Streetlight effect

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Not to be confused with Street light interference phenomenon.

The streetlight effect is a type of observational bias where people only look for whatever they are searching by looking where it is easiest.[1][2][3]

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."[2]

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).

Other Examples[edit]

Nasreddin's keys

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?"