Folie a deux versus genetically driven delusional disorder: case reports and nosological considerations.

Folie a deux is thought to occur in persons who live close together, so that delusions of the primarily ill patient are adopted by the partner. Alternatively, if both patients are related, they may share the same genetically driven psychiatric illness. We present the cases of two psychotic couples (monozygotic twins and a mother and daughter pair, respectively), in which both subjects featured an almost identical psychopathological syndrome with the same delusional content and thus fulfilled the diagnostic criteria of folie a deux. When Leonhard's diagnostic system was applied, all four patients were found to suffer from affect-laden paraphrenia, arguing against the presence of folie a deux in those dyads. Thus, thorough psychopathological examination in suspected folie a deux settings can help to obtain valid diagnoses.