Public Goods

A public good is non-rival and non-excludable. Non-rival: your consumption does not reduce mine. Non-excludable: the provider cannot practically exclude non-payers from enjoying it. National defence, clean air, lighthouses, open-source software, and basic research fit the definition. Once the good exists, everyone can use it; no one can be barred. That creates a funding problem. If you cannot exclude non-payers, you cannot charge them. If you cannot charge them, voluntary provision fails. The market underprovides or does not provide at all.

Paul Samuelson formalised the concept in the mid-1950s. His point was mathematical: for a pure public good, the efficient quantity is where the sum of everyone's marginal benefits equals marginal cost. But no private firm can capture that sum through voluntary payment, because each user has an incentive to free-ride — to enjoy the good without paying. So private provision stops short of the socially efficient level. The result is underinvestment in things society values: defence, disease eradication, climate stability, fundamental science, open protocols.

The strategic implication is twofold. First, when you are analysing why something valuable is underprovided, check whether it has public-good character. If benefits are non-rival and non-excludable, market failure is the default. Second, when you are building or funding such a good, you need a mechanism that does not rely on voluntary payment: government provision, mandatory contributions, bundling with an excludable private benefit, or philanthropy. Wikipedia survives on donations and institutional grants, not on charging readers. GPS is provided by the U.S. government; the private sector builds applications on top. The good is free at the point of use; the funding comes from elsewhere.

Many goods are only partly public. A road is non-rival until it congests, then it becomes rival. A software library is non-excludable if it is open-source, excludable if it is proprietary. Club goods are excludable but non-rival within the club (e.g. a paywalled journal). The more excludable or rival a good becomes, the more likely private provision works. The purer the public good, the more you need a non-market solution.

In business, the public-good lens helps explain underinvestment in shared infrastructure. Industry standards, open protocols, and certification schemes benefit all participants but are underprovided by any single firm because the benefit is diffuse and non-excludable. The firms that do invest often do so strategically: they fund the good to build adoption or lock-in and capture value through complements (products, services, or membership) that are excludable. The model does not say that public goods are never provided privately; it says they are underprovided unless a non-market mechanism or a strategic bundling makes provision rational for at least one actor.