folie à deux

plural folies à deux · alternative folie a deux · alternative rare folly à deux · alternative rare folly a deux

1. A situation in which two people behave abnormally together in a way that neither of them would individually.

   And so it came to pass. Ann’s psychiatrist at the time came to believe that the pair were psychotic, and diagnosed Ann with folie à deux, a disorder where two people reinforce each other’s mental illness until it reaches extreme proportions.

   2025 October 8, Stuart Heritage, “‘Their chemistry made one of their deaths inevitable’: how a billionaire couple’s life in paradise turned deadly”, in The Guardian, →ISSN

2. strictlyhuman sciencesA rare syndrome in which a symptom of psychosis (particularly a paranoid or delusional belief) is transmitted from one person to another.