This has been a source of some ambiguity in discussions of "natural monopoly".
"The superiority of reward is not here the consequence of competition, but of its absence: not a compensation for disadvantages inherent in the employment, but an extra advantage; a kind of monopoly price, the effect not of a legal, but of what has been termed a natural monopoly... independently of... artificial monopolies [i.e. grants by government], there is a natural monopoly in favour of skilled labourers against the unskilled, which makes the difference of reward exceed, sometimes in a manifold proportion, what is sufficient merely to equalize their advantages. If unskilled labourers had it in their power to compete with skilled, by merely taking the trouble of learning the trade, the difference of wages might not exceed what would compensate them for that trouble, at the ordinary rate at which labour is remunerated. But the fact that a course of instruction is required, of even a low degree of costliness, or that the labourer must be maintained for a considerable time from other sources, suffices everywhere to exclude the great body of the labouring people from the possibility of any such competition.
So Mill's initial use of the term concerned natural abilities, in contrast to the common contemporary usage, which refers solely to market failure in a particular type of industry, such as rail, post or electricity. Mill's development of the idea is that what is true of labour is true of capital.
"All the natural monopolies (meaning thereby those which are created by circumstances, and not by law) which produce or aggravate the disparities in the remuneration of different kinds of labour, operate similarly between different employments of capital. If a business can only be advantageously carried on by a large capital, this in most countries limits so narrowly the class of persons who can enter into the employment, that they are enabled to keep their rate of profit above the general level. A trade may also, from the nature of the case, be confined to so few hands, that profits may admit of being kept up by a combination among the dealers. It is well known that even among so numerous a body as the London booksellers, this sort of combination long continued to exist. I have already mentioned the case of the gas and water companies.
Mill also used the term in relation to land, for which the natural monopoly could be extracted by virtue of it being the only land like it. Furthermore, Mill referred to network industries, such as electricity and water supply, roads, rail and canals, as "practical monopolies", where "it is the part of the government, either to subject the business to reasonable conditions for the general advantage, or to retain such power over it, that the profits of the monopoly may at least be obtained for the public. So, a legal prohibition against competition is often advocated and rates are not left to the market but are regulated by the government.
Companies that grow to take advantage of economies of scale often run into problems of bureaucracy; these factors interact to produce an "ideal" size for a company, at which the company's average cost of production is minimized. If that ideal size is large enough to supply the whole market, then that market is a natural monopoly.
A further discussion and understanding requires more microeconomics:
Two different types of cost are important in microeconomics: marginal cost, and fixed cost. The marginal cost is the cost to the company of serving one more customer. In an industry where a natural monopoly does not exist, the vast majority of industries, the marginal cost decreases with economies of scale, then increases as the company has growing pains (overworking its employees, bureaucracy, inefficiencies, etc.) Along with this, the average cost of its products will decrease and then increase again. A natural monopoly has a very different cost structure. A natural monopoly has a high fixed cost for a product that does not depend on output, but its marginal cost of producing one more good is roughly constant, and small.
A firm with high fixed costs will require a large number of customers in order to retrieve a meaningful return on their initial investment. This is where economies of scale become important. Since each firm has large initial costs, as the firm gains market share and increases its output the fixed cost (what they initially invested) is divided among a larger number of customers. Therefore, in industries with large initial investment requirements, average total cost declines as output increases over a much larger range of output levels.
Once a natural monopoly has been established because of the large initial cost and that, according to the rule of economies of scale, the larger corporation (to a point) has lower average cost and therefore a huge advantage. With this knowledge, no firms attempt to enter the industry and an oligopoly or monopoly develops.
A natural monopoly and a monopoly are not the same concept. A natural monopoly describes a firm's cost structure (high fixed cost, extremely low constant marginal cost). A monopoly describes market share and market power; the two are not synonymous.
As a quid pro quo for accepting government oversight, private suppliers may be permitted some monopolistic returns, through stable prices or guaranteed through limited rates of return, and a reduced risk of long-term competition. (See also rate of return pricing). For example, an electric utility may be allowed to sell electricity at price that will give it a 12% return on its capital investment. If not constrained by the public utility commission, the company would likely charge a far higher price and earn an abnormal profit on its capital.
Regulatory responses:
Since the 1980s there is a global trend towards utility deregulation, in which systems of competition are intended to replace regulation by specifying or limiting firms' behaviour; the telecommunications industry is a leading example globally.
Arguments from public choice suggest that regulatory capture is likely in the case of a regulated private monopoly. Moreover, in some cases the costs to society of overzealous regulation may be higher than the costs of permitting an unregulated private monopoly. (Although the monopolist charges monopoly prices, much of the price increase is a transfer rather than a loss to society.)
More fundamentally, the theory of contestable markets developed by Baumol and others argues that monopolists (including natural monopolists) may be forced over time by the mere possibility of competition at some point in the future to limit their monopolistic behaviour, in order to deter entry. In the limit, a monopolist is forced to make the same production decisions as a competitive market would produce. A common example is that of airline flight schedules, where a particular airline may have a monopoly between destinations A and B, but the relative ease with which in many cases competitors could also serve that route limits its monopolistic behaviour. The argument even applies somewhat to government-granted monopolies, as although they are protected from competitors entering the industry, in a democracy excessively monopolistic behaviour may lead to the monopoly being revoked, or given to another party.
Nobel economist Milton Friedman, said that in the case of natural monopoly that "there is only a choice among three evils: private unregulated monopoly, private monopoly regulated by the state, and government operation." He said "the least of these evils is private unregulated monopoly where this is tolerable." He reasons that the other alternatives are "exceedingly difficult to reverse," and that the dynamics of the market should be allowed the opportunity to have an effect and are likely to do so (Capitalism and Freedom). In a Wincott Lecture, he said that if the commodity in question is "essential" (for example: water or electricity) and the "monopoly power is sizeable," then "either public regulation or ownership may be a lesser evil." However, he goes on to say that such action by government should not consist of forbidding competition by law. Friedman has taken a stronger laissez-faire stance since, saying that "over time I have gradually come to the conclusion that antitrust laws do far more harm than good and that we would be better off if we didn’t have them at all, if we could get rid of them" (The Business Community's Suicidal Impulse).
Advocates of laissez-faire capitalism, such as libertarians, typically say that permanent natural monopolies are merely theoretical. Economists from the Austrian school claim that governments take ownership of the means of production in certain industries and ban competition under the false pretense that they are natural monopolies.
Equally, competition may be used for part of the market (eg IT services), through outsourcing contracts; some water companies outsource a considerable proportion of their operations. The extreme case is Welsh Water, which outsources virtually its entire business operations, running just a skeleton staff to manage these contracts. Franchising different parts of the business on a regional basis (eg parts of a city) can bring in some features of "yardstick" competition (see below), as the performance of different contractors can be compared. See also water privatization.
Such a system may be considered a form of deregulation, but in fact it requires active government creation of a new system of competition rather than simply the removal of existing legal restrictions. The system may also need continuing government finetuning, for example to prevent the development of long-term contracts from reducing the liquidity of the generation market too much, or to ensure the correct incentives for long-term security of supply are present. See also California electricity crisis. Whether such a system is more efficient than possible alternatives is unclear; the cost of the market mechanisms themselves are substantial, and the vertical de-integration required introduces additional risks. This raises the cost of finance - which for a capital intensive industry (as natural monopolies are) is a key issue. Moreover, such competition also raises equity and efficiency issues, as large industrial consumers tend to benefit much more than domestic consumers.
In practice, the notorious short-termism of the stock market may be antithetical to appropriate spending on maintenance and investment in industries with long time horizons, where the failure to do so may only have effects a decade or more hence (which is typically long after current chief executives have left the company). By way of example, the UK's water economic regulator, Ofwat, sees the stock market as an important regulatory instrument for ensuring efficient management of the water companies.