Analysis of Delusional Statements from 15 Japanese Cases of ‘Folie à Deux’

Background: Although many studies have reported folie à deux (FAD) cases, there has not yet been a study systematically examining patients’ delusional statements. In the present study, we analyzed the changes in patients’ delusional statements across the clinical course from the perspective of discourse analysis. Sampling and Methods: First, we presented a case of FAD in a married Japanese couple. Second, we examined 14 other cases of FAD from Japanese literature and analyzed changes in subjective pronoun (SP) use in each patient’s delusional statements. Results: Observed delusions of FAD were classified into the following two categories, based on the SP: (1) We-type: ‘We are persecuted’, and (2) Non We-type: mostly ‘I am persecuted’. Interestingly, We-type was generally observed in paranoid schizophrenia, paranoid disorder, and shared psychotic disorder. In contrast, Non We-type was predominantly observed in nonparanoid schizophrenia. Conclusions: The present classification might reflect two different mechanisms of delusional association in FAD. Possible mechanisms of delusional transmission in schizophrenia were discussed. The present study has a clear limitation in the small number of cases from only Japanese literature. Furthermore, the relatively high occurrence of FAD between married couples in the Japanese literature may have introduced some bias into the present results. Further studies on FAD are needed, however, the present method of examining the SP seems to be useful in order to investigate the psychological mechanism of delusion formation between plural subjects.