5.3 Short-run and long-run equilibrium in monopolistic competition

Monopolistic competition blends elements of perfect competition and monopoly. Firms have some price-setting power because their products are differentiated, but they face competition from many rivals selling similar (not identical) products. Understanding how equilibrium works in both the short run and long run is central to seeing why this market structure produces characteristic inefficiencies alongside genuine benefits like product variety.

Short-run Equilibrium in Monopolistic Competition

Profit Maximization and Demand Characteristics

A monopolistically competitive firm maximizes profit the same way any firm does: produce the quantity where $MR = MC$. But the shape of the demand curve is what makes this market structure distinctive.

Because each firm sells a slightly different product, it faces a downward-sloping demand curve. That slope gives the firm some ability to raise price without losing all its customers. The demand curve is more elastic than a monopolist's (because close substitutes exist) but less elastic than in perfect competition (because products aren't identical).

In the short run, three outcomes are possible:

• Economic profit: $P > ATC$ at the profit-maximizing quantity. A clothing brand charges $50 for a shirt that costs $35 per unit on average to produce, earning $15 profit per shirt.
• Economic loss: $P < ATC$. The firm loses money on each unit but may continue operating if $P > AVC$ (covering variable costs and some fixed costs).
• Break even: $P = ATC$. The firm earns zero economic profit.

Price always exceeds marginal cost ($P > MC$) because the downward-sloping demand curve means $MR < P$, so setting $MR = MC$ guarantees a markup.

Excess Capacity and Market Position

Even in the short run, firms don't produce at the quantity that minimizes ATC. They produce less than that efficient scale, meaning they carry excess capacity. A restaurant might have seating for 100 but typically serves 70 customers during peak hours.

How elastic the firm's demand curve is depends on how many close substitutes are available and how differentiated the product is. In the smartphone market, for instance, many competing brands make each individual firm's demand fairly elastic. In a market with fewer close substitutes, demand is less elastic and the firm has more pricing power.

Long-run Equilibrium in Monopolistic Competition

Market Entry and Exit Dynamics

The key mechanism driving the long run is free entry and exit.

When existing firms earn economic profits, the adjustment works like this:

1. New firms enter the market, attracted by those profits.
2. Each new entrant draws some customers away from incumbents, shifting each existing firm's demand curve leftward.
3. As demand shifts left, the profit-maximizing price and quantity both fall, squeezing profits.
4. Entry continues until economic profit reaches zero: $P = ATC$ at the profit-maximizing output.

The reverse happens when firms suffer losses:

1. Some firms exit the market.
2. Remaining firms pick up customers, shifting their demand curves rightward.
3. Exit continues until losses disappear and $P = ATC$.

At long-run equilibrium, the firm's demand curve is tangent to the ATC curve at the profit-maximizing quantity. This tangency condition is the defining graphical result. The firm still sets $MR = MC$, but the price it charges exactly equals its average total cost, so economic profit is zero.

Notice what tangency means geometrically: the demand curve touches the ATC curve at exactly one point. If it intersected the ATC curve (crossing through it), the firm would earn positive profit at some quantities, inviting further entry. Tangency is the only configuration consistent with zero profit and a downward-sloping demand curve.

As new entrants offer similar products, each firm's demand becomes more elastic, reducing individual market power. Think of a neighborhood where three coffee shops become six: each shop now faces customers who can more easily switch.

Persistent Characteristics and Inefficiencies

Even with zero economic profit, two features persist in the long run:

• Markup over marginal cost: Price still exceeds $MC$ because the demand curve is still downward-sloping. At the tangency point, $P = ATC > MC$. (This follows from the fact that ATC is declining at the tangency point, which means $MC < ATC$ at that quantity.)
• Excess capacity: The tangency point sits to the left of the minimum of the ATC curve. Firms produce less output than the quantity that would minimize average cost. A gym built for 500 members might operate sustainably with only 400, never reaching its most cost-efficient scale.

Why does the tangency always occur on the downward-sloping portion of ATC? Because the demand curve is downward-sloping, the only way it can be tangent to the U-shaped ATC curve (touching it without crossing) is on the left side of ATC's minimum. If the tangency occurred at the minimum of ATC, the demand curve would have to be flat (horizontal) at that point, which would mean perfect competition.

Product differentiation also persists. Firms continue investing in distinguishing their products because that differentiation is what gives them their downward-sloping demand curve in the first place. If a firm stopped differentiating, its demand would become perfectly elastic and it would lose all pricing power.

Efficiency: Monopolistic Competition vs Perfect Competition

Inefficiencies in Monopolistic Competition

Compared to the perfectly competitive benchmark, monopolistic competition produces two types of inefficiency:

• Allocative inefficiency: $P > MC$, meaning society values additional units more than they cost to produce. In perfect competition, $P = MC$, so this distortion doesn't arise.
• Productive inefficiency: Firms don't produce at minimum ATC. In perfect competition, long-run equilibrium occurs at the minimum of the ATC curve.

Together, these inefficiencies create deadweight loss. If a firm charges $10 for a product with a marginal cost of $8, some consumers who value the product between $8 and $10 don't buy it. Those mutually beneficial transactions don't happen.

Potential Benefits of Monopolistic Competition

The efficiency comparison doesn't tell the whole story. Monopolistic competition delivers real benefits that perfect competition cannot:

• Product variety: Consumers get meaningful choices. Multiple car brands offer different designs, features, and price points rather than a single homogeneous product. Economists sometimes frame this as a tradeoff: the excess capacity "cost" is what society pays for variety.
• Innovation: The need to differentiate pushes firms to develop new products and improve existing ones. Smartphone manufacturers continuously introduce new features precisely because standing still means losing customers to rivals.
• Non-price competition: Firms compete through advertising, branding, and quality improvements. This provides information to consumers and shapes the competitive landscape in ways absent from perfect competition.

Whether the variety benefits outweigh the efficiency costs is an empirical question that depends on the specific market. Economists generally view the deadweight loss in monopolistic competition as relatively small because demand is fairly elastic, which keeps the markup over $MC$ modest.

Entry and Exit in Monopolistic Competition

Long-run Market Adjustments

Free entry and exit are what drive the market toward zero economic profit. The adjustment process works through demand shifts rather than price changes imposed by the market (as in perfect competition).

• Entry effect: A new artisanal bakery opens in a neighborhood with established bakeries. Each existing bakery loses some customers, its demand curve shifts left, and its profits shrink.
• Exit effect: A local bookstore closes. The remaining competitors absorb its former customers, their demand curves shift right, and profitability improves.

This process continues until the market reaches the tangency condition: each firm's demand curve is tangent to its ATC curve, yielding zero economic profit.

Industry Dynamics and Efficiency

The entry and exit process also generates efficiency gains over time. Less efficient firms tend to be the ones that exit, replaced by more capable entrants. The decline of video rental stores and rise of streaming services is a dramatic example of this process at work.

The threat of entry matters even before new firms actually arrive. Established restaurants improve their menus and service quality partly because they know a new competitor could open nearby at any time. This competitive pressure keeps incumbents from becoming complacent, which is one reason monopolistic competition tends to perform better than pure monopoly despite sharing some structural features with it.