Folie à deux may seem familiar due to the Fall Out Boy album by that name, but in reality it is a real psychological phenomenon!
The phrase is French for “madness shared by two,” and is when two people share the same delusion, often transmitted from one to the other.
The same syndrome shared by more than two people may be called folie à trois, folie à quatre, folie en famille or even folie à plusieurs ("madness of many").
They all classify under the official names ‘shared psychotic disorder’ and ‘induced delusional disorder’. It’s largely still referred to by its original name in research literature.
There are two types of presentation this syndrome can take on.
The first is where a dominant person imposes their delusion on someone who otherwise wouldn’t have seen one.
The other is when all parties are previously disposed to have delusions. The syndrome is most commonly diagnosed when the two or more individuals concerned live in proximity and may be socially or physically isolated and have little interaction with other people.