Which of the following best defines a pure public good?

• A. A good whose benefits are enjoyed by many at the same time and from which consumers cannot be excluded
• B. A good that is excludable but nonrival
• C. A good with high private demand but zero externalities
• D. A good that is rival in consumption

Answer: (A)

A pure public good is defined by being non-excludable and nonrivalrous: many people can consume it at the same time, and there’s no practical way to prevent someone from enjoying the benefit. The description that says benefits are enjoyed by many at the same time and from which consumers cannot be excluded matches both of these features, so it is the best definition.

The other descriptions describe goods that are excludable (you can prevent some people from using them) or rival in consumption (one person’s use reduces another’s), which are not characteristics of a pure public good. For example, a club good is excludable but nonrival, and a rival good can’t be consumed by everyone simultaneously. A brief example: national defense illustrates a pure public good, since everyone benefits and one person’s benefit doesn’t exclude others.