Public Good vs Private Good
Public Good and Private Good are two Market Failure & Government concepts in AP Economics that students often mix up. A public good is non-excludable and non-rival: no one can be excluded from it, and one person's use does not reduce another's. A private good is both excludable and rival: people can be prevented from using it, and one person's use reduces what is left for others. Here is how they compare side by side.
National defense and street lighting are classic examples. Because users cannot be excluded, markets underprovide public goods due to the free-rider problem. They are usually funded by government through taxation.
Most goods, such as food and clothing, are private goods. Markets generally provide them efficiently because sellers can charge a price and exclude non-payers. They contrast with public goods, which are non-excludable and non-rival.
Public vs Private Goods: Rivalry and Excludability
| Public good | Private good | |
|---|---|---|
| Rival in consumption | No. One person's use does not reduce what is left for others | Yes. A unit consumed by one person is gone |
| Excludable | No. Non-payers cannot practically be kept out | Yes. Sellers can withhold it from non-payers |
| Market outcome | Underprovided or not provided at all | Provided efficiently in a competitive market |
| Why the market fails | The free-rider problem | It does not, absent externalities or market power |
| Usual provider | Government, funded by taxation | Private firms, funded by prices |
| Examples | National defence, a lighthouse, a public radio broadcast | A sandwich, a haircut, a pair of shoes |
Two questions, asked in order, classify anything
Ask whether the good is rival: if I consume it, is there less for you? Then ask whether it is excludable: can a seller stop a non-payer from consuming it? A private good is yes and yes. A public good is no and no. The two mixed cases matter too and appear on exams: a common resource is rival but not excludable, like ocean fish, which is why it gets overused, and a club good is excludable but not rival, like a streaming service or a toll road below capacity. Note that public goods are defined by these two properties, not by who happens to provide them. A government-run bus service is a private good by this definition, because seats are rival and fares exclude.
The free-rider problem is the whole reason this category exists
If you cannot exclude non-payers, everyone has an incentive to enjoy the good without paying and let someone else fund it. Each individual is behaving rationally, and the collective result is that too little is produced or none at all, even when everyone would be better off with it. National defence is the standard example: it protects you whether or not you contributed, so a voluntary market would badly underfund it. This is a genuine market failure, meaning the free market outcome is inefficient rather than merely unequal, and it is the standard justification for tax-funded provision. Be precise on the exam: the problem is not that people cannot afford the good, it is that they cannot be made to pay for it.
Why the efficient quantity is found by adding vertically
For a private good, market demand is the horizontal sum of individual demands: at each price you add up the quantities each person wants. For a public good it is the vertical sum: because everyone consumes the same single unit, you add up what each person is willing to pay for that unit. The efficient quantity is where that summed willingness to pay equals marginal cost. This is why a public good can be worth providing even when no individual values it enough to fund it alone. If a question gives you two people's valuations of a bridge and the bridge's cost, add the valuations and compare to cost rather than comparing either one separately.
Frequently asked questions
What makes a good a public good?
Two properties together: it is non-rival, meaning one person's consumption does not reduce what is available to others, and non-excludable, meaning non-payers cannot practically be prevented from consuming it. National defence and a lighthouse are the standard examples. Being provided by a government does not by itself make something a public good.
Why do markets underprovide public goods?
Because of the free-rider problem. Since non-payers cannot be excluded, each person has an incentive to consume without contributing and let others fund it. If enough people reason that way, too little revenue is raised and the good is underprovided or not provided at all, even though everyone would prefer to have it.
Is education a public good?
Not by the technical definition. A classroom seat is rival, since one more student uses capacity, and it is excludable, since schools can turn people away. Education is usually described instead as a good with large positive externalities, which is a different market failure and calls for subsidy rather than direct public provision on free-rider grounds.
Related comparisons
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