How likely is it that you father has been replaced by an imposter? Or that you are the Emperor of Antarctica? These beliefs are instances of delusions: fixed, irrational beliefs that are not amenable to change in the face of compelling evidence to the contrary.
In popular culture, delusions are considered to be the mark of madness, while psychiatry usually takes them to be the symptoms of a serious mental illness. However, in countertendency to the narrative that sees delusions as pathological, some researchers working in the field of mental health have argued that delusions are biologically adaptive, i.e., that they are part of an evolutionary mechanism devised to increase the chances of survival and reproduction of an organism in a given environment. More specifically, delusions would be answers to already existing problems of a biological or psychological nature rather than problems in themselves.
Among the proponents of delusions as an adaptive mechanism there are the predictive coding theorists Fineberg and Corlett, who, in a 2016 paper, have argued that schizophrenic and some neurological delusions such as Capgras would allow ongoing function in the face of paralysing difficulty (p. 73).
Here is an illustration of how Capgras delusion would be adaptive in their model. When someone suffers from a neurological impairment in the emotional processing of familiar faces, he expects to experience feelings of familiarity when looking at the face of a beloved person, but actually he does not get those feelings. This anomaly generates a mismatch between what one expects and what one perceives – a prediction error – which should be eliminated in order to keep the learning system going.
By coming to believe that one’s beloved person has been replaced by an imposter – what people with Capgras delusion believe - the prediction error is eliminated, and thus the learning system is rescued. As keeping the learning system going is key to surviving and reproducing in a given environment, Capgras delusion is biologically adaptive, because it allows this in the face of paralysing difficulty, i.e., a neurological impairment in the processing of familiar faces.
In my paper, I move two objections to Fineberg and Corlett’s view. The first is that their view is less parsimonious than the more traditional maladaptive view of delusions, and thus, ceteris paribus, the latter should be preferred. The second objection is that the claim that delusions are adaptive requires empirical evidence that Fineberg and Corlett do not provide.
Does this mean that the biological adaptiveness of delusions is doomed? I argue that a more definitive answer to this question can only come from empirical studies of a comparative kind, whose form I sketch out in my paper.