Streetlight Effect
Drunkard's Search
A type of observational bias that occurs when people only search for something where it is easiest to look. Named for the well-known joke where a policeman sees a drunk man searching for something under a streetlight and asks what the drunk has lost. He says he lost his keys and they both look under the streetlight together. After a few minutes the policeman asks if he is sure he lost them here, and the drunk replies, no, and that he lost them in the park. The policeman asks why he is searching here, and the drunk replies, "this is where the light is."
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Origin
The original anecdote is attributed to Nasreddin Hodja, a Muslim world folkore character from the 13th century. The anecdote was then used metaphorically in the social sciences since at least 1964, when Abraham Kaplan referred to it as "the principle of the drunkard's search".