Clinical Delusions: What Are They? (by guest blogger Lisa Bortolotti)

(Lisa Bortolotti is a Senior Lecturer in Philosophy at University of Birmingham and author of Delusions and Other Irrational Beliefs.)

In the last five years I have been working on the nature of clinical delusions, and have asked what they can tell us about the philosophy and psychology of belief. Clinical delusions are a symptom of a variety of psychiatric disorders, among which are schizophrenia and dementia. Some delusions have fairly mundane content, such as delusions of persecution or jealousy. Other delusions are very bizarre, and people may come to assert that they are dead (Cotard delusion) or that their spouse or family member has been replaced by an impostor (Capgras delusion).

In the Diagnostic and Statistical Manual of Mental Disorders (American Psychiatric Association DSM-IV-TR, 2000), delusions are defined in epistemic terms, as beliefs that are false, insufficiently supported by the available evidence, resistant to counterevidence and not shared by other people belonging to the same cultural group. In the philosophical literature it is an open question whether delusions should be considered as genuinely instances of belief.

According to the two-factor theory of delusions, delusions are explanatory hypotheses for an abnormal experience which is due to brain damage. The first factor contributing to the formation of the delusion is a neuropsychological deficit and the second factor is an impairment in the evaluation of hypotheses. Imagine that, overnight, Julia’s sister appears different to Julia, and this is powerful experience. One possible explanation is that an alien has abducted Julia’s sister during the night and replaced her with an almost identical replica without anybody else noticing. This hypothesis is implausible and even Julia would consider it as highly improbable, but if her hypothesis-evaluation system doesn’t work properly and doesn’t dismiss it, Julia may endorse it as something she truly believes. As a result, she may become hostile and even aggressive towards the alleged impostor.

Even from the oversimplified example above, one can see where the tension is. On the one hand, delusions seem to be just like any other beliefs. They are reported with sincerity and they can affect the person’s other intentional states and behaviour. They “make sense” of a very unusual experience. On the other hand, there is a neuropsychological deficit at the origin of delusions that is not present in the non-clinical population. The affective channel of Julia’s facial recognition process is damaged. The good functioning of the hypothesis-evaluation system is also compromised, maybe due to exaggerated versions of common reasoning biases. Julia “jumped to conclusions” as she accepted her initial hypothesis on the basis of insufficient evidence and without considering other, more probable, alternatives. This unusual aetiology and the apparent extreme irrationality might seem to be in tension with the view that delusions are “beliefs” in the ordinary sense of that term.

However, in my view, the main difference between clinical delusions and other irrational beliefs is that delusions severely undermine well-being. People with schizophrenia are often isolated and withdrawn and their life plans are disrupted. But on purely epistemic grounds we can’t easily tell delusions apart from the false beliefs that we ourselves report and ascribe to others on an everyday basis, such as: “Women make poor scientists” or “I failed the exam because the teacher hates me”. Irrationality is indeed all around us.