ABSTRACT
Public, or collective, goods are characterized by non-excludability in use, non-rivalry in consumption, or both these characteristics combined. A good is non-excludable if it is costly and counterproductive to exclude individuals from using it. It is non-rivalrous if one person's consumption cannot reduce or impede the ability of others to consume the good. When individuals do not take into account the marginal social cost of consumption, the public good becomes congested. Generally, a public good is also indivisible; one cannot meaningfully speak of units of a public good (e.g., ambient air).
