A natural monopoly arises when the technology of production exhibits subadditive costs over the relevant range of demand — meaning one firm can serve the market more cheaply than any combination of smaller firms. This typically occurs in industries with very high fixed or sunk costs and low marginal costs, so average cost keeps falling as output expands. Classic examples are water distribution, electricity transmission, natural gas pipelines, sewerage, and fixed-line rail infrastructure: duplicating the network would waste capital without lowering prices.
Because competition is inefficient (or self-eliminating) in these sectors, governments rarely leave them to the market. Three regulatory responses dominate:
- Public ownership, common in European water and rail systems for much of the 20th century.
- Rate-of-return or price-cap regulation, used for U.S. electric utilities under state public utility commissions, and for UK utilities after privatisation in the 1980s–90s (e.g., Ofwat for water, Ofgem for energy, using the RPI–X formula pioneered by Stephen Littlechild).
- Structural separation and third-party access, where the monopoly "bottleneck" (the grid or track) is unbundled from competitive activities (generation, train operation). The EU's electricity and gas directives and the UK's 1993 rail restructuring are leading examples.
Economists associated with the concept include John Stuart Mill, who described "natural monopolies" in Principles of Political Economy (1848), and later Harold Demsetz, whose 1968 article "Why Regulate Utilities?" proposed franchise bidding as an alternative to direct regulation. William Baumol's work on contestable markets (1982) refined the definition by emphasising cost subadditivity rather than mere scale economies.
For IR and policy researchers, natural monopolies matter beyond domestic regulation: cross-border pipelines, undersea cables, and satellite slots raise similar bottleneck problems at the international level, feeding debates on infrastructure geopolitics, WTO services commitments, and energy-market integration.
Example
When the UK privatised British Gas in 1986, regulators created Ofgas (later Ofgem) to oversee the pipeline network as a natural monopoly while opening gas supply to competition.
Frequently asked questions
An ordinary monopoly may result from barriers like patents, mergers, or state grants; a natural monopoly arises from the cost structure itself, where one firm is genuinely cheaper than many.
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