What is a public good? And the free rider problem

A public good, in economics, is something with two properties at once: it is non-excludable, meaning nobody can practically be kept from using it, and non-rival, meaning one person's use leaves just as much for everyone else. National defense, a fireworks show over a town and a street light are the classic examples. Because people can enjoy a public good without paying, each is tempted to let others pay, and markets tend to provide too little of it.

In short

• A public good is non-excludable (you can't keep people out) and non-rival (my use doesn't use it up for you). Most goods are neither.
• The free rider problem follows: if you get the benefit whether or not you pay, paying looks like a loss, so a good everyone wants may not get made.
• Societies pay for public goods with taxes, but also with advertising, donations, clubs and pledges that only count if enough people join.

What makes a good "public"?

Economists sort goods by two questions. Can people who don't pay be kept from using it? That is excludability. Does one person using it leave less for others? That is rivalry. OpenStax's Principles of Economics puts the definition plainly: "public goods have two defining characteristics: they are nonexcludable and non-rival."

The two questions give four kinds of good, not two:

	Rival: use by one leaves less for others	Non-rival: use by one leaves the same for others
Excludable: non-payers can be kept out	Private goods: a sandwich, a pair of shoes, a parking space	Club goods: a cinema screening, a streaming service, a toll bridge with no traffic
Non-excludable: non-payers can't be kept out	Common resources: fish in the open sea, groundwater, a free road at rush hour	Public goods: national defense, a lighthouse's beam, a flood barrier, a published mathematical theorem

A sandwich is the opposite of a public good: if you eat it, nobody else can, and the shop can refuse to hand it over until you pay. The theorem is a public good: once it's published, anyone can use it, and your using it costs nobody anything. Club goods, a term from the economist James Buchanan's 1965 paper on clubs, are non-rival up to a point but have a gate. Common resources have no gate but do get used up, which is the setting for the tragedy of the commons.

The economist Paul Samuelson set out the modern theory in 1954, in a short paper called "The Pure Theory of Public Expenditure", where he called them "collective consumption goods". Few goods are perfectly pure. A public park is non-rival on a quiet Tuesday and rival on a sunny holiday, and a road shifts from one box to another as traffic builds. The categories describe how a good behaves, not what it's made of, and technology can move a good across them: scrambling a TV signal turned broadcast television, a public good, into a club good.

What is the free rider problem?

The free rider problem is the gap between what a group wants and what each member, acting alone, will pay for. Since a public good can't be withheld from non-payers, each person can reason that their own payment makes little difference and they'll get the benefit anyway. A free rider is someone who takes the benefit without paying their share.

A worked example makes the trap concrete. Ten households share a dark cul-de-sac. A street light would cost $1,000 a year to install and run, and each household values it at $150 a year in safety and convenience.

1. Is the light worth it? Yes. Together the ten households value it at 10 × $150 = $1,500, which is more than the $1,000 it costs. Society is $500 a year better off with it.
2. Would anyone buy it alone? No. One household paying the whole $1,000 for a benefit of $150 would be $850 worse off.
3. Will they chip in? Someone suggests a voluntary fund of $100 each. Now each household thinks it through. If the other nine pay, the fund is $900, exactly $100 short, so if I chip in we get the light and I gain $150 − $100 = $50. Better still, if someone else covers the gap, I gain the full $150 and pay nothing. If too few others pay, my $100 is wasted either way.
4. What happens? Every household is tempted to wait for the others. If enough of them wait, the fund falls short and the street stays dark, though every single household would have been better off paying $100 for a $150 light.

Each household acts sensibly for itself, and the group ends up with nothing. That structure, where individual and group interests pull apart, is the prisoner's dilemma played by many people at once. It is a story about incentives, not about selfish people: even generous neighbors face the uncertainty of whether their $100 will be wasted.

How do societies pay for public goods?

There are more answers than "the government", and each fits some goods better than others.

• Taxes. A government can require everyone to contribute. In the street example, a town charging each household $100 gets the light built and leaves every household $50 better off. This is how national defense, courts and public health are usually funded. Tyler Cowen's Econlib entry notes that almost all economists think defense has to be paid for this way.
• Bundling. Sell something excludable and give the public good away with it. Broadcast radio and television are paid for by advertising, and shopping malls provide free lighting, security and benches because they raise what shoppers spend.
• Exclusion by technology or law. Find a way to put up a gate. Encryption turned broadcasts into subscriptions, and patents and copyright give inventors and authors a time-limited right to exclude.
• Pledges that only count if enough join. An assurance contract collects promises that are binding only if the total reaches the cost. In the street example, no household risks a wasted $100, because the money is spent only if all $1,000 arrives. Crowdfunding sites use a version of this.
• Donations and social pressure. Small groups where everyone knows everyone can shame free riders, and some public goods run on gifts: open-source software and Wikipedia are built largely by volunteers and paid for by donations.

The lighthouse, long the textbook case of a good no market could supply, shows how messy the boundary is. In a 1974 paper, The Lighthouse in Economics, the economist Ronald Coase showed that many British lighthouses from the 17th to 19th centuries were built and run privately, paid for by "light dues" collected from ships at nearby ports. Critics answered that those dues depended on royal patents and port authorities to collect them, so the government was involved all along. Both sides agree on the useful point: a public good can sometimes be made excludable by tying it to something that is.

Is a public good anything the government provides?

No, and this is the most common mix-up. Many things governments provide are private goods: a place in a state school, a bed in a public hospital or a public housing apartment is rival, since one person's place is another's lost one, and excludable, since entry is controlled. Governments provide them for other reasons, such as fairness or the benefits to others, which economists call positive externalities. Meanwhile some true public goods are provided privately, from free-to-air radio to the open-source code underneath most websites.

The everyday phrase "the public good", meaning the general welfare, is a different idea too. Economists use "public good" as a technical label for how a good behaves, and it says nothing on its own about who should provide it or how much is worth having.

Still argued

How much should be provided by government is argued, not settled. One side stresses free riding: without compulsory taxes, defense, basic research and clean air will be underfunded. The other, set out by writers such as Cowen and drawing on cases like Coase's lighthouses, stresses that private and voluntary arrangements solve more public-goods problems than textbooks suggest, and that governments have their own failures, from lobbying to spending decided by politics rather than value. Most economists accept both points and argue about where the balance lies for each good.

Where does the idea lead next?

Public goods are one of the standard cases where markets left alone produce the wrong amount, alongside externalities and monopolies. They sit in the economics of scarcity as a twist: the good itself isn't used up, but the money and effort to make it are. Their close relative, the shared resource that does get used up, leads to overfishing and depleted aquifers, and to the question of how communities manage them.

Check yourself

• Without looking back, explain the two tests that make a good public, and give one good that passes only one of them.
• In the street-light example, why might the light not get built even though everyone would gain from it?
• Why is a place in a public hospital not a public good in the economist's sense?

Sources

• 13.3 Public Goods — OpenStax, Principles of Economics 3e, 2022
• Public Goods — Tyler Cowen, The Concise Encyclopedia of Economics, Econlib, 2008
• Public good (economics) — Wikipedia
• The Lighthouse in Economics — Wikipedia, on Ronald Coase's 1974 paper
• The Free Rider Problem — Garrett Cullity, Stanford Encyclopedia of Philosophy, 2025