The streetlight effect, or the drunkard's search principle, is a type of observational bias that occurs when people only search for something where it is easiest to look.[1] Both names refer to a well-known joke:

Three lamp globes on a park.
It is harder to find something on the part of the floor that is not well lit.

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".[2]

The anecdote is attributed to Nasreddin. According to Idries Shah, this tale is used by many Sufis, commenting upon people who seek exotic sources for enlightenment.[3] Outside of the Nasreddin corpus, the anecdote goes back at least to the 1920s,[4] and has been used metaphorically in the social sciences since at least 1964, when Abraham Kaplan referred to it as "the principle of the drunkard's search".[5] Noam Chomsky, for instance, uses the tale as a picture of how science operates: "Science is a bit like the joke about the drunk who is looking under a lamppost for a key that he has lost on the other side of the street, because that's where the light is. It has no other choice."[6]

See also



  1. ^ David H. Freedman (August 1, 2010). "The Streetlight Effect". Discover. Retrieved August 24, 2010.
    - Sufism/Nasrudin on Wikibooks
    - Battaglia, Manuela; Atkinson, Mark A. (April 1, 2015). "The Streetlight Effect in Type 1". Diabetes. 64 (4): 1081–1090. doi:10.2337/db14-1208. ISSN 0012-1797. PMC 4375074. PMID 25805758.
  2. ^ David H. Freedman (2010). Wrong: Why Experts Keep Failing Us. Little, Brown and Company. ISBN 978-0-316-02378-8 – via Google Books.
    - Sufism/Nasrudin on Wikibooks
  3. ^ Shah, Idries (1964). The Sufis. Doubleday. pp. 62. ISBN 9780385079662.
  4. ^ "'Did You Lose the Keys Here?' 'No, But the Light Is Much Better Here'", Quote Investigator April 4, 2013
    - "Bulletin of the Atomic Scientists". Bulletin of the Atomic Scientists: Science and Public Affairs. Educational Foundation for Nuclear Science, Inc.: 9 December 1960. ISSN 0096-3402. Retrieved January 8, 2017.
    - United States Senate Committee on Military Affairs (1945). Hearings on Science Legislation (S. 1297 and Related Bills): Hearings Before a Subcommittee of the Committee on Military Affairs, United States Senate, Seventy-ninth Congress, First Session, Pursuant to S. Res. 107 (78th Congress) and S. Res. 146 (79th Congress) Authorizing a Study of the Possibilities of Better Mobilizing the National Resources of the United States. U.S. Government Printing Office. Retrieved January 8, 2017 – via Google Books.
    - Alabama State Bar Association (July 1926). Report of the Organization and of the... Annual Meeting of the Alabama State Bar Association. Smith & Armstrong. p. 94. Retrieved January 8, 2017 – via Google Books.
  5. ^ Kaplan, Abraham (1964). The Conduct of Inquiry: Methodology for Behavioral Science. Transaction Publishers. p. 11. ISBN 9781412836296. Retrieved October 8, 2014 – via Google Books.
  6. ^ Barsky, Robert F. (1998). Noam Chomsky: a life of dissent (Repr. ed.). Cambridge, Mass.: MIT Press. p. 95. ISBN 0262024187. Retrieved July 23, 2022 – via Google Books.

Further reading