folies à deux

Noun

folie à deux (plural folies à 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. (strictly) A rare syndrome in which a symptom of psychosis (particularly a paranoid or delusional belief) is transmitted from one person to another.