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Revisiting Maher’s one-factor theory of delusion

Today's post is by Chenwei Nie on his recent paper, "Revisiting Maher’s one-factor theory of delusion" (Neuroethics, 2023). Currently, he is a Teaching Fellow in Philosophy at the University of Warwick, UK.

Chenwei Nie


Suppose your friend, Ava, is suffering from a delusional belief that her partner is replaced by an imposter. Despite overwhelming evidence to the contrary - such as the supposed imposter's identical appearance to her partner and knowledge of intimate details, and reassurances from you and her other friends - Ava steadfastly maintains this belief. In this unfortunate circumstance, it is obvious that something must have gone wrong in the formation and maintenance of Ava’s belief. It is likely that Ava would be diagnosed with the Capgras delusion. Unravelling what exactly has gone wrong, however, has proven challenging.

According to a classic one-factor theory of delusion developed by Brendan Arnold Maher (1924-2009), the only factor, i.e. departure from normality, is the patient’s anomalous experience, of which the delusion is a normal explanation. Let us call this sort of anomalous experience the Maherian experience. If this theory is true, then Ava’s belief that her partner is an imposter should be considered as a normal explanation of her particular Maherian experience. Here, ‘normal’ means that if a typical, non-delusional person were to have Ava’s experience, they might also formulate the imposter hypothesis and come to believe that their partner is an imposter.

But is the one-factor theory a satisfactory explanation for delusions? In the past decade or so, a growing number of papers have argued in its favour. My paper, ‘Revisiting Maher’s One-Factor Theory of Delusion’, scrutinizes these one-factor arguments and reveals two fundamental issues.

First, the notion of the Maherian experience is too elusive. For example, advocates of the one-factor theory have been unable to articulate what exactly the Maherian experience is in the Capgras delusion. More often than not, they suggest that the Maherian experience could be an ‘intense’, ‘prolonged’, and ‘profound’ feeling of unfamiliarity. However, these descriptors are not informative enough to tell us why a far-fetched hypothesis like the imposter belief could somehow become a normal explanation.

Second, the notion of normal explanation is overly encompassing. In recognition of the above issue, some supporters of the one-factor theory suggest that suboptimal reasoning might play a part in delusion formation, but they maintain that these suboptimal reasoning processes are still within ‘the normal range’. Yet, without a more tangible account of the Maherian experience, it remains obscure how the reasoning processes that result in delusional explanations are normal.

Unless one-factor theory advocates can adequately address these issues, I think a more promising approach may involve abandoning the idea that the Maherian experience is the only factor in the aetiology of delusion and instead concentrate on searching for additional contributing factors.


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