Intermediate Microeconomic Theory | Perloff 5th Ed., Chapter 14 (Section 14.6)
Tags: monopolistic competition, free entry, zero profit, downward-sloping demand, excess capacity, minimum efficient scale, fixed costs, product differentiation, food trucks
Difficulty: Intermediate | Prerequisites: Parts 1–3 notes (Market Structures, Cournot, Stackelberg, Bertrand), Chapter 7 (Costs), Chapter 11 (Monopoly)
Monopolistic competition is the last market structure in this chapter and in many ways the most common one in daily life: restaurants, clothing brands, local services, books. It combines elements of monopoly (downward-sloping demand, price above marginal cost) with elements of competition (free entry, zero long-run economic profit). This section explains how those two features coexist and why the number of firms in equilibrium depends on fixed costs.
In a monopolistically competitive market, firms face downward-sloping demand curves (due to few rivals or differentiated products) and set MR = MC to maximise profit. But free entry means new firms keep entering until economic profit is driven to zero (price equals average cost). The result: firms have market power but no long-run profit, and they operate below full capacity.
Monopolistic competition
A market structure in which firms have market power (downward-sloping demand, price above MC) but no additional firm can enter and earn a positive profit. Zero long-run economic profit, but price above marginal cost. Think of it as monopoly pricing with competitive profits.
Full capacity / minimum efficient scale
The smallest quantity at which the average cost curve reaches its minimum. In monopolistic competition, firms operate to the left of this point (on the downward-sloping portion of AC), meaning they always have excess capacity in long-run equilibrium.
Excess capacity
The gap between a monopolistically competitive firm's actual output and its minimum efficient scale. The firm could produce more at a lower average cost, but doing so would push price below average cost and create losses.
Downward-sloping residual demand
In monopolistic competition, each firm faces a downward-sloping demand curve because it has relatively few rivals, or because its product is differentiated, or both. This is what gives the firm market power, in contrast to perfectly competitive firms, which face horizontal demand.
Two reasons a monopolistically competitive firm has a downward-sloping residual demand curve:
Few rivals relative to the market. When economies of scale are significant relative to market demand, the market supports only a limited number of firms. Each firm is large enough to affect price.
Product differentiation. If consumers perceive products as different (Tide vs Cheer, one restaurant vs another), a firm does not lose all sales when it raises its price slightly. Loyal customers stick around.
The fewer the firms, the less elastic the residual demand. The elasticity of the residual demand is nε, where n is the number of firms and ε is the market demand elasticity.
A monopolistically competitive equilibrium requires two conditions to hold simultaneously:
Profit maximisation: MR^r = MC (marginal revenue from the residual demand equals marginal cost). This is the same condition as for any profit-maximising firm.
Zero economic profit: p = AC (price equals average cost). Free entry ensures this. If price exceeded AC, positive profits would attract entrants. If AC exceeded price, firms would exit.
Graphically, these conditions hold where the average cost curve is tangent to the residual demand curve. At that tangency point, the firm produces output q, charges price p = AC, and earns zero profit.
Because the monopolistically competitive equilibrium occurs where the downward-sloping demand curve is tangent to the downward-sloping portion of the AC curve, the firm produces at a quantity less than the minimum of AC. This is the "excess capacity" result: the firm could lower its average cost by producing more, but it cannot sell the extra output without cutting price below average cost.
This is a key difference from perfect competition, where firms produce at the minimum of AC in the long run.
The number of firms in a monopolistically competitive equilibrium depends on fixed costs. Higher fixed costs mean fewer firms can break even, so the equilibrium has fewer firms.
Using the same demand and cost parameters as the Cournot model (a = 339, b = 1, m = $147), but now allowing free entry and adding a fixed cost F:
No fixed costs (F = 0): Firms earn positive profit in the Cournot equilibrium. Entry would continue indefinitely, approaching perfect competition.
F = $2.3 million: With two firms, each earns profit of about $1.8M (above zero). This attracts a third entrant. With three firms, each produces q = 48, charges p = $195, and AC = $195 exactly. Zero profit. Three firms is the monopolistically competitive equilibrium.
F = $3 million: With two firms, each earns $1.1M profit. A third entrant would cause losses of $0.7M per firm. So two firms is the equilibrium, each earning a small positive profit that is too small to attract entry. (This is technically a monopolistically competitive equilibrium because no further firm wants to enter.)
F = $4.1 million: Fixed cost exactly absorbs the Cournot duopoly profit. Two firms, zero profit. Same output and price as the Cournot equilibrium (q = 64, p = $211), but now with zero economic profit.
The lower the fixed costs, the more firms in the monopolistically competitive equilibrium. This is because lower fixed costs make it easier for entrants to break even, so more firms can coexist.
The gourmet food truck market illustrates monopolistic competition well:
Low fixed costs ($50,000 to $250,000) compared to brick-and-mortar restaurants, so entry is easy and many firms operate.
Product differentiation is strong (cuisines, branding, location).
Free entry: when lines grow long, new trucks appear. When demand shifts, trucks can physically move to a different neighbourhood.
Zero or near-zero long-run profit for the marginal entrant.
Average cost with fixed costs:
AC = m + F/q
where m is the constant marginal cost and F is the fixed cost.
Cournot equilibrium profit (per firm, n firms, no fixed costs):
π = [(a - m) / (n + 1)]² / b
This is the profit that fixed costs must absorb for the monopolistically competitive equilibrium to have exactly n firms.
"Monopolistic competition is a contradiction in terms." The name combines "monopoly" (each firm has some market power from its downward-sloping demand) with "competition" (free entry drives profit to zero). The two features coexist: firms have pricing power but no long-run profit.
"Monopolistically competitive firms are efficient." They are not, in the sense that they produce below minimum efficient scale (excess capacity). This is an inherent inefficiency of the market structure.
"Price equals marginal cost in monopolistic competition." Price equals average cost (p = AC), not marginal cost. Price is above marginal cost, creating a deadweight loss. The zero-profit condition comes from p = AC, not p = MC.
"More firms always means lower prices." More firms generally does mean lower prices (the residual demand becomes more elastic). But the relationship between fixed costs and the equilibrium number of firms means that in some markets, high fixed costs keep prices high by limiting the number of viable entrants.
⚠️ The two equilibrium conditions (MR = MC and p = AC) must both hold simultaneously. Be able to identify these on a graph where the AC curve is tangent to the residual demand curve.
⚠️ Understand why monopolistically competitive firms operate at less than full capacity (excess capacity theorem). This is the tangency condition on the downward-sloping part of AC.
⚠️ Be able to calculate the fixed cost that results in a given number of firms in equilibrium: set the Cournot profit equal to the fixed cost.
⚠️ Know the difference between monopolistic competition (free entry, zero profit, price above MC) and perfect competition (free entry, zero profit, price equals MC). The key distinction is the shape of the demand curve.
⚠️ The relationship "lower fixed costs = more firms in equilibrium" is commonly tested.
True or False: Monopolistically competitive firms earn positive long-run economic profit. (False, free entry drives profit to zero.)
Fill in the blank: In monopolistic competition, the zero-profit condition is p = __________. (AC / average cost.)
True or False: Monopolistically competitive firms produce at the minimum of their average cost curve. (False, they produce to the left of the minimum, on the downward-sloping portion.)
Fill in the blank: The larger the fixed cost, the __________ firms in the monopolistically competitive equilibrium. (Fewer.)
True or False: A monopolistically competitive firm faces a horizontal demand curve. (False, it faces a downward-sloping demand curve.)
Q: In the airline example, what fixed cost results in a three-firm monopolistically competitive equilibrium?
A: With three firms and no fixed costs, each firm earns a Cournot profit of about $2.3 million [using q = 48, p = $195, profit = (195 - 147) × 48 = $2,304,000]. So a fixed cost of $2.3 million produces exactly three firms with zero profit.
Q: Why does a monopolistically competitive firm have market power even though it earns zero profit?
A: Market power means the ability to set price above marginal cost, which comes from the downward-sloping demand curve. The firm uses this power by setting MR = MC, which gives a price above MC. But free entry means other firms enter until price is driven down to average cost. The firm charges above MC (market power) but at the same price as AC (zero profit).
Q: What is the monopolistically competitive equilibrium if firms face no fixed costs?
A: With no fixed costs and constant marginal cost, AC = MC at all output levels. The zero-profit condition (p = AC) then requires p = MC, which is the competitive outcome. Firms would keep entering until the market is perfectly competitive. Fixed costs are necessary for the typical monopolistically competitive outcome with downward-sloping AC.
Q: How does monopolistic competition create a market failure?
A: Because firms face downward-sloping demand curves, they set price above marginal cost. This means consumers buy less than the socially efficient quantity (where willingness to pay equals marginal cost). The gap between price and marginal cost is the source of deadweight loss. This inefficiency exists even though firms earn zero economic profit.
Monopolistic competition ties together several threads from the chapter. The Cournot model (Part 2) provides the profit function that determines how many firms the market can support. The relationship between the number of firms and the Lerner Index (from the Cournot analysis) applies here too: more firms means more elastic residual demand and lower markups. The excess capacity result connects to Chapter 7 (Costs) and the distinction between short-run and long-run cost structures. The market failure (price above MC) connects to Chapter 9's discussion of inefficient consumption.
monopolistic competition, free entry, zero economic profit, excess capacity, minimum efficient scale, full capacity, downward-sloping demand, residual demand, product differentiation, fixed costs, number of firms, market failure, deadweight loss, food trucks, tangency condition, long-run equilibrium, average cost pricing, barriers to entry, economies of scale