As part of this series, Martin Davies (Wilde Professor of Mental Philosophy at the University of Oxford and a Fellow of Corpus Christi College) kindly agreed to answer some questions about delusions. The interview will be published in three parts.
LB: Thank you for helping us launch PERFECT! When you started working on delusions, philosophical literature on the topic was scarce. Why did you find delusions interesting to start with?
MKD: Congratulations, Lisa, on the launch of your new research project, supported by an ERC Consolidator Grant of nearly two million euros! Perfect, indeed. And thank you for the opportunity to answer some questions on your blog.
I started to learn about delusions in the early 1990s from talks that Andy Young (now Professor of Neuropsychology at the University of York) gave at early meetings of the European Society for Philosophy and Psychology.
The phenomenology and the theories were of evident interest to philosophers and psychologists and Tony Stone (a philosopher) joined with Andy Young to write a seminal interdisciplinary paper about monothematic and circumscribed delusions following brain injury (Stone and Young, 1997). My first paper on delusions was a long editorial introduction, written with Max Coltheart, for a special issue of Mind & Language, reprinted as Pathologies of Belief. Our discussion was organised around two questions: (1) Can the delusional idea or hypothesis be understood as arising in a folk psychologically intelligible way from the subject’s experience? (2) Why is that hypothesis adopted and maintained as a belief despite its utter implausibility and the uniform scepticism with which other people greet it?
LB: How much was your own thinking influenced by the Stone and Young paper? And do those two questions indicate that you and Max Coltheart were already proposing the two-factor account of delusions (summarised by Max here)?
MKD: Much of our paper was an extended reflection on Stone and Young (1997) and we broadly followed them – as they, in turn, followed Brendan Maher (1974) – in our answer to the first of the two questions. Neuropsychological impairment following brain injury results in an anomalous and disturbing experience. The delusional idea or hypothesis then comes to mind as the patient tries to explain or make sense of this anomalous experience (e.g. an experience with the representational content, ‘This woman looks just like my wife but there is something different or odd about her’, might be explained by the hypothesis that the woman who looks like my wife and says she is my wife is not my wife, but an impostor). Later in our paper, we distinguished a second way in which the delusional hypothesis might arise from the subject’s experience: the core content of the delusion might already be part of the content of the experience itself (e.g. an experience with the content, ‘This woman looks just like my wife but is not her’).
Stone and Young themselves argued that an anomalous experience does not, by itself, explain why the patient adopts the delusional hypothesis as a belief. Thus, additional elements would be needed to explain the patient’s misinterpretation of the anomalous experience and Stone and Young proposed 'reasoning biases' for this explanatory role.
Specifically, they suggested that a subject’s externalising or internalising attributional style might influence which explanatory hypotheses were considered and that the ‘jumping to conclusions’ data-gathering bias might then lead to early adoption of a hypothesis as a belief. It is of some interest that they did not exclude the possibility that variation within the normal range (e.g. in attributional style and in data gathering) might explain the difference between two patients with the same anomalous experience, only one of whom had a delusion. Tony Stone sometimes suggested that an account of why some patients with anomalous experiences are, and some are not, delusional might be ‘a chapter in the psychology of individual differences’.
Our answer to the second of the two questions departed somewhat from Stone and Young. We focused, not so much on the initial adoption of the delusional belief, but on its persistence – sometimes for months or years – in the face of its implausibility and of evidence against it. This failure to reject the belief seemed to be more than a bias (see also Langdon and Coltheart 2000) and we found it tempting to suppose that the failure was to be explained in terms of a second neuropsychological impairment – a deficit in some component of the cognitive machinery that underpins belief evaluation and revision.
Thus – coming to the second part of your question – the two-factor framework for explaining delusions, and its specifically neuropsychological version, the two-deficit framework, was already present in that first paper. It was explicit in Robyn Langdon and Max Coltheart’s (2000) paper in Pathologies of Belief, and featured in the title of a subsequent paper (Davies, Coltheart, Langdon and Breen, 2001).
LB: Nearly fifteen years on from your first paper on the topic, what still interests you about delusions?
MKD: There are many things about delusions that I still find interesting and sometimes write about: the two-factor framework itself, the cognitive nature of the second factor or deficit, reasoning in individuals with delusions, and anosognosia considered as a delusion – a topic on which my wife, Anne Aimola Davies, and I have written some papers. Let me start with the first of these issues.
I am interested in the question whether we can expect a single explanatory framework to encompass all delusions, with their varying contents and different aetiologies. Relatedly, I am interested in variations on the two-factor theme – that is, ‘parametric variation’ within the two-factor framework (Aimola Davies and Davies, 2009). Here is one example.
In response to an earlier question, I mentioned two ways in which a delusional hypothesis might arise from an anomalous experience. The hypothesis might come to mind as a possible explanation of the experience or it might already be part of the content of the experience itself. In the first case, the hypothesis might be adopted as a belief because the subject judges it to be, not just a possible explanation, but the best available explanation of the anomalous experience. In the second case, adoption of the delusional belief is simply a matter of accepting the experience as veridical. We described this as a ‘prepotent doxastic response’ (Davies, Coltheart, Langdon and Breen, 2001). The two-factor framework allows at least these two accounts of the transition from anomalous experience to delusional belief. In the literature on delusions they are usually described as the ‘explanationist’ versus ‘endorsement’ accounts.
The second part of the interview will be published next Thursday.
Ellis, H.D. and Young, A.W. 1990: Accounting for delusional misidentifications. British Journal of Psychiatry, 157, 239–48.
Wright, S., Young, A.W. and Hellawell, D.J. 1993: Sequential Cotard and Capgras delusions. British Journal of Clinical Psychology, 32, 345–9.
Stone, T. and Young, A.W. 1997: Delusions and brain injury: The philosophy and psychology of belief. Mind & Language, 12, 327–64.
Coltheart, M. and Davies, M. (eds) 2000: Pathologies of Belief. Oxford: Blackwell Publishers.
Maher, B.A. 1974: Delusional thinking and perceptual disorder. Journal of Individual Psychology, 30, 98–113.
Langdon, R. and Coltheart, M. 2000: The cognitive neuropsychology of delusions. Mind & Language, 15, 184–218. Reprinted in Coltheart and Davies (2000).
Davies, M., Coltheart, M., Langdon, R. and Breen, N. 2001: Monothematic delusions: Towards a two-factor account. Philosophy, Psychiatry, and Psychology, 8, 133–58.
Aimola Davies, A.M. and Davies, M. 2009: Explaining pathologies of belief. In M.R. Broome and L. Bortolotti (eds), Psychiatry as Cognitive Neuroscience: Philosophical Perspectives, pp. 285–323. Oxford: Oxford University Press.
Davies, M. and Coltheart, M. 2000: Introduction: Pathologies of belief. Mind & Language, 15, 1–46. Reprinted in Coltheart and Davies (2000).