This is “Perfect Competition in the Long Run”, section 6.3 from the book Managerial Economics Principles (v. 1.0).
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As described in Chapter 4 "Cost and Production", a long-run time frame for a producer is enough time for the producer to implement any changes to its processes. In the short run, there may be differences in size and production processes of the firms selling in the market. Some sellers may be able to make a healthy economic profit, whereas others may only barely make enough to justify continued operation and, as noted earlier, may not have sustainable operations although they may continue to operate for a while since a substantial portion of their short-run costs are sunk costs.
Due to the assumption of perfect information, all sellers know the production techniques of their competitors. As a result, any firm that intends to remain in the market will revise its operations to mimic the operations of the most successful firms in the market. In theory, in the long run all firms would either have the most cost-efficient operations or abandon the market.
However, when all firms use the same processes, the possibility for firms to continue to earn positive economic profits will disappear. Suppose all firms are earning a positive profit at the going market price. One firm will see the opportunity to drop its price a small amount, still be able to earn an economic profit, and with the freedom to redefine itself in the long run, no longer be constrained by short-run production limits. Of course, when one firm succeeds in gaining greater profit by cutting its prices, the other firms will have no choice but to follow or exit the market, since buyers in perfect competition will only be willing to purchase the good from the seller who has the lowest price. Since the price has been lowered, all firms will have a lower economic profit than they had collectively before they lowered the price.
Some firms may realize they can even drive the price lower, again take sales from their competitors, and increase economic profit. Once again, all firms will be required to follow their lead or drop out of the market because firms that do not drop the price again will lose all their customers. And once again, as all firms match the lowered price, the economic profits are diminished.
In theory, due to competition, homogeneous goods, and perfect information, firms will continue to match and undercut other firms on the price, until the price drops to the point where all remaining firms make an economic profit of zero. As we explained earlier, an economic profit of zero is sufficient to sustain operations, but the firm will no longer be earning an accounting profit beyond the opportunity costs of the resources employed in their ventures.
Another necessary development in the long run under perfect competition is that all firms will need to be large enough to reach minimum efficient scale. Recall from Chapter 4 "Cost and Production" that minimum efficient scale is the minimum production rate necessary to get the average cost per item as low as possible. Firms operating at minimum efficient scale could charge a price equal to that minimum average cost and still be viable. Smaller firms with higher average costs will not be able to compete because they will have losses if they charge those prices yet will lose customers to the large firms with lower prices if they do not match their prices. So, in the long run, firms that have operations smaller than minimum efficient scale will need to either grow to at least minimum efficient scale or leave the market.