The "Free Rider Problem" (or "Free-Rider Problem") is faced when a good is provided to a group, but not every individual within this group is obligated to aid in the provision of this good, despite the fact that every individual benefits from it.
A common Free Rider problem is national defense: everyone within a polity benefits from the protection its armed forces provide, but only a fraction of the total population actually provides resources to the country's defenses.
The severity of the problem is a reflection of the ratio of contributors to diminishers.
Other "problems" that are commonly encountered in either Game Theory or the social sciences:
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