Tuesday, 20 April 2021

Delusional Evidence-Responsiveness

This post is by Carolina Flores on evidence responsiveness in delusions. A paper on the same topic has been published open access in Synthese.

Carolina Flores

Delusions are deeply evidence-resistant. Patients with delusions are unmoved by evidence that is in direct conflict with the delusion, often responding to such evidence by offering obvious, and strange, confabulations. As a consequence, the standard view is that delusions are not evidence-responsive. This claim has been used as a key argumentative wedge in debates on the nature of delusions. Some have taken delusions to be beliefs and argued that this implies that belief is not constitutively evidence-responsive. Others hold fixed the evidence-responsiveness of belief and take this to show that delusions cannot be beliefs.

Against this common assumption, I argue that (for the most part) delusions are evidence-responsive—in the sense that subjects have the capacity to rationally respond to evidence on their delusion. This is compatible with the patient rarely successfully exercising that capacity in the actual world, and hence with routine delusional evidence-resistance. Analogously, I might have the capacity to run a sub-40 minute 10k, but rarely be well-rested, calm, and motivated enough to actually do so.

Looking closely, there are many behavioral signs of the capacity to rationally respond to counter-evidence to delusions. For example, patients put substantial effort into avoiding counter-evidence, suggesting that, if they were to acquire that evidence, it might force them to abandon the delusion, or require much effort to avoid revising it. If that evidence would have no effect, why avoid it? More decisively, in successful CBT, patients actually respond to counter-evidence to their delusion, and thereby come to abandon it. CBT’s relatively high success rate shows that many patients have the capacity to rationally respond to such evidence. Check out the paper for a much wider range of evidence, and more detailed discussion.

If patients have such capacities, how come they fail to revise their delusional beliefs in the face of counter-evidence? The reason is that internal factors—such as strange perceptual experiences, motivation to retain the delusion, and cognitive biases that require effort to override—mask these capacities. For example, many patients have persistent altered perceptual experiences which yield apparent evidence for their delusions. And patients are often highly motivated to maintain their delusion, either because it is a pleasant one, or because abandoning it would require the painful realization that something has gone seriously amiss with them. Delusions, then, result from the layering of capacities to respond to evidence and perceptual, motivational, and cognitive masks on those capacities—something, by the way, that is also true of ordinary beliefs.

As a result, we can hold both that belief is constitutively evidence-responsive and that delusions are beliefs. This does justice to the long-standing view that believing essentially involves tracking the way things are while acknowledging that real beliefs often fall drastically short of ideal rationality, and fitting the intuitive, and useful, classification of delusions as beliefs.

Perhaps most importantly, this view has significant implications for how we treat people with delusions. It entails that patients with delusions are not outside the space of reasons—even when it comes to their delusion. Delusions do not justify seeing the patient as only an object to be managed, controlled, and studied under an objective, coldly scientific glace. It can be appropriate to reason with the patient, work to understand the reasons for their delusion, and hold them to epistemic standards. Not only should we continue to invest in CBT for delusions, but we should also reconceive the social standing accorded to people with delusions.



  1. CBT is not as effective for delusions as claimed. In Jauhar et al.'s meta analysis CBT has a non-significant effect on positive symptoms including delusions in studies where outcome raters were masked to treatment group. You can get poor quality systematic reviews showing CBT is effective against delusions but the better quality ones show at most a small effect. This means that you cannot rely on CBT as proving that delusions can respond to evidence as CBT has little effect on delusions.


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