Folie à Deux

Folie à deux, which translates from French as 'dual madness' or 'madness of two' (and not 'madness for two' as Wikipedia erroneously claims), has lately been renamed as shared psychosis or Shared Delusional Disorder (SDD). The syndrome has not been included in the most recent edition of the Diagnostic and Statistical Manual of Mental Disorders, the DSM-5.

Here, I will continue to address the problem as folie à deux, as it is still the best known terminology. Folie à deux is a psychiatric syndrome in which the symptoms of a delusional belief (or belief system) is shared by two persons. It is not magically transmitted as some sources seem to think, but if both individuals are susceptible to the same delusion at the same time, both may suffer from the same psychiatric problems. One (the inducer) influences the other (acceptor).

This syndrome is most commonly diagnosed when the two or more individuals of concern live in close proximity, may be socially, physically or religiously isolated, and have little interaction with other people.

Even though folie à deux is a rare disorder, socially or physically isolated psychiatrists have managed to discover a number of sub-classifications, such as folie imposée, folie simultanée and folie partagée.

As with most psychological disorders, the extent and type of delusion varies, but the non-dominant person's delusional symptoms usually resemble those of the inducer. Prior to therapeutic interventions, the inducer typically does not realize that they are causing harm but instead believe they are helping the second person to become aware of vital or otherwise notable information. The cause or cause of folie à deux are still unknown, but the main contributors are pre-existing psychological issues, stress and social isolation.

The best known, most depressing case is that of the Swedish twin sisters Ursula Eriksson and Sabina Eriksson (both born 3 November 1967). Their behaviour quickly became more erratic in 2008. Sabina stabbed a man to death, fled and then jumped from a 12-metre high bridge. She was taken to hospital with two broken ankles and a fractured skull. Her sister tried to kill herself by running onto the motorway, which succeeded and a passing lorry crushed her legs. A distressing documentary was made of these incidents and can be watched here

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