Taxonomising delusions: content or aetiology?

ABSTRACT

Introduction: Many theoretical treatments assume (often implicitly) that delusions ought to be taxonomised by the content of aberrant beliefs. A theoretically sound, and comparatively under-explored, alternative would split and combine delusions according to their underlying cognitive aetiology.

Methods: We give a theoretical review of several cases, focusing on monothematic delusions of misidentification and on somatoparaphrenia.

Results: We show that a purely content-based taxonomy is empirically problematic. It does not allow for projectability of discoveries across all members of delusions so delineated, and lumps together delusions that ought to be separated. We demonstrate that an aetiological approach is defensible, and further that insofar as content-based approaches are plausible, it is only to the extent that they implicitly link content to aetiology.

Conclusions: We recommend a more explicit focus on cognitive aetiology as the grounds for delusion taxonomy, even when that would undermine traditional content-based boundaries. We also highlight the iterative and complex nature of evidence about aetiologically grounded taxonomies.

KEYWORDS:

• Delusions
• aetiology
• taxonomy
• belief
• content

Acknowledgements

Thanks to Tim Bayne and Max Coltheart as well as a reading group in philosophy and the belief formation Group in the CCD for helpful feedback on previous drafts. A portion of this content developed out of material presented at “The Value of Suffering” workshop, jointly funded by the Value of Suffering Project and the Macquarie University Research Centre for Agency, Values, and Ethics (CAVE).

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Peter Clutton http://orcid.org/0000-0002-2040-5107

Stephen Gadsby http://orcid.org/0000-0002-2302-844X

Colin Klein http://orcid.org/0000-0002-7406-4010

Notes

1 For some examples of previous work, see Marková and Berrios (1994) and Rodrigues and Banzato (2006). We endorse this kind of overt discussion of the taxonomy of individual delusions and aim to advance the discussion with some theoretical and practical principles regarding the use of the aetiological approach we advocate.

2 When discussing biological species, “taxonomy” is often used to refer to methods other than the cladistic. We follow that convention when discussing cladistics.

3 Consider the analogy that Wakefield draws to the process of automobile repair. Wakefield claims that auto repair manuals are “strikingly unparsimonious” because “problems do not hang together the way the car’s parts do” (1998, p. 847). Yet standard repair manuals do organise symptoms by the failures of the systems which cause them (Choate & Haynes, 1992, 20ff).

4 As Breen et al. (2001, p. 250) point out, their patient did not in fact have difficulty recognising faces in photographs, which led them to suggest that there might be something functionally unique about recognising faces in the mirror.

5 Our point here can be more succinctly stated: the content definition of De Clérembault’s may suffer from Berkson’s (1946) fallacy, in which a contingent factor that influences which patients are brought to clinical attention is incorrectly taken as an important part of the disorder.

6 Note that we do not currently have sufficient understanding or sufficient technology to do so. The point is illustrative: it shows the kind of evidence that would be relevant.

7 Note that when we refer to cognitive functions and their breakdown, we have in mind a causal role sense of “function” rather than the role that something was selected to do. So for example, pure alexia is undoubtedly a breakdown of some of the functional underpinnings of visual word recognition, even if the underlying machinery was not evolutionarily selected for reading. For aficionados, we here use “function” in the sense of Cummins (1975) rather than of Wright (1973), though we are ultimately pluralists about this issue (Godfrey-Smith, 1993).

8 Of course, determining whether two brain regions have the same function is a nontrivial task in its own right; further, the best taxonomies of brain regions for many neuroscientific disciplines may not be in terms of function but (e.g.) connectivity or cytoarchitecture. The point is merely that a relatively “pure” neural classification for delusions seems somewhat rare. In practice, lesions are grouped together based on their functional effects, rather than the other way around. Thanks to an anonymous reviewer for pressing us to clarify this point.

9 Thanks to an anonymous reviewer for this point.

Additional information

Funding

This work was partially supported by the Australian Research Council [grant number FT140100422 (to C.K.)], an Australian Government Research Training Program (RTP) Scholarship (to P.C and S.G), and membership of the ARC Centre of Excellence in Cognition and its Disorders (P.C).