Thursday, 13 March 2008

Psychiatric taxonomy

I’ve been writing a commentary on a recent article on psychiatric taxonomy in the American Journal of Psychiatry (Zachar, P. and Kendler, K. (2007) ‘Psychiatric Disorders: A Conceptual Taxonomy’ American Journal of Psychiatry 164: 557-565) for the Association for the Advancement of Philosophy and Psychiatry.

In the article, Zachar and Kendler set out a number of dimensions on which to locate any possible psychiatric taxonomy(or perhaps syndromes within a taxonomy which might not be uniform). They say:

These dimensions are 1) causalism-descriptivism, 2) essentialism-nominalism, 3) objectivism-evaluativism, 4) internalism-externalism, 5) entities-agents, and 6) categories-continua. [ibid: 557]

Given that the main aim of the article is to provide a framework for future discussion rather than a robust argument for a particular position, I was at first unsure how to comment on it. But two things struck me.

1) The framework blurs the important distinction between causal and constitutive forms of externalism.

Should psychiatric disorders be defined solely by processes that occur inside the body (internalism), or can events outside the skin also play an important (or exclusive) defining role (externalism)? [ibid: 558]

If one, plausibly, thinks that environmental factors sometimes cause mental illness then one is a causal externalist. But one may think that they cause mental illness by affecting states – perhaps neurological – within the body. If so, whilst a causal externalist, one is also a constitutive internalist. (Constitution is not quite the same thing as what defines a mental illness. Even a constitutional internalist may find it helpful to label illnesses by their causes.)

Combining a constitutive form of externalism with evaluativism gives a position in which mental illness is partly constituted by value judgements. But this leaves open two interpretations of constitutive evaluativist externalism depending on whether one thinks that the values are real or merely subjective, disciplined or undisciplined by real values out there.

2) An undisciplined constitutive evaluativist externalist account is not so much one way to frame a taxonomy, it undermines the very idea of psychiatric taxonomy. Mental illnesses are constituted, at least in part, by matters external to the body. In addition, these matters are not features of the world, broadly construed, but rather expressions of subjectivity. If this were the correct approach to the nature of mental illness, however, it fits uneasily with the very idea of a psychiatric taxonomy. Whilst one the aims of taxonomy is validity – to cut nature at the joints – so as to enable the framing of true judgements, on an undisciplined evaluativist approach, that idea of correctness is missing.

But this impacts interestingly on the challenge Pat Bracken raised at the WPA meeting in London last Autumn. How should psychiatry respond to those who argue that their experiences, such as hearing internal voices, whilst fitting a psychiatric diagnostic category, are not really pathological?

Imagine, for example, that objectivists succeeded in developing a consistent and intuitively plausible account of mental illness, reducing concepts of mental disorder to simple facts. Suppose that on this account, hearing voices turned out to be pathological. Suppose also that undisciplined evaluativists succeeded in developing a rival account on which hearing voices was not in itself pathological. How should the two accounts be assessed. One problem, of course, is that whilst the status of hearing voices is evidence one way or the other, it is contested. If one somehow knew, antecendently, its pathological status that would be a crucial test for the two accounts. But, as Neil Pickering argues, no such pre-theoretical knowledge is possible [Pickering 2006]. In fact, however, the problem goes deeper.

Setting out the debate like this suggests that whether or not mental illness is simply factual or whether it is irreducibly evaluative – and if so of what sort – is itself a deeper level factual matter. But it is open to an undisciplined evaluativist to argue that that deeper level matter is not factual but rather, also, evaluative. (It is a case of ‘values all the way down’.) They can argue that we should, for reasons expressive of better subjective value, choose their model of mental illness not because it is true but because it is (evaluatively) right. And that is why assessing Bracken’s question runs so deep.

PS: an update is here.