Shared psychosis, known to many as folie à deux, is an uncommon psychiatric disorder in which two people feed off of one another’s delusional beliefs or hallucinations. It was first conceptualized in 19th Century French psychiatry by Charles Lasègue and Jean-Pierre Falret.

Essentially, one person can have a delusion and, through the positive reception of another, can further that delusion if both parties are decently isolated from society. That delusion can fester and grow into something far worse than what might have arisen if the two parties were apart.

Take Joseph Kallinger and his son, Michael. In 1974, Kallinger took this then 15-year-old son on a crime spree throughout Philadelphia, Baltimore, and New Jersey where the two robbed, assaulted, and sexually abused four families and murdered three people.

Almost six months later, the two started out again on January 8 when they invaded a home in Leonia, New Jersey. There, the Kallingers bound three residents, four more who arrived later, and killed the eighth after she reprimanded the elder Kallinger. After one captive escaped and was able to call for police, the Kallingers fled on a city bus, dumping a bloody shirt and the weapon along the way.

Unfortunately for the father-son crime duo, they were caught and arrested. Joseph, who had been previously diagnosed with paranoid schizophrenia, plead insanity. He was found sane and sentenced to life in prison in 1976.

Michael, being a child at the time, was judged to be under his father’s control, though a willing participant in the crimes. After being released at the age of 21, Michael moved out of state and changed his name. Joseph made several suicide attempts while in prison, but never succeeded, dying of heart failure in 1988.

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Another, more recent example of folie à deux, is one you’ve probably heard about. On May 21, 2014, in Waukesha, Wisconsin, two 12-year-old girls, Anissa Weier and Morgan Geyser, lured their friend of the same age, Payton Leutner, into the woods and stabbed her 19 times in order to appease the fictional, and super creepy, character Slender Man.

Anissa and Morgan, two malleable young girls isolated by bullying, came together and bonded over their love of the character from Creepypasta. As their fandom turned into obsession, the two girls decided that they needed to run away from their current lives and into the world of Slender Man. In order to do that, they would have to offer their friend as a sacrifice.

Payton, though she suffered serious injury, survived the vicious, premeditated attack. Both girls initially plead not guilty by insanity, of which they were found, despite later pleading guilty in a plea deal. They will both spend at least three years in a mental hospital.

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But, perhaps the most severe example of shared psychosis is also probably the most disturbing. In 1977, while in prison, Lawrence Bittaker and Roy Norris became loosely acquainted. By 1978, the pair had grown close and soon discovered that they shared a common interest in sexual violence and misogyny. As they shared fantasies, their desires escalated into plans to assault and murder young women once they had been released from their sentences.

Sure enough, upon release, Bittaker and Norris got to work. During 1979, the two picked up at five different women in a van they had nicknamed “Murder Mac,” where they beat, raped, and tortured them with objects you’d find in a toolbox, earning their moniker: The Toolbox Killers.

Their last victim, Shirley Lynette Ledford, had to suffer not just twisted and sadistic methods of torture, but the indignity of having it recording it on a particularly gruesome cassette tape. While it was played at his trial, Bittaker smiled at his handiwork.

Bittaker was sentenced to death for the five murders he committed with Norris. He currently resides at San Quentin State Prison. While Norris accepted a plea deal where he agreed to testify against his former co-killer and was sentenced to life imprisonment at Richard J. Donovan Correctional Facility.

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Would any of these horrific acts occurred if one hadn’t met the other? Were any of these individuals dangerous on their own, or were they only turned into monsters by their new best friend? It’s hard to say.

So, the next time you hang out with your best friend, you might want to invite a third or fourth. Just in case things start to get a little … dark.