Over the long run (a span of time long enough for firms to freely enter or leave the market with no barriers or to change their output level), another assumption of perfect competition is that profits will tend toward zero. To understand why profits will tend toward zero under this system, it is first necessary to understand the difference between explicit costs and implicit costs and the type of profit with which each is associated.
An explicit cost is a cost involving a payment. Explicit costs are paid by a firm directly, such as costs paid to suppliers of materials, to its labor force, and to landlords, as well as taxes, tariffs, and other fees. Depreciation of assets, or a decrease in value of assets, is also counted as an explicit cost of doing business. The amount left over from total revenue once explicit cost is subtracted is known as accounting profit and is the profit reported by a firm when paying taxes or reporting to shareholders.
An implicit cost is a cost that does not require the buyer to pay cash, or that cannot easily be assigned a monetary value. Implicit costs are incurred by a firm through the loss of opportunity to generate revenue in other ways, such as using the firm's assets to do other kinds of business and management's opportunities to make money doing other kinds of work. These opportunity costs are included along with explicit costs in the calculations that give rise to economic profit, which is the type of profit considered in analyses of perfect competition.The concepts of economic profit and opportunity cost demonstrate why, in a situation of perfect competition, profits must tend toward zero in the long run. If producers in an industry are earning greater economic profits than those in another industry, producers in the second industry are paying an opportunity cost by not producing goods in the first industry. Because perfect competition assumes there are no barriers to entry, producers in the second industry are likely to enter the market in the first industry. As producers enter the first industry, they will increase the supply in the first industry and reduce profits correspondingly. An increase in supply reduces the market price so that profits, being equal to , will go down for each individual firm, where P is price, ATC is average total cost, and Q is quantity sold. This process will continue until there are zero economic profits, because with zero economic profits, there is no longer motivation for new producers to enter the first industry.
Meanwhile, in the second industry, supply will have dwindled as producers left for more fertile economic ground. Perfect competition assumes that demand is constant, so price (and therefore economic profits) will increase in the second industry until the point of zero economic losses is reached. With zero economic losses, producers will no longer be motivated to leave the second industry.
Exit and Entry in the Long Run
Observing this situation, some clever juice pop makers decide to leave their industry and get into the candy bar business because candy bar firms seem to be doing pretty well by comparison. Assume firms that have been producing a total of 2,000 juice pops leave juice pop production for candy bar production, and with no economic barriers or additional costs, they are able to manufacture the same number of candy bars as they did juice pops.These firms are now producing 2,000 candy bars, so there is now a total of 12,000 candy bars being produced and sold. This greater supply causes the price of candy bars to fall to $0.75. The average cost of candy bars is still $0.75, so zero economic profit is being made. Meanwhile, over in the juice pop business, now only 8,000 juice pops are being made. This reduction in supply moves the market price up to $0.46 per juice pop. Losses are still being incurred in the juice pop business, but now at only $0.04 per juice pop instead of $0.05, a reduction in losses of a full 20%. The theory and conditions of perfect competition hold that this migration of producers from less profitable situations to more profitable ones will continue until profits on the one hand and losses on the other are both reduced to zero. The candy bar market further highlights the process of exit in the short-run. Some time in the future, the market price becomes $1.00, but the cost structure has changed, and average total cost is now $1.25 due to rising input costs. In this case, firms in the candy bar market are incurring short-term economic losses. Firms will begin to exit the market to reduce losses or search for profits elsewhere. Firms will exit, which reduces supply of candy bars in the market, and thus increases the market price. These exits will occur until the market reaches long-run equilibrium, where price equals average total cost and firms are making zero economic profit. In this new example, the equilibrium market price is $1.25 and average total cost is $1.25.