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. Both names refer to a well-known joke:
- 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".
The anecdote goes back at least to the 1920s, 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". The anecdote has also been attributed to Nasreddin. According to Idries Shah, this tale is used by many Sufis, commenting upon people who seek exotic sources for enlightenment.
- David H. Freedman (August 1, 2010). "The Streetlight Effect". Discover. Retrieved 2010-08-24.
- David H. Freedman (2010). Wrong: Why Experts Keep Failing Us. Little, Brown and Company. ISBN 0-316-02378-7.
- Sufism/Nasrudin on Wikibooks
- Battaglia, Manuela; Atkinson, Mark A. (2015-04-01). "The Streetlight Effect in Type 1". Diabetes. 64 (4): 1081–1090. doi:10.2337/db14-1208. ISSN 0012-1797. PMC 4375074. PMID 25805758.
- "'Did You Lose the Keys Here?' 'No, But the Light Is Much Better Here'", Quote Investigator April 4, 2013
- Bulletin of the Atomic Scientists. Educational Foundation for Nuclear Science, Inc. December 1960. p. 9. ISSN 0096-3402. Retrieved 2017-01-08.
- United States. Congress. 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 2017-01-08.
- Alabama State Bar Association (July 1926). "daddy%27s+watch" Report of the Organization and of the... Annual Meeting of the Alabama State Bar Association. Smith & Armstrong. p. 94. Retrieved 2017-01-08.
- Kaplan, Abraham (1964). The Conduct of Inquiry: Methodology for Behavioral Science. Transaction Publishers. p. 11. ISBN 9781412836296. Retrieved 2014-10-08.
- Shah, Idries (1964). The Sufis. Doubleday. pp. 70. ISBN 9780385079662.
- Iyengar, Shanto (1993). "The Drunkard's Search". Explorations in Political Psychology. Duke Studies in Political Psychology. Duke University Press. ISBN 978-0-8223-1324-3.
- Popkin, Samuel L. (1991). "Going beyond the data". The reasoning voter: communication and persuasión in presidential campaigns (2nd ed.). University of Chicago Press. pp. 92–95. ISBN 978-0-226-67545-9.