Fundamentals

Natural Monopoly: Meaning, Examples, and Limits

By Daniel Sardá · Published on September 07, 2026

7 min read1,433 words

One network may supply a market at a lower cost than several competing networks. Understanding why means separating costs from prices and efficiency from legal privileges.

A natural monopoly exists when a single provider can meet a market's demand at a lower total cost than several providers dividing the same output among them, under comparable conditions. “Natural” describes a relationship between costs and demand; it does not mean inevitable or inherently beneficial.

Imagine a town that needs a water network. Digging up streets, laying pipes, and connecting homes requires substantial investment. If two companies build parallel networks, each serving half the residents, they could duplicate infrastructure without a proportionate improvement in service.

That potential saving explains the concept. A further question remains: how to ensure that users benefit through reasonable prices, service quality, and proper maintenance.

Why one network can cost less

In our example, much of the cost of installing the network must be incurred before the first cubic meter of water flows through it. Within a given capacity, those fixed costs are spread across more units as supply increases.

This can bring down average cost: total cost divided by the quantity supplied. A fall in average cost as output expands is known as an economy of scale. It does not require each additional unit to be cheaper than the previous one: marginal cost can remain constant while the average falls.

High fixed costs alone, however, do not establish that a natural monopoly exists. The total cost of meeting all demand with one firm must be compared with the cost of doing so with several, considering the different ways output could be divided among them.

This condition is called cost subadditivity. For a single product, declining average cost across the entire relevant range of output is sufficient for subadditivity, although it is not a necessary condition. The academic formulation is set out in Paul L. Joskow's *Regulation of Natural Monopolies*.

Returning to the pipes, the comparison would need to hold coverage, pressure, continuity of supply, and water quality constant. A network that appears cheap because it leaves neighborhoods unserved is not providing the same service. Maintenance and replacement costs would also need to be included, alongside initial construction costs.

Key idea: The test is the cost of meeting the same demand at comparable quality. The presence of just one firm does not prove that a natural monopoly exists.

Which activity has the cost advantage—and what creates exclusivity?

The analysis needs clear boundaries: what service is being provided, in which area, and for how many users. A local network's cost advantage does not establish that every activity related to water must belong to a single organization.

In our hypothetical town, building two pipe systems could be costly, while rival suppliers could compete for maintenance contracts or orders for certain equipment. Each activity needs its own comparison. A similar distinction applies to electricity networks: a finding about one piece of infrastructure should not automatically be extended to the entire sector.

Economies of scale should also be distinguished from network effects. The former describe a decline in average cost as output rises; the latter describe an increase in a service's value to a user as others participate. These are different mechanisms, though they can coexist.

Another crucial distinction is between natural monopoly and legal monopolies. The first concerns costs. The second concerns exclusivity created by rules that prevent or restrict competitors from entering.

A town can have both: a network that would be costly to duplicate and a concession that reserves the service for one operator. But demonstrating the advantage of a shared network does not establish that every restriction on entry into any related activity is justified.

From a classical liberal perspective, this distinction calls for scrutiny of the scope of the privileges granted. Freedom of entry allows alternatives to be explored; evaluating their results requires taking infrastructure costs seriously. Neither exclusivity nor duplication deserves automatic approval.

Low production costs do not guarantee low prices

Suppose a single network is the least costly option. Its operator could still charge high rates if residents lack effective alternatives. Savings in production and benefits to consumers are related but distinct issues.

Productive efficiency means providing a service at the lowest possible resource cost while holding quantity and quality constant. Allocative efficiency requires the quantities produced and consumed to reflect both the benefit of each additional unit and its cost to society. A firm can have low costs yet set a price that prevents consumption whose benefit exceeds the additional cost of supplying it.

This creates a pricing problem. Marginal cost is the cost of supplying one additional unit. If it is below average cost, charging that amount for each unit will not recover all costs under a uniform per-unit rate with no fixed charge or compensating transfers. This is the problem illustrated in UNED's explanation of natural monopoly regulation (in Spanish).

In our network, charging only the additional cost of delivering more water could leave part of the infrastructure unfunded. A fixed charge or a transfer changes the calculation, but also raises questions about who pays and how access is affected. Recovering costs does not mean accepting every expense the operator reports.

Key idea: Saving resources, recovering investment, and keeping rates affordable are distinct goals. A pricing rule must explain how it reconciles them.

Organizing the service with incentives in mind

Economic regulation provides tools for addressing these tensions. Their usefulness depends on the information available, the characteristics of the service, and the ability to enforce agreed obligations.

Regulating rates and quality

One option is to link rates to recognized costs. A problem arises if the operator expects to pass every increase in spending on to users: it loses incentives to save, and the regulator must distinguish necessary expenses from inefficiency.

A price cap can encourage cost reductions by allowing the firm to retain part of the savings for a period. However, the design must also include oversight of quality and maintenance. Savings from operational improvements and savings from allowing pipes to deteriorate have different consequences. UNED examines these pricing incentives (in Spanish).

Allowing access to shared infrastructure

Where separating the network from related services is feasible, different providers can use it under defined access conditions. This requires technical rules, usage charges, and safeguards against discriminatory treatment.

Access cannot be settled simply by issuing a general instruction to share. An inappropriate charge can hinder competition or weaken investment in the infrastructure. The OECD examines these challenges of access and vertical separation. Feasibility must be assessed for each activity; it cannot be inferred from the example of the pipes.

Making firms compete for the right to provide the service

Another option is to award the right to operate for a fixed term through competitive bidding. Instead of building parallel networks, firms compete to provide the service under defined conditions.

The bid must be viable, and obligations covering service coverage, investment, and quality must be verifiable. If the successful bidder offers terms that are difficult to meet and later manages to renegotiate them, the initial advantage of the bidding process can disappear. The OECD analyzes these limits of competition for the market.

These decisions are distinct from choosing who owns the assets. A concession can assign responsibility for operating the network without selling it. Public or private ownership alone does not specify how rates will be set or how the operator will be held accountable for poor service.

Key idea: Every option needs incentives to maintain the network and mechanisms to enforce performance. The label attached to a management model is no substitute for either.

When to revisit the assessment

The economic conclusion can change. Higher demand changes the relevant scale of output; new technology can make alternatives cheaper; a substitute can reduce dependence on an infrastructure network. None of these guarantees that a natural monopoly will disappear, but each calls for a fresh comparison.

To evaluate a specific proposal, it helps to ask for clarity on three points: which activity costs less with a single provider, which alternatives were compared, and what conditions would warrant revisiting the finding.

In our imaginary town, a defensible decision requires more than pointing to the existing pipes. It calls for evidence of what it would cost to serve the same residents through other means, at the same quality, and how the operator would be held accountable for the service. That scrutiny can turn a cost advantage into a benefit for users and prevent it from being treated as a permanent privilege.