Some of these beliefs can be quite striking, such as believing you are dead or don’t exist – known as Cotard’s delusion, or believing that a close relative, usually a spouse, has been replaced by an identical looking impostor – known as the Capgras delusion.
These forms are relatively uncommon though, with the more prevalent types including (for example) the belief that you are being persecuted, or that people on the television or radio are talking about you.
Although the diagnostic criteria that define delusions describe them as false, fixed and culturally out-of-place beliefs, it is becoming increasingly clear that this is not an adequate definition.
For example, you can be pretty sure that ‘being dead’ is a false belief, but it’s much more difficult for a clinician to judge whether someone is or isn’t the subject of a conspiracy.
Furthermore, psychiatrist and philosopher Bill Fulford has pointed out that some cases of delusion may turn out to be true beliefs, noting that about 10% of cases of delusional jealousy involve actual infidelity.
Some beliefs diagnosed as delusional may not even be falsiable. For example, someone who has the distressing and unshakeable belief that “The devil is listening to my thoughts” cannot be proved wrong on the basis of any objective evidence.
The programme visits the researchers and discusses some of the pressing scientific issues and unusual beliefs they encounter.