Cognitive theories of delusion
Martin Davies
Faculty of Philosophy and Department of Experimental Psychology
University of Oxford

A dominant cognitive theory of delusion is the two-factor theory developed by Coltheart and colleagues (e.g. Coltheart, 2007, 2010; Coltheart et al., 2011). The leading idea of the two-factor theory is that an explanation of a delusion can be achieved by answering two questions (Coltheart, 2007): ‘Where did the delusion come from?’ and ‘Why does the patient not reject the belief?’. The answers to these questions are supposed to identify two factors (two departures from normality) in the aetiology of the delusion. The first is an anomalous experience (or, in more explicitly neuropsychological versions, a neuropsychological deficit that might give rise to an anomalous experience). The second factor is an impairment of belief evaluation.

The strength of the two-factor theory is the evidence that a first factor is present in patients with delusions, yet is not itself sufficient to explain the delusion – so that there must be (at least) a second factor. A weakness of the two-factor theory is the inadequate characterisation of the cognitive nature of the second factor.

Coltheart’s two questions may also obscure the complexity of explanations of delusions. There are at least three processing stages to be considered: from anomalous experience to delusional idea or hypothesis; from the generation of the hypothesis to its adoption as a belief; and the persistence of the belief despite its implausibility and the evidence against it. So there are at least three questions that an explanation of a delusion needs to answer: ‘Where did the delusional idea or hypothesis come from?’; ‘Why was the delusional idea or hypothesis adopted as a belief, rather than being rejected?’; and ‘Why does the delusional belief – once it is adopted – persist, rather than being rejected?’. So it is not only the cognitive nature, but also the role, of the second factor that is inadequately characterised.

In this talk, I shall consider the role and nature of the second factor (or factors) in delusion, drawing on (1) the empirical literature on reasoning in individuals with delusions; (2) recent work on Bayesian inference and compartmentalisation (Davies and Egan, 2013); and (3) evidence from patients with anosognosia for motor impairments (failure to acknowledge paralysis following stroke; Aimola Davies et al., 2009, 2010; Vocat et al., 2013).

References
Aimola Davies, A.M., Davies, M., Ogden, J.A., Smithson, M. and White, R.C. 2009: Cognitive and motivational factors in anosognosia. In T. Bayne and J. Fernández (eds), Delusion and Self-Deception: Affective Influences on Belief-Formation. Hove, East Sussex: Psychology Press, 187–225.
Aimola Davies, A.M., White, R.C. and Davies, M. 2010: Assessment of anosognosia for motor impairments. In J.M. Gurd, U. Kischka and J.C. Marshall (eds), Handbook of Clinical Neuropsychology (Second Edition). Oxford: Oxford University Press, 436– 68.
Coltheart, M. 2007: Cognitive neuropsychiatry and delusional belief. Quarterly Journal of Experimental Psychology, 60, 1041–62.
Coltheart, M. 2010: The neuropsychology of delusions. Annals of the New York Academy of Sciences, 1191, 16–26.
Coltheart, M., Langdon, R. and McKay, R. 2011: Delusional belief. Annual Review of Psychology, 62, 271–98.
Davies, M. and Egan, A. 2013: Delusion, cognitive approaches: Bayesian inference and compartmentalisation. In K.W.M. Fulford, M. Davies, R.G.T. Gipps, G. Graham,
J. Sadler, G. Stanghellini and T. Thornton (eds), The Oxford Handbook of Philosophy and Psychiatry. Oxford: Oxford University Press, 689–727.
Vocat, R., Saj, A. and Vuilleumier, P. 2013: The riddle of anosognosia: Does unawareness of hemiplegia involve a failure to update beliefs? Cortex, 49, 1771–81.
2