The streetlight effect is a type of observational bias that occurs when people are searching for something and look only where it is easiest. Another term for this is 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:
David H. Freedman apparently coined the phrase "streetlight effect," but concept was used in the social sciences since at least 1964 when Abraham Kaplan refers to it as "the principle of the drunkard's search". An earlier version occurs in the Bulletin of the Atomic Scientists (1950), and an even earlier version in the 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)