Markets work best when goods have clear owners, clear prices, and buyers who pay for what they use. Some important resources do not fit that pattern, which is why economists study public goods and common resources. These goods matter because they include parks, clean air, fisheries, roads, flood control, and national defense.
When markets miss their value or fail to protect them, society can get too little of some goods and too much use of others.
Key Facts
- Excludability asks whether people can be prevented from using a good.
- Rivalry asks whether one person’s use reduces what is available for others.
- Public goods are non-excludable and non-rival, such as national defense or a lighthouse signal.
- Common resources are non-excludable and rival, such as open-access fisheries or shared groundwater.
- Free-rider problem: people can benefit without paying, so private markets may underprovide public goods.
- Tragedy of the commons: individual users have an incentive to overuse a shared rival resource, causing depletion.
Vocabulary
- Public good
- A public good is a good that is difficult to exclude people from using and that one person can use without reducing another person’s use.
- Common resource
- A common resource is a good that is difficult to exclude people from using but is reduced when one person uses it.
- Excludability
- Excludability is the ability to prevent people who do not pay from using a good or service.
- Rivalry
- Rivalry means one person’s use of a good reduces the amount or quality available to others.
- Free rider
- A free rider is someone who receives the benefit of a good without paying for its cost.
Common Mistakes to Avoid
- Calling every government-provided service a public good is wrong because public goods are defined by non-excludability and non-rivalry, not by who provides them.
- Confusing public goods with common resources is wrong because public goods are not used up by one more user, while common resources are rival and can be depleted.
- Assuming free markets always provide the efficient amount is wrong because free-riding can make public goods unprofitable for private sellers even when society values them.
- Ignoring incentives in common resources is wrong because each user may gain personally from extra use while spreading the cost of depletion across everyone.
Practice Questions
- 1 Classify each good as a private good, club good, public good, or common resource: a toll road with little traffic, national defense, ocean fish in international waters, and a slice of pizza.
- 2 A village has 100 residents. A flood warning siren costs 30. What is the total social benefit, and should the village buy the siren if decisions are based on total benefit versus total cost?
- 3 A lake can sustainably support 1,000 fish caught per month, but 50 fishers each catch 30 fish per month. How many fish are caught, by how much does this exceed the sustainable level, and what policy could reduce overuse?
- 4 Explain why a lighthouse can create a free-rider problem, and describe one way a community or government could fund it fairly.