We start with the standard DSM definition of delusions, and explain that clinical delusions are characterised by surface features of two kinds, epistemic (fixity, implausibility) and psychological (negative impact on functioning). Then we ask whether we can decide whether a type of belief is delusional by using those criteria. We consider three cases of belief that match at least some of the criteria: the belief that some thoughts have been inserted in one's mind by a third party; the belief that one has been abducted by aliens; and the belief that one is better than average at just about everything.
Discussing the similarities and differences among such beliefs -- Are they based on evidence? Are they common or rare? Do they bring any benefit? Do they help explain anomalous experience? -- is relevant to two projects that have been featured on this blog before: the Costs and Benefits of Optimism project and the PERFECT project. In the end, our goal is to show that neither the epistemic nor psychological features mentioned in the definition of delusions can help us sort delusional beliefs from non-delusional ones. Some delusions do not meet the criteria and some beliefs that we would not class as delusions do.
Rachel Gunn (who successfully completed her PhD at the University of Birmingham, working on delusions) examines the phenomenon of thought insertion by using very interesting first-person reports by people who used internet forums to share their unusual experiences. She observes that the experience affects people in different ways. For some but not for all the thought is felt like an unwelcome intrusion. For some but not for all the content of the thought also generates a compulsion to think or do certain things. Thought insertion is regarded as a symptom of schizophrenia, but whether it involves a pathological experience, whether it is a delusional belief, or whether it is harmful cannot be established without reflecting on the specifics of the individual case.
Here is one of the first-person reports Rachel analyses:
i truly do have unwanted thoughts that are forced into my head from somewhere... I mean I will have a thought saying my grandmother is a bitch. I would never ever think of my grandmother as a bitch. She is one of the greatest women I know and I adore her. So how is that a delusion? It is an intrusive thought! I sure didnt imagine it!.... i really do not think my grandmother is a bitch. i think these thoughts are evil and came from an evil being. Some thoughts however that pop into my head all of a sudden are my own thoughts and i can recognize that even though they are unwanted, but some are just plain ridiculous and mean and i know must be from an outside force.
Star-28, ‘Forum’, Mental Health Forum (2010)