The Tinkerbell Effect refers to things that only exist because people believe in them- like taking a placebo pill and feeling better because you’ve convinced yourself it was real medicine. There’s also a Reverse Tinkerbell Effect, where the more you believe in something the more it’s bound to vanish – like being so convinced it’s safe to drive that cars actually become more dangerous because your illusion of safety causes you to drive with less caution. Source Source 2 Source 3