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 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".
- One late evening Nasreddin found himself walking home. It was only a very short way and upon arrival he can be seen to be upset about something. Alas, just then a young man comes along and sees the Mullah's distress.
- "Mullah, pray tell me: what is wrong?"
- "Ah, my friend, I seem to have lost my keys. Would you help me search them? I know I had them when I left the tea house."
- So, he helps Nasreddin with the search for the keys. For quite a while the man is searching here and there but no keys are to be found. He looks over to Nasreddin and finds him searching only a small area around a street lamp.
- "Mullah, why are you only searching there?"
- "Why would I search where there is no light?"
- 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.
- http://en.wikibooks.org/wiki/Sufism/Nasrudin Missing or empty
- Kaplan, Abraham (1964). The Conduct of Inquiry: Methodology for Behavioral Science. Transaction Publishers. p. 11. ISBN 9781412836296. Retrieved 2014-10-08.
- 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.