If one monopolistic competitor earns positive economic profits, other firms will be tempted to enter the market. A gas station with a great location must worry that other gas stations might open across the street or down the road—and perhaps the new gas stations will sell coffee or have a carwash or some other attraction to lure customers. A successful restaurant with a unique barbecue sauce must be concerned that other restaurants will try to copy the sauce or offer their own unique recipes. A laundry detergent with a great reputation for quality must be concerned that other competitors may seek to build their own reputations.
The entry of other firms into the same general market (like gas, restaurants, or detergent) shifts the demand curve faced by a monopolistically competitive firm. As more firms enter the market, the quantity demanded at a given price for any particular firm will decline, and the firm’s perceived demand curve will shift to the left. As a firm’s perceived demand curve shifts to the left, its marginal revenue curve will shift to the left, too. The shift in marginal revenue will change the profit-maximizing quantity that the firm chooses to produce, since marginal revenue will then equal marginal cost at a lower quantity.
Figure 10.4 (a) shows a situation in which a monopolistic competitor was earning a profit with its original perceived demand curve (D0). The intersection of the marginal revenue curve (MR0) and marginal cost curve (MC) occurs at point S, corresponding to quantity Q0, which is associated on the demand curve at point T with price P0. The combination of price P0 and quantity Q0 lies above the average cost curve, which shows that the firm is earning positive economic profits.
Figure 10.4. Monopolistic Competition, Entry, and Exit (a) At P0 and Q0, the monopolistically competitive firm shown in this figure is making a positive economic profit. This is clear because if you follow the dotted line above Q0, you can see that price is above average cost. Positive economic profits attract competing firms to the industry, driving the original firm’s demand down to D1. At the new equilibrium quantity (P1, Q1), the original firm is earning zero economic profits, and entry into the industry ceases. In (b) the opposite occurs. At P0 and Q0, the firm is losing money. If you follow the dotted line above Q0, you can see that average cost is above price. Losses induce firms to leave the industry. When they do, demand for the original firm rises to D1, where once again the firm is earning zero economic profit.