Public Goods: Two Defining Characteristics
Defines a public good using its two defining characteristics -- non-excludability and non-rivalry -- and illustrates both with the classic lighthouse example. The key insight is that a good must satisfy both conditions simultaneously to be a true public good, distinguishing it from merely publicly-provided services like healthcare. Contains: text explanation, a worked example applying the lighthouse case, a key_concept callout summarising the two conditions, and a common-mistake callout on confusing public goods with public services.
A public good is a good or service that is both non-excludable and non-rival in consumption. These two characteristics together make public goods fundamentally different from the private goods that markets normally allocate efficiently, and understanding each one precisely is the foundation for explaining why public goods are a source of market failure.
Non-excludability means it is impossible, or prohibitively expensive, to prevent anyone from consuming the good once it is provided -- including people who have not paid for it. National defence is a clear example: a government cannot selectively protect only tax-paying citizens while leaving non-payers exposed. Everyone within the country's borders is protected, whether they contributed to funding the armed forces or not.
Non-rivalry means one person's consumption of the good does not reduce its availability to anyone else. The marginal cost of supplying the good to one additional consumer is zero. Street lighting illustrates this well: whether one pedestrian or a hundred walk beneath a lamp on a given night, the cost of providing that light is unchanged, and no one's use diminishes the amount of light available to others.
The lighthouse as a classic public good
- A lighthouse emits a beam of light to warn ships of rocky coastlines and guide them safely.
- Test for non-excludability: could the lighthouse operator stop a particular ship from seeing and using the light? No -- the light is visible to any vessel within range, regardless of whether that ship's owner paid towards the lighthouse's upkeep. The good is non-excludable.
- Test for non-rivalry: if one ship uses the light to navigate, is there any less light available for the next ship that passes? No -- the beam is not depleted or diminished by use. The good is non-rival.
- Because the lighthouse satisfies both conditions simultaneously, it is a textbook example of a pure public good, historically used by economists (including Paul Samuelson) to illustrate the concept.
- A public good must be BOTH non-excludable AND non-rival -- missing either condition means it is not a true public good.
- Non-excludable: non-payers cannot be prevented from consuming the good (e.g. national defence).
- Non-rival: one person's consumption does not reduce what is available to others; marginal cost of an extra user is zero (e.g. street lighting).
- The lighthouse is the classic textbook example: any ship can use the light (non-excludable), and one ship's use doesn't reduce the light for others (non-rival).
- A public good is defined by these two economic characteristics, not by who provides it -- government provision alone does not make something a public good.
Non-Excludability
Explains non-excludability, one of the two defining characteristics of a public good, meaning it is impossible or prohibitively costly to prevent non-payers from consuming the good, illustrated through national defence. The key insight is that non-excludability breaks the payment-for-consumption link that private markets rely on, enabling free-riding and causing underprovision. Contains: text explanation, national defence worked example, key-concept callout distinguishing excludability from rivalry, and a common-mistake callout on conflating public goods with public services.
Non-excludability is one of the two defining characteristics of a pure public good (the other being non-rivalry, covered elsewhere in this subtopic). A good is non-excludable when it is technically impossible, or prohibitively expensive, to prevent someone who has not paid from consuming it. Once the good is supplied to anyone, it is effectively supplied to everyone within reach of it.
This matters economically because markets normally allocate goods through a simple mechanism: if you don't pay, the seller withholds the good from you. Excludability is what lets a firm charge a price and earn revenue. When a good is non-excludable, this mechanism collapses -- there is no way to withhold the good from a non-payer, so there is no reliable way to charge for it.
National defence as a non-excludable good
- A government funds an army, navy and air force to protect the country's territory and citizens from external threats.
- Consider a citizen who deliberately evades paying income tax. Ask: can the government feasibly switch off military protection for this one individual while continuing to protect everyone else living around them?
- In practice this is not feasible -- defence operates over an entire territory and population simultaneously. A missile defence shield or a deployed navy protects whoever happens to be within its coverage, taxpayer or not.
- Therefore, the tax evader still benefits fully from national defence, exactly like every taxpaying citizen. This is what makes national defence non-excludable: non-payers cannot be prevented from consuming the benefit.
- Conclusion: because firms cannot exclude non-payers, no private firm could profitably sell national defence directly to individual households, since anyone could simply wait for others to pay while still receiving protection.
Key concept: Non-excludability is about who can be stopped from consuming, while non-rivalry (discussed elsewhere in this subtopic) is about whether consumption reduces availability for others. A good only counts as a pure public good if it satisfies both conditions simultaneously. Testing only one condition and concluding a good is or isn't a public good is a common analytical shortcut that misses half the definition.
Common mistake: students often assume that anything provided by the government must be a public good. Government-provided healthcare, for example, is excludable (you can be denied treatment without an appointment, insurance or payment in many systems) and rival (a hospital bed or a doctor's time used by one patient is unavailable to another). It is a publicly provided merit good, not a public good, precisely because it fails the non-excludability test that national defence passes.
Non-excludability is the root cause of the free-rider problem: since non-payers cannot be shut out, individuals have no financial incentive to voluntarily pay for the good, expecting to benefit anyway from others' contributions. Because a private firm cannot charge a price it can reliably collect, it cannot earn revenue or profit from supplying the good, so the free market tends to supply it at a quantity of zero, even though the good has genuine value to society. This is why non-excludable goods like national defence are typically provided and funded collectively by government through taxation rather than sold in a market.
- Non-excludability: impossible or prohibitively costly to stop non-payers from consuming the good.
- National defence is a classic example -- tax evaders still receive military protection.
- Non-excludability breaks the pay-to-consume link that private markets depend on to earn revenue.
- It directly causes the free-rider problem, since non-payers can still benefit.
- Non-excludability alone does not make a good a public good -- non-rivalry must also hold.
- Publicly provided does not equal non-excludable: healthcare is government-provided but still excludable.