||It has been suggested that Perfect market be merged into this article. (Discuss) Proposed since April 2012.|
In economic theory, perfect competition (sometimes called pure competition) describes markets such that no participants are large enough to have the market power to set the price of a homogeneous product. Because the conditions for perfect competition are strict, there are few if any perfectly competitive markets. Still, buyers and sellers in some auction-type markets, say for commodities or some financial assets, may approximate the concept. Perfect competition serves as a benchmark against which to measure real-life and imperfectly competitive markets.
Basic structural characteristics 
Generally, a perfectly competitive market exists when every participant is a "price taker", and no participant influences the price of the product it buys or sells. Specific characteristics may include:
- Infinite buyers and sellers – An infinite number of consumers with the willingness and ability to buy the product at a certain price, and infinite producers with the willingness and ability to supply the product at a certain price.
- Zero entry and exit barriers – A lack of entry and exit barriers makes it extremely easy to enter or exit a perfectly competitive market.
- Perfect factor mobility – In the long run factors of production are perfectly mobile, allowing free long term adjustments to changing market conditions.
- Perfect information - All consumers and producers are assumed to have perfect knowledge of price, utility, quality and production methods of products.
- Zero transaction costs - Buyers and sellers do not incur costs in making an exchange of goods in a perfectly competitive market.
- Profit maximization - Firms are assumed to sell where marginal costs meet marginal revenue, where the most profit is generated.
- Homogenous products - The qualities and characteristics of a market good or service do not vary between different suppliers.
- Non-increasing returns to scale - The lack of increasing returns to scale (or economies of scale) ensures that there will always be a sufficient number of firms in the industry.
- Property rights - Well defined property rights determine what may be sold, as well as what rights are conferred on the buyer.
In the short run, perfectly-competitive markets are not productively efficient as output will not occur where marginal cost is equal to average cost (MC=AC). They are allocatively efficient, as output will always occur where marginal cost is equal to marginal revenue (MC=MR). In the long run, perfectly competitive markets are both allocatively and productively efficient.
In perfect competition, any profit-maximizing producer faces a market price equal to its marginal cost (P=MC). This implies that a factor's price equals the factor's marginal revenue product. It allows for derivation of the supply curve on which the neoclassical approach is based. This is also the reason why "a monopoly does not have a supply curve". The abandonment of price taking creates considerable difficulties for the demonstration of a general equilibrium except under other, very specific conditions such as that of monopolistic competition.
Approaches and conditions 
In neoclassical economics there have been two strands of looking at what perfect competition is. The first emphasis is on the inability of any one agent to affect prices. Usually justified by the fact that any one firm or consumer is so small relative to the whole market that their presence or absence leaves the equilibrium price very nearly unaffected. This assumption of negligible impact of each agent on the equilibrium price has been formalized by Aumann (1964) by postulating a continuum of infinitesimal agents. The difference between Aumann's approach and that found in undergraduate textbooks is that in the first, agents have the power to choose their own prices but do not individually affect the market price, while in the second it is simply assumed that agents treat prices as parameters. Both approaches lead to the same result.
The second view of perfect competition conceives of it in terms of agents taking advantage of – and hence, eliminating – profitable exchange opportunities. The faster this arbitrage takes place, the more competitive a market. The implication is that the more competitive a market is under this definition, the faster the average market price will adjust so as to equate supply and demand (and also equate price to marginal costs). In this view, "perfect" competition means that this adjustment takes place instantaneously. This is usually modeled via the use of the Walrasian auctioneer (see article for more information). The widespread recourse to the auctioneer tale appears to have favored an interpretation of perfect competition as meaning price taking always, i.e. also at non-equilibrium prices; but this is rejected e.g. by Arrow (1959) or Mas-Colell et al.
Steve Keen notes, following George Stigler, that if firms do not react strategically to one another, the slope of the demand curve that a firm faces is the same as the slope of the market demand curve. Hence, if firms are to produce at a level that equates marginal cost and marginal revenue, the model of perfect competition must include at least an infinite number of firms, each producing an output quantity of zero. As noted above, an influential model of perfect competition in neoclassical economics assumes that the number of buyers and sellers are both of the power of the continuum, that is, an infinity even larger than the number of natural numbers. K. Vela Velupillai quotes Maury Osborne as noting the inapplicability of such models to actual economies since money and the commodities sold each have a smallest positive unit.
Thus nowadays the dominant intuitive idea of the conditions justifying price taking and thus rendering a market perfectly competitive is an amalgam of several different notions, not all present, nor given equal weight, in all treatments. Besides product homogeneity and absence of collusion, the notion more generally associated with perfect competition is the negligibility of the size of agents, which makes them believe that they can sell as much of the good as they wish at the equilibrium price but nothing at a higher price (in particular, firms are described as each one of them facing a horizontal demand curve). However, also widely accepted as part of the notion of perfectly competitive market are perfect information about price distribution and very quick adjustments (whose joint operation establish the law of one price), to the point sometimes of identifying perfect competition with an essentially instantaneous reaching of equilibrium between supply and demand. Finally, the idea of free entry with free access to technology is also often listed as a characteristic of perfectly competitive markets, probably owing to a difficulty with abandoning completely the older conception of free competition. In recent decades it has been rediscovered that free entry can be a foundation of absence of market power, alternative to negligibility of agents.
Free entry also makes it easier to justify the absence of collusion: any collusion by existing firms can be undermined by entry of new firms. The necessarily long-period nature of the analysis (entry requires time!) also allows a reconciliation of the horizontal demand curve facing each firm according to the theory, with the feeling of businessmen that "contrary to economic theory, sales are by no means unlimited at the current market price" (Arrow 1959 p. 49). Sraffian economists see the assumption of free entry and exit as characteristic of the theory of free competition in Classical economics, an approach that is not expressed in terms of schedules of supply and demand.
As mentioned above, the perfect competition model, if interpreted as applying also to short-period or very-short-period behaviour, is approximated only by markets of homogeneous products produced and purchased by very many sellers and buyers, usually organized markets for agricultural products or raw materials. In real-world markets, assumptions such as perfect information cannot be verified and are only approximated in organized double-auction markets where most agents wait and observe the behaviour of prices before deciding to exchange (but in the long-period interpretation perfect information is not necessary, the analysis only aims at determining the average around which market prices gravitate, and for gravitation to operate one does not need perfect information).
In the absence of externalities and public goods, perfectly competitive equilibria are Pareto-efficient, i.e. no improvement in the utility of a consumer is possible without a worsening of the utility of some other consumer. This is called the First Theorem of Welfare Economics. The basic reason is that no productive factor with a non-zero marginal product is left unutilized, and the units of each factor are so allocated as to yield the same indirect marginal utility in all uses, a basic efficiency condition (if this indirect marginal utility were higher in one use than in other ones, a Pareto improvement could be achieved by transferring a small amount of the factor to the use where it yields a higher marginal utility).
A simple proof assuming differentiable utility functions and production functions is the following. Let wj be the 'price' (the rental) of a certain factor j, let MPj1 and MPj2 be its marginal product in the production of goods 1 and 2, and let p1 and p2 be these goods' prices. In equilibrium these prices must equal the respective marginal costs MC1 and MC2; remember that marginal cost equals factor 'price' divided by factor marginal productivity (because increasing the production of good by one very small unit through an increase of the employment of factor j requires increasing the factor employment by 1/MPji and thus increasing the cost by wj/MPji, and through the condition of cost minimization that marginal products must be proportional to factor 'prices' it can be shown that the cost increase is the same if the output increase is obtained by optimally varying all factors). Optimal factor employment by a price-taking firm requires equality of factor rental and factor marginal revenue product, wj=piMPji, so we obtain p1=MC1=wj/MPj1, p2=MCj2=wj/MPj2.
Now choose any consumer purchasing both goods, and measure his utility in such units that in equilibrium his marginal utility of money (the increase in utility due to the last unit of money spent on each good), MU1/p1=MU2/p2, is 1. Then p1=MU1, p2=MU2. The indirect marginal utility of the factor is the increase in the utility of our consumer achieved by an increase in the employment of the factor by one (very small) unit; this increase in utility through allocating the small increase in factor utilization to good 1 is MPj1MU1=MPj1p1=wj, and through allocating it to good 2 it is MPj2MU2=MPj2p2=wj again. With our choice of units the marginal utility of the amount of the factor consumed directly by the optimizing consumer is again w, so the amount supplied of the factor too satisfies the condition of optimal allocation.
Monopoly violates this optimal allocation condition, because in a monopolized industry market price is above marginal cost, and this means that factors are underutilized in the monopolized industry, they have a higher indirect marginal utility than in their uses in competitive industries. Of course this theorem is considered irrelevant by economists who do not believe that general equilibrium theory correctly predicts the functioning of market economies; but it is given great importance by neoclassical economists and it is the theoretical reason given by them for combating monopolies and for antitrust legislation.
In contrast to a monopoly or oligopoly, it is impossible for a firm in perfect competition to earn economic profit in the long run, which is to say that a firm cannot make any more money than is necessary to cover its economic costs. In order not to misinterpret this zero-long-run-profits thesis, it must be remembered that the term 'profit' is also used in other ways. Neoclassical theory defines profit as what is left of revenue after all costs have been subtracted, including normal interest on capital plus the normal excess over it required to cover risk, and normal salary for managerial activity. Classical economists on the contrary defined profit as what is left after subtracting costs except interest and risk coverage; thus, if one leaves aside risk coverage for simplicity, the neoclassical zero-long-run-profit thesis would be re-expressed in classical parlance as profits coinciding with interest in the long period, i.e. the rate of profit tending to coincide with the rate of interest. Profits in the classical meaning do not tend to disappear in the long period but tend to normal profit. With this terminology, if a firm is earning abnormal profit in the short term, this will act as a trigger for other firms to enter the market. As other firms enter the market the market supply curve will shift out causing prices to fall. Existing firms will react to this lower price by adjusting their capital stock downward. This adjustment will cause their marginal cost to shift to the left causing the market supply curve to shift inward. However, the net effect of entry by new firms and adjustment by existing firms will be to shift the supply curve outward. The market price will be driven down until all firms are earning normal profit only.
It is important to note that perfect competition is a sufficient condition for allocative and productive efficiency, but it is not a necessary condition. Laboratory experiments in which participants have significant price setting power and little or no information about their counterparts consistently produce efficient results given the proper trading institutions.
The shutdown point 
In the short run, a firm operating at a loss [R < TC (revenue less than total cost) or P < ATC (price less than unit cost)] must decide whether to continue to operate or temporarily shutdown. The shutdown rule states "in the short run a firm should continue to operate if price exceeds average variable costs." Restated, the rule is that for a firm to continue producing in the short run it must earn sufficient revenue to cover its variable costs. The rationale for the rule is straightforward. By shutting down a firm avoids all variable costs. However, the firm must still pay fixed costs. Because fixed cost must be paid regardless of whether a firm operates they should not be considered in deciding whether to produce or shutdown. Thus in determining whether to shut down a firm should compare total revenue to total variable costs (VC) rather than total costs (FC + VC). If the revenue the firm is receiving is greater than its total variable cost (R > VC) then the firm is covering all variable cost plus there is additional revenue ("contribution"), which can be applied to fixed costs. (The size of the fixed costs is irrelevant as it is a sunk cost. The same consideration is used whether fixed costs are one dollar or one million dollars.) On the other hand if VC > R then the firm is not even covering its production costs and it should immediately shut down. The rule is conventionally stated in terms of price (average revenue) and average variable costs. The rules are equivalent (If you divide both sides of inequality TR > TVC by Q gives P > AVC). If the firm decides to operate, the firm will continue to produce where marginal revenue equals marginal costs because these conditions insure not only profit maximization (loss minimization) but also maximum contribution.
Another way to state the rule is that a firm should compare the profits from operating to those realized if it shutdown and select the option that produces the greater profit. A firm that is shutdown is generating zero revenue and incurring no variable costs. However, the firm still has to pay fixed cost. So the firm's profit equals fixed costs or −FC. An operating firm is generating revenue, incurring variable costs and paying fixed costs. The operating firm's profit is R − VC − FC. The firm should continue to operate if R − VC − FC ≥ −FC, which simplified is R ≥ VC. The difference between revenue, R, and variable costs, VC, is the contribution to fixed costs and any contribution is better than none. Thus, if R ≥ VC then firm should operate. If R < VC the firm should shut down.
A decision to shut down means that the firm is temporarily suspending production. It does not mean that the firm is going out of business (exiting the industry).] If market conditions improve, and prices increase, the firm can resume production. Shutting down is a short-run decision. A firm that has shut down is not producing. The firm still retains its capital assets; however, the firm cannot leave the industry or avoid its fixed costs in the short run. Exit is a long-term decision. A firm that has exited an industry has avoided all commitments and freed all capital for use in more profitable enterprises.
However, a firm cannot continue to incur losses indefinitely. In the long run, the firm will have to earn sufficient revenue to cover all its expenses and must decide whether to continue in business or to leave the industry and pursue profits elsewhere. The long-run decision is based on the relationship of the price and long-run average costs. If P ≥ AC then the firm will not exit the industry. If P < AC, then the firm will exit the industry. These comparisons will be made after the firm has made the necessary and feasible long-term adjustments. In the long run a firm operates where marginal revenue equals long-run marginal costs.
Short-run supply curve 
The short run supply curve for a perfectly competitive firm is the marginal cost (MC) curve at and above the shutdown point. Portions of the marginal cost curve below the shut down point are not part of the SR supply curve because the firm is not producing in that range. Technically the SR supply curve is a discontinuous function composed of the segment of the MC curve at and above minimum of the average variable cost curve and a segment that runs with the vertical axis from the origin to but not including a point "parallel" to minimum average variable costs.
Though there is no actual perfectly competitive market in the real world, a number of approximations exist:
Perhaps the closest thing to a perfectly competitive market would be a large auction of identical goods with all potential buyers and sellers present. By design, a stock exchange resembles this, not as a complete description (for no markets may satisfy all requirements of the model) but as an approximation. The flaw in considering the stock exchange as an example of Perfect Competition is the fact that large institutional investors (e.g. investment banks) may solely influence the market price. This, of course, violates the condition that "no one seller can influence market price".
Horse betting is also quite a close approximation. When placing bets, consumers can just look down the line to see who is offering the best odds, and so no one bookie can offer worse odds than those being offered by the market as a whole, since consumers will just go to another bookie. This makes the bookies price-takers. Furthermore, the product on offer is very homogeneous, with the only differences between individual bets being the pay-off and the horse. Of course, there are not an infinite amount of bookies, and some barriers to entry exist, such as a license and the capital required to set up.
Free software works along lines that approximate perfect competition as well. Anyone is free to enter and leave the market at no cost. All code is freely accessible and modifiable, and individuals are free to behave independently. Free software may be bought or sold at whatever price that the market may allow.
Some believe[who?] that one of the prime examples of a perfectly competitive market anywhere in the world is street food in developing countries. This is so since relatively few barriers to entry/exit exist for street vendors. Furthermore, there are often numerous buyers and sellers of a given street food, in addition to consumers/sellers possessing perfect information of the product in question. It is often the case that street vendors may serve a homogenous product, in which little to no variations in the product's nature exist.
Another very near example of perfect competition would be the fish market and the vegetable or fruit vendors who sell at the same place, the bars in "Le Carré" (Liège, Belgium) or the "kebab street" near the Grand Place in Brussels:
- There are large number of buyers and sellers.
- There are no entry or exit barriers.
- There is perfect mobility of the factors, i.e. buyers can easily switch from one seller to the other.
- The products are homogeneous.
The use of the assumption of perfect competition as the foundation of price theory for product markets is often criticized as representing all agents as passive, thus removing the active attempts to increase one's welfare or profits by price undercutting, product design, advertising, innovation, activities that - the critics argue – characterize most industries and markets. These criticisms point to the frequent lack of realism of the assumptions of product homogeneity and impossibility to differentiate it, but apart from this the accusation of passivity appears correct only for short-period or very-short-period analyses, in long-period analyses the inability of price to diverge from the natural or long-period price is due to active reactions of entry or exit.
Some economists have a different kind of criticism concerning perfect competition model. They are not criticizing the price taker assumption because it makes economic agents too "passive", but because it then raises the question of who sets the prices. Indeed, if everyone is price taker, there is the need for a benevolent planner who gives and sets the prices, in other word, there is a need for a "price maker". Therefore, it makes the perfect competition model appropriate not to describe a decentralize "market" economy but a centralized one. This in turn means that such kind of model has more to do with communism than capitalism.
Another frequent criticism is that it is often not true that in the short run differences between supply and demand cause changes in price; especially in manufacturing, the more common behaviour is alteration of production without nearly any alteration of price.
The critics of the assumption of perfect competition in product markets seldom question the basic neoclassical view of the working of market economies for this reason. The Neo-Austrian school insists strongly on this criticism, and yet the neoclassical view of the working of market economies as fundamentally efficient, reflecting consumer choices and assigning to each agent his contribution to social welfare, is esteemed to be fundamentally correct Some non-neoclassical schools, like Post-Keynesians, reject the neoclassical approach to value and distribution, but not because of their rejection of perfect competition as a reasonable approximation to the working of most product markets; the reasons for rejection of the neoclassical 'vision' are different views of the determinants of income distribution and of aggregated demand.
In particular, the rejection of perfect competition does not generally entail the rejection of free competition as characterizing most product markets; indeed it has been argued that competition is stronger nowadays than in 19th century capitalism, owing to the increasing capacity of big conglomerate firms to enter any industry: therefore the classical idea of a tendency toward a uniform rate of return on investment in all industries owing to free entry is even more valid today; and the reason why General Motors, Texon or Nestle do not enter the computers or pharmaceutical industries is not insurmountable barriers to entry but rather that the rate of return in the latter industries is already sufficiently in line with the average rate of return elsewhere as not to justify entry. On this few economists, it would seem, would disagree, even among the neoclassical ones. Thus when the issue is normal, or long-period, product prices, differences on the validity of the perfect competition assumption do not appear to imply important differences on the existence or not of a tendency of rates of return toward uniformity as long as entry is possible, and what is found fundamentally lacking in the perfect competition model is the absence of marketing expenses and innovation as causes of costs that do enter normal average cost.
The issue is different with respect to factor markets. Here the acceptance or denial of perfect competition in labour markets does make a big difference to the view of the working of market economies. One must distinguish neoclassical from non-neoclassical economists. For the former, absence of perfect competition in labour markets, e.g. due to the existence of trade unions, impedes the smooth working of competition, which if left free to operate would cause a decrease of wages as long as there were unemployment, and would finally ensure the full employment of labour: labour unemployment is due to absence of perfect competition in labour markets. Most non-neoclassical economists deny that a full flexibility of wages would ensure the full employment of labour and find a stickiness of wages an indispensable component of a market economy, without which the economy would lack the regularity and persistence indispensable to its smooth working. This was, for example, John Maynard Keynes's opinion.
Particularly radical is the view of the Sraffian school on this issue: the labour demand curve cannot be determined hence a level of wages ensuring the equality between supply and demand for labour does not exist, and economics should resume the viewpoint of the classical economists, according to whom competition in labour markets does not and cannot mean indefinite price flexibility as long as supply and demand are unequal, it only means a tendency to equality of wages for similar work, but the level of wages is necessarily determined by complex sociopolitical elements; custom, feelings of justice, informal allegiances to classes, as well as overt coalitions such as trade unions, far from being impediments to a smooth working of labour markets that would be able to determine wages even without these elements, are on the contrary indispensable because without them there would be no way to determine wages.
Equilibrium in perfect competition 
Equilibrium in perfect competition is the point where market demands will be equal to market supply. A firm's price will be determined at this point. In the short run, equilibrium will be affected by demand. In the long run, both demand and supply of a product will affect the equilibrium in perfect competition. A firm will receive only normal profit in the long run at the equilibrium point.
See also 
- Contestable market
- Imperfect competition
- Monopolistic competition
- Bertrand competition
- Cournot competition
- Mas-Colell, Andreu; Whinston, Michael D.; Green, Jerry R. (1995). Microeconomic Theory. Oxford University Press, New York and Oxford. p. 315. ISBN 978-0-19-507340-9.
- Steve Keen, Debunking Economics: The Naked Emperor of the Social Sciences, Pluto Press Australia, 2001
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- Binger & Hoffman, Microeconomics with Calculus, 2nd ed. (Addison-Wesley 1998) at 312–314. A firm's production function may display diminishing marginal returns at all production levels. In that case both the MC curve and the AVC curve would originate at the origin and there would be no minimum avc )or min avc = 0) Consequently the entire MC curve would be the SR supply curve.
- Perfect Competition
- This was the kind of criticism made by the "autisme economie" movement Example of this kind of criticisms: http://www.paecon.net/PAEtexts/Guerrien1.htm
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- Novshek, W., and H. Sonnenschein (1987), "General Equilibrium with Free Entry: A Synthetic Approach to the Theory of Perfect Competition", Journal of Economic Literature, Vol. 25, No. 3, September, pp. 1281–1306.
- Petri, F. (2004), General Equilibrium, Capital and Macroeconomics, Cheltenham: Edward Elgar.
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