A free rider, in the broad sense of the term, is anyone who enjoys a benefit provided, probably unwittingly, by others. In the narrow sense, a free rider is someone who receives the benefits of a cooperative venture without contributing to the provision of those benefits. A person who does not participate in a cooperative effort to reduce air pollution by driving less, for instance, will still breathe cleaner air—and thus be a free rider—if the effort succeeds.

In this sense, free riders are a major concern of the theory of collective action. As developed by economists and social theorists, this theory rests on a distinction between private and public (or collective) goods. A public good differs from a private good because it is indivisible and nonrival. A public good, such as clean air or national defense, is indivisible because it cannot be divided...