Here’s a famous definition of delusion:
‘A belief held with unusual conviction that is unamenable to logic whose erroneousness is manifestly obvious to others’ – Jaspers (1959)
This came to mind the other day when I was reading about Mohamed Al Fayed’s peformance in court at the inquest into the death of Princess Diana. Al Fayed spent time outlining the extent of the international conspiracy which had been involved her death and that of his son, stating that conspirators included Tony Blair, Robin Cook, MI5, MI6, the CIA, the French intelligence service and the French ambulance service, who drove to the hospital deliberately slowly so that she might die.
This doesn’t strike me as very likely and this view appears to be shared by the inquiry’s coroner, who asked Mr Al Fayed: ‘Do you think that there is any possibility, however remote that your beliefs about conspiracies may be wrong and that the deaths of Dodi and Diana were in truth no more than a tragic accident?’
Fayed replied: ‘No way. I am 100% certain’.
I think this would count as ‘unusual conviction’. In ICD-10, there is a diagnosis of ‘delusional disorder‘, which is defined by the presence of persistent, non bizarre, delusions. A non-bizarre delusion is plausible; this is in contrast to a bizarre delusion which is not. For instance a person who thinks that they are under survelliance by the security force may be delusional, but this does happen to a small number of people. This is non-bizarre; a person would hold a bizarre delusion if this had no chance of being true, for instance if they felt that there was a goat living on their head. Delusions also need to be outside what is considered to be culturally accepted for instance, in isolation, some religious practices might be considered odd, but they are widely accepted and so not delusional.
If you were to meet a person with a delusional disorder you might not notice anything obviously odd about them. This is in contrast to someone who is suffering from a psychotic delusional disorder, when their behaviour may appear manifestly odd. They are able to continue functioning normally, although may make some strange decisions based on their world view.
Finally, sometimes people are labelled as being delusional, when in fact they are not. This is called the Martha Mitchell Effect this is when a psychiatrist mistakes a patients perception of real events as delusional and misdiagnoses accordingly. It is named after the wife of the attorney general in the Nixon administration who alleged that White House staff were engaged in illegal activities. Her claims were attributed to mental illness, but the outcome of the Watergate scandal vindicated her.