Countless horror films claim to be based on real-life events, but which of these really happened and which are telling us porkies in the name of good marketing? When so many claim to be based on facts, it’s hard to know which are telling the truth. We’ve examined some of the most popular scary films, from stories of serial killers to the oddly common exorcism of little girls, to uncover just how steeped in reality they really are.

Got an appetite for the grim and unsettling? Check out our list of the most disturbing documentaries next.