ultrafacts:

The Tinkerbell effect is an American English expression describing things that are thought to exist only because people believe in them. The effect is named after Tinker Bell, the fairy in the play Peter Pan, who is revived from near death by the belief of the audience.

Another form is called the Reverse Tinkerbell effect, a term coined by David Post in 2003. It stipulates that the more you believe in something the more likely it is to vanish. For example, as more people believe that driving is safe, more people will drive carelessly, in turn making driving less safe.

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