The streetlight effect is named after an old 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.”

It is also called the drunkard’s search.

Joke

Whether we find any given joke funny depends on the joke and it depends on us — and the experiences, beliefs and background knowledge that we bring to it.

We get the point of this joke to the extent that we understand that the drunkard is looking for something at place where he already knows it is not — it is obvious that he cannot possibly find what he is looking for there.

Perhaps we even feel a brief sense of superiority: This is so clear to us – the drunkard truly had his senses dulled!

Drunkard

The person scrambling about, looking for his keys is contending with a few challenges:

1. He lost his keys.
2. He is intoxicated (and not thinking clearly).
3. He believes he is thinking clearly.
4. He is looking for his keys at the wrong place.

Not to dwell too much on the obvious, but he is looking for his keys at the wrong place, because that is where the light is. Put differently: He is looking, where he can see.

The keys are, where he cannot see. We are blind to the places that we cannot see. The joke is illustrative, but this is not about that, it is about the streetlight effect. Lights in the dark will attract our attention.

This joke is in fact probably on you (and me).

Blind spots

Our perspectives depend on where we look. The looking is easiest, where our vision is best. The streetlight effect is a useful name for the observational bias we experience, when we look for information or solutions only at the places, where we can see best. We focus on what we can see, perhaps fully ignoring what is in darkness — and likely not even realizing the fact of our ignorance.

Sometimes that is, where the correct answer can be found. Often we could really benefit from additional light.