Curiosity: Psychotic condition could be transmitted.

When you live in close proximity to someone, it's common for illness to spread from one person to another. But mental illness? It's possible. The phenomenon is technically called shared psychotic disorder, but it's most famously known as folie à deux.

The first case of the condition was documented in the 19th century, and described a 30-something married couple, Margaret and Michael. The couple shared a delusion that people were sneaking into their house at night spreading dust, dropping pieces of fluff, and wearing down the soles of the couple's shoes.


In another instance, not two, but three sisters experienced what could be called "folie à trois." Two of the sisters moved into a house near a third sister to help her care for her children, and over time, all three became closer and more religious. At one point, the youngest began believing that there were troubling discrepancies between different versions of the Bible, and became determined to make them right. For three days, the sisters prayed nonstop without sleeping until they believed that God wanted them to have a particular house in the town. Even though the house didn't belong to them, the sisters went to the house and demanded to be let in, even breaking windows and attacking the occupant until police arrived.

According to a review published in the Canadian Journal of Psychiatry, shared psychotic disorder most often affects people in very close relationships, such as married couples, siblings, and parents and children. They're also usually socially isolated, and often have pre-existing mental illness. The condition takes several forms, the most common and oldest known of which is "folie imposée," or "imposed madness." In that form, the more dominating person in the pair spreads his or her delusion to the more submissive, who doesn't resist the ideas.

There's also "folie simultanée", or "simultaneous madness," where two people with a deep connection both experience the delusion at once; "folie communiquée," or "communicated madness," which is like imposed madness except there's a period of resistance from the second person; and "folie induite" or "induced madness," which is like communicated madness except that extra delusions are spread from a second person.

Luckily, in most of these forms, the cure is simple: just separate the two people. When that doesn't work — which, especially in cases of folie communiquée and folie induite, it may not — psychiatrists can resort to medication or electroconvulsive therapy. But, as Esther Inglis-Arkell of io9 points out, the tendency to share mental eccentricities isn't always bad — and we all do it. "There are few old married who don't share eccentricities. There are few families, or even close friendships, that don't require both people to work with the various mental glitches of the other. We all go a little crazy for the other people in our lives."

Curiosity.