Mass hallucination

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A mass hallucination is a phenomenon in which a large group of people, usually in physical proximity to each other, all experience the same hallucination simultaneously. Mass hallucination is a common explanation for mass UFO sightings, appearances of the Virgin Mary, and other paranormal phenomena.

In most cases, mass hallucination refers to a combination of suggestion and pareidolia, wherein one person will see, or pretend to see, something unusual (like the face of Jesus in the burn-marks on a tortilla, or the face of a kidnapped girl on a blank billboard) and point it out to other people. Having been told what to look for, those other people will consciously or subconsciously convince themselves to recognize the apparition, and will in turn point it out to others.

It is commonly used in Science-Fiction stories as a means of preventing the human race from becoming aware of alien existence after a major event, with almost the entire population of Earth convincing themselves that such events as the Vogon Constructor Fleet or Battle between Daleks and Cybermen were nothing more than hallucinations caused by drugs planted in the water supply.

See also: Power of suggestion