Somatic delusions as motivated beliefs?

When seeking to explain any case of monothematic delusional belief, it has proven fruitful to ask two questions. What prompted the delusional idea in the first place? And, given that there is generally much evidence against this idea, why is it nevertheless adopted as a belief? For at least six types of delusional beliefs (Capgras Delusion, Cotard Delusion, Fregoli Delusion, somatoparaphrenia, mirrored-self misidentification and delusion of alien control), it has been possible to identify answers to these two questions, and hence to explain what causes each of these delusions (Coltheart, 2007; Coltheart et al., 2011).

For all six of these delusions, the answers to the two questions have been frankly neuropsychological in nature. In each case, there is some localized region of the brain where neuropathology has been identified which plausibly explains why the specific delusional idea first arose, and another localized region of the brain with identified neuropathology (there is a reason to believe that this region may be right dorsolateral prefrontal cortex; Coltheart et al., 2018) has been identified which plausibly explains why this idea came to be accepted as a belief.

However, there are forms of monothematic delusion which occur even when there is no evidence of such localized neuropathologies in the brain: for example, erotomania, folie a deux and the delusion of alien abduction. Might the two-factor theory of delusional belief apply even in cases where this no localized neuropathology, such as the case reported by Connors and Lehmann-Waldau (2018)?

One might ask both questions in relation to this case.

What could possibly have caused this man to entertain the idea that his penis had been stolen and replaced with someone else’s?

And given not only the implausibility of this idea but also the data the patient himself collected that conflicts with the idea (‘he cut his penis and poured boiling water on [his penis]. He found these acts painful and was unable to explain how he could feel sensation from the supposedly foreign penis attached to him’), why was this idea nevertheless adopted as a belief?

We have seen a somewhat similar somatic delusion in a young man Patient X, who believed that his gums were ‘rotting’. Again, one can ask: (a) where did this idea come from and (b) why was it that no amount of inspecting his gums in the mirror weakened his belief, and nor did his being told by several oral pathologists he consulted that his gums were normal?

Some progress here might be achieved by acknowledging that people in general sometimes adopt beliefs for motivational reasons and by proposing that motivational factors might be at work in some cases of delusion. Prior to the onset of his delusion, Patient X had applied to go to University but had not been accepted, though other family members had succeeded in such applications. This had disappointed him greatly and triggered a depression and a general feeling of inadequacy and of not coping with life.

If it were the case that he had a disfiguring physical condition which he would not want others to see, then that would provide him with a reason why there was an obstacle to his going to University, a reason that did not reflect any personal lack of ability. So the answer to our second question ‘Given that there is generally much evidence against this idea, why is it nevertheless adopted as a belief?’ may have been that his adoption of the belief about his gums allowed him to interpret his not going to University as something that did not imply a personal rejection.

However, a belief about any kind of illness would serve this purpose. What prompted the idea that it was, specifically, a disfigurement of his gums? We don’t know. But he did tell us that his gum disease started in his late teens after he’d cut his gum while he was eating. If this accidental injury to his gum really did occur, it could have been this experience which prompted the specific delusional idea that his gums were disfigured.

A similar two-factor motivated-belief account might be offered in the case of the patient described by Connors and Lehmann-Waldau (2018). That patient had had long-standing concerns involving insecurities with women. If these took the form of concerns about sexual inadequacy and specifically a belief that his own penis was too small, such concerns would no longer apply if it were the case that the penis he could see attached to his body was not his own but someone else’s. So the answer to our second question ‘Given that there is generally much evidence against this idea, why is it nevertheless adopted as a belief?’ may have been that his adoption of this belief about the penis resolved his distress about the size of his own penis.