In most investigations (skeptical, criminal and otherwise), two eyewitnesses are better than one. A second person brings a second perspective to the incident, and might have noticed different details than the first eyewitness. But having a second person corroborate an eyewitness account does not always bring us closer to understanding what happened. This is because sometimes one person’s interpretation will be adopted and reinforced by another person.
Sometimes two people-usually people who are related, have longstanding relationships, or have common interests and opinions-will reinforce each other’s beliefs. Two, three or more people can share in a collective delusion. Often the subjects report minor, odd experiences, such as strange, untraceable noises, feelings of skin crawling or nausea, etc.
Folie a deux is a form of mass hysteria, and though it is rare, it does occur and should be considered as a possible explanation for an event that could not have occurred as claimed, yet two eyewitnesses swear happened as described. In chapter 6 of my book Scientific Paranormal Investigation I describe an instance of folie a deux in a haunted house I investigated.
An excellent, fictional example of folie a deux can be seen in the 2006 horror film Bug, directed by William Friedkin (The Exorcist, Sorcerer) and starring Ashley Judd and Michael Shannon. In the film, a man begins having hallucinations about being attacked by insects and being watched; because he is sincere, a friend of his also begins experiencing his hallucinations. Because I have a background in psychology, I recognized that the writer clearly did his research on cases of delusional parasitosis. The film, which was adapted from a stage play, masterfully evokes paranoia, delusions, and claustrophobia. Not only is Bug a very good horror film, but it also provides a fairly realistic (though of course fictionalized) layperson’s look at these disorders.
For more on folie a deux, see Arnone, D., A. Patel, and G.M. Tan. 2006. The nosological significance of Folie a deux: a review of the literature, in Annual of General Psychiatry, 5:11; Shimizu, M., Y. Kubota, M. Toichi, and H. Baba. 2007. Folie a deux and shared psychiatric disorder, in Current Psychiatry Reports, 9:200.