what exactly we are doing when we classify and to make sense of how it is that some of our classifications seem to be better than others. What, moreover, does the world fundamentally have to be like for nominalism to be true? How is it that the world lends itself to so many different ways of classifying? Zachar and Kendler distinguish between a radical nominalism and a more “moderate” one that hold simply that “there is some structure of psychiatric illness in the world but there is no one unique categorization that stands above the others on a priori grounds” (558). But what is it, then, about the world and about our own categorizing practices that lends itself to structure without unique categorization?

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