Folie à Deux
Folie à deux is also known as Shared Psychosis or Shared Delusional Disorder (SDD). Those afflicted with SDD experience shared delusions, usually with someone close to them, such as a spouse or family member. In some cases, the delusions are shared by more than two people and are, therefore, called folie à famille (the folly of family).
Source: Pinterest
There have been many recorded cases of SDD throughout history, though rarely have they ended in such tragic and baffling circumstances as the case of the Eriksson twins. What possessed the twins to act the way they did on that fateful day?