Explain why non-excludability is not a sufficient condition for a public good.

• A. Non-excludability alone guarantees a public good.
• B. Non-excludability always means non-rivalry.
• C. Non-excludability plus non-rivalry is required; some non-excludable goods may be congestible or rival.
• D. Non-excludability implies markets will always underprovide.

Answer: (C)

Non-excludability alone does not make a good a public good. A true public good must be non-excludable and non-rival in consumption. If a good is non-excludable but rivals as more people use it, then one person’s use reduces another’s enjoyment or access, so it isn’t a pure public good. For example, a road can be hard to exclude people from using, but when many drivers are on it, each additional driver slows everyone else, creating congestion and making the good rival. In contrast, a good that remains non-rival even as more people use it—like a freely available broadcast at normal levels—fits the public-good idea. Because non-excludability by itself doesn’t ensure non-rivalry, it’s not sufficient to classify something as a public good. That’s why the correct view is that both properties are required, and some non-excludable goods can be congestible or rival.