Perfect Competition

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. This is 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...