The streetlight effect is a type of observational bias where people only look for whatever they are searching by looking where it is easiest. The search itself may be referred to as a drunkard's search.
Taken from an old joke about a drunkard who is searching for something he has lost, the parable is told several ways but typically 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, 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."
David H. Freedman apparently coined the phrase "streetlight effect," but the story and concept were used in the social sciences since at least 1964, by Abraham Kaplan, where he refers to this as "the principle of the drunkard's search"  although an earlier version occurs in the Bulletin of the Atomic Scientists (1950) Volume 6, p9. and an even earlier version in 1945 US Senate Committee Hearings. An expanded version occurs in the Proceedings of the Forty-fifth Annual Meeting of the Alabama State Bar Association (1922) 
- David H. Freedman (August 1, 2010). "The Streetlight Effect". Discover magazine. 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 . PMID 25805758.
- Kaplan, Abraham (1964). The Conduct of Inquiry: Methodology for Behavioral Science. Transaction Publishers. p. 11. ISBN 9781412836296. Retrieved 2014-10-08.
- Bulletin of the Atomic Scientists. Educational Foundation for Nuclear Science, Inc. 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 (1922). Report of the Organization and of the... Annual Meeting of the Alabama State Bar Association. Smith & Armstrong. Retrieved 2017-01-08.
- 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.