Streetlight effect

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. 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".