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The parable is told several ways but includes the following details:
- 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, 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."
David Freedman apparently coined the phrase "streetlight effect," but the story, and concept, were used much earlier, e.g., by Abraham Kaplan, in his classic work The Conduct of Inquiry: Methodology for Behavioral Science (Harper & Row, 1964), where he refers to this as "the principle of the drunkard's search" (p. 11).