Called ‘shared psychotic disorder’ in DSMIV and ‘induced delusional disorder’ in ICD10, this phenomenon occurs when mental symptoms, usually delusions, are communicated from a psychiatrically ill individual to another individual, who accepts them as truth. The two people are nearly always closely associated, especially husband and wife, siblings, or parent–child dyad (Silveira and Seeman, 1995), and social isolation is usually present (Layman and Cohen, 1957; Silveira and Seeman, 1995). The content of shared belief varies from case to case and includes feelings of persecution (Fernando and Frieze, 1985; Kendler and colleagues, 1986; Brooks, 1987), delusional parasitosis (Gieler and Knoll, 1990; Musalek and Kutzer, 1990), conviction of having a child who does not exist (Fishbain, 1987), misidentification of the Capgras type (Hart and McClure, 1989; Ananth, Kaur and Djenderedjian, 1990), and many others. Myers (1988) discusses the phenomenon of shared persecutory beliefs in a cult setting, and refers to Cameron's (1959) description of the ‘paranoid pseudocommunity’ – the ever-present ‘they’ who, so far as the patient is concerned, are carrying out the persecution which is not apparent to others.

Folie à deux is not common, but neither is it unduly rare. In 1974, Soni and Rockley reported on 162 cases they had traced up to that time in the English language literature, and Silveira and Seeman (1995) described 123 cases identified from 1942 to 1993.