Scarcity heuristic

In human psychology, the scarcity heuristic is a mental heuristic in which the mind values something based on how easily it may lose it, especially to competitors.

For example, take a group of boys playing marbles. Each player has at least one of every color marble except blue. Only one boy has a blue marble. By the scarcity heuristic, that boy and his playmates will value the blue marble more because there is only one, regardless of whether the blue marble is "better" (more aesthetically attractive, or better in the marbles game, for instance).