Free-Rider Problem
Etymology
The concept was crystallized by economist Mancur Olson in “The Logic of Collective Action” (1965), though the “free ride” idiom for getting something without paying is older American slang. The metaphor is the passenger who rides without buying a ticket.
noun phrase · /FREE-ry-der PROB-lum/
The tendency for people to enjoy a shared benefit without paying for it, leaving it underfunded or undersupplied.
What it names
A handle for why goods that everyone benefits from — clean air, public radio, herd immunity — are chronically hard to fund, because each person is tempted to let others foot the bill.
Example
If everyone assumes a neighbor will organize the cleanup, the free-rider problem leaves the park a mess.
Related idea
It is the engine behind the tragedy of the commons: the commons is depleted precisely because each user free-rides on a shared resource no one is paid to protect.