Monopolistic Competition Explained: Short Run, Long Run, Graph

Monopolistic competition is a market structure with many firms selling differentiated products and free entry and exit. Each firm faces its own downward-sloping demand curve, so it produces where MR = MC and charges a price above marginal cost. In the long run, entry pushes economic profit to zero, which happens when the firm's demand curve is tangent to its ATC curve.

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A monopolistically competitive firm earning a short-run economic profit. Tap it to drag the curves, open the full version, or put it on your own site free, or turn it into a five-minute class activity.

Restaurants, coffee shops, hair salons, clothing brands and dentists are the standard examples. This guide covers the four features of the structure, why P > MC, the short-run and long-run graphs, a fully worked numeric example, excess capacity, advertising, and how AP Micro topic 4.4 tests it. If you want the side-by-side with price takers, read perfect competition vs monopolistic competition.

The four features

Many sellers. Each firm is small relative to the market, so one firm's price change does not provoke a strategic response from rivals. That is the line between this structure and oligopoly, where a few firms watch each other closely.

Differentiated products. Firms sell close substitutes that are not identical. They differ by quality, style, location, brand or service. This is product differentiation, and it is the feature that gives each firm a small amount of market power.

Easy entry and exit. There are no significant barriers. A new taco shop can open next to an old one.

Some control over price. Because the product is a little different, a firm that raises its price loses some customers, not all of them.

The name describes the mix. The structure is "monopolistic" because each firm is the only seller of its exact product and faces a downward-sloping demand curve. It is "competition" because many close substitutes and free entry limit what any firm can earn over time.

Why P > MC

To sell one more unit, a firm with a downward-sloping demand curve has to lower its price on every unit it sells, not only the last one. So marginal revenue is less than price, and the MR curve lies below demand. For a straight-line demand curve, MR has the same intercept and is twice as steep.

The firm picks the quantity where MR = MC, then reads the price straight up on the demand curve. At that quantity, P is above MR, and MR equals MC, so P is above MC. That gap between price and marginal cost is the firm's markup.

Compared with a monopolist, a monopolistically competitive firm faces a flatter (more elastic) demand curve, because buyers have many close substitutes. Its markup is usually smaller. It is still positive, and that is enough to make the outcome allocatively inefficient. The profit maximization guide explains the MR = MC rule in detail.

The short run: profit, break even or loss

In the short run the number of firms is fixed, and the graph looks exactly like a monopoly graph. Find Q where MR = MC, read P on demand, and compare P with ATC at that quantity.

• If P > ATC, the firm earns economic profit, shown as a rectangle with height P minus ATC and width Q.
• If P = ATC, the firm breaks even.
• If P < ATC, the firm takes a loss. It keeps producing as long as P covers AVC.

The long run: entry until demand is tangent to ATC

Short-run profit attracts new firms. Each new firm sells a close substitute, so some customers leave the existing firms. Each existing firm's demand curve (and its MR curve) shifts left, and it usually becomes more elastic as substitutes multiply. Entry continues until economic profit is zero. Losses work the other way: firms exit, and demand for each remaining firm shifts right until losses disappear.

Zero economic profit means P = ATC at the profit-maximizing quantity. A downward-sloping demand curve can only touch ATC at one point without crossing it if the two curves are tangent there. Because demand slopes down, the tangency has to fall on the downward-sloping part of ATC, to the left of minimum ATC.

A correct long-run graph has five things:

1. A downward-sloping demand curve tangent to ATC at the firm's quantity.

2. An MR curve below demand.

3. MC crossing MR directly below the tangency point.

4. Price on the vertical axis at the tangency, equal to ATC.

5. MC crossing ATC at its minimum, to the right of the firm's quantity.

You can drag the demand curve into the tangency yourself in the monopolistic competition sandbox.

Worked example: from profit to the tangency

A coffee shop has total cost TC = 100 + 2Q + 0.25Q², where Q is cups per hour. That gives:

• Fixed cost: $100
• MC = 2 + 0.5Q
• ATC = 100/Q + 2 + 0.25Q

Minimum ATC. ATC is at its minimum where MC = ATC. Setting 2 + 0.5Q = 100/Q + 2 + 0.25Q gives 0.25Q = 100/Q, so Q² = 400 and Q = 20. ATC there is 5 + 2 + 5 = $12.

Short run. Demand is P = 28 - 0.75Q, so MR = 28 - 1.5Q. Setting MR = MC: 28 - 1.5Q = 2 + 0.5Q, so 26 = 2Q and Q = 13. Price is 28 - 0.75(13) = 28 - 9.75 = $18.25. Total revenue is 18.25 x 13 = $237.25. Total cost is 100 + 26 + 42.25 = $168.25. Economic profit is 237.25 - 168.25 = $69.

Long run. That $69 attracts new coffee shops, and this shop's demand shifts left to P = 22 - 0.75Q. (For simplicity the slope stays the same here; in practice it often gets flatter too.) Now MR = 22 - 1.5Q. Setting MR = MC: 22 - 1.5Q = 2 + 0.5Q, so 20 = 2Q and Q = 10. Price is 22 - 7.5 = $14.50. ATC at 10 cups is 10 + 2 + 2.5 = $14.50. Total revenue is $145 and total cost is 100 + 20 + 25 = $145, so economic profit is zero.

Check the tangency. ATC minus price is (100/Q + 2 + 0.25Q) - (22 - 0.75Q) = 100/Q + Q - 20. At Q = 10 that is 10 + 10 - 20 = 0. At any other quantity it is positive (for example, at Q = 5 it is 20 + 5 - 20 = 5). Demand touches ATC at one point and lies below it everywhere else, which is exactly what tangency means.

Three results come straight out of the long-run column:

Markup. P - MC = 14.50 - 7.00 = $7.50. As a percentage of marginal cost, that is (7.50 / 7.00) x 100, about 107%. You can rerun this with the markup over marginal cost calculator.

Excess capacity. The shop sells 10 cups, but ATC bottoms out at 20 cups. Excess capacity is 20 - 10 = 10 cups, and each cup costs 14.50 - 12.00 = $2.50 more than it would at minimum ATC. The excess capacity calculator walks through the same steps.

Deadweight loss. The allocatively efficient quantity is where demand meets MC: 22 - 0.75Q = 2 + 0.5Q, so Q = 16 and P = $10. The deadweight loss triangle is 0.5 x (16 - 10) x (14.50 - 7.00) = $22.50. The market does not reach 16 cups because at that output ATC is 6.25 + 2 + 4 = $12.25, above the $10 price, and the shop would lose money.

Excess capacity and efficiency

In long-run equilibrium, a monopolistically competitive firm fails both efficiency tests:

• Not allocatively efficient, because P > MC. Some cups that buyers value above their marginal cost never get made.
• Not productively efficient, because the firm produces left of minimum ATC. That gap is excess capacity, the restaurant with empty tables on a Tuesday night.

The usual defense is variety. Buyers get many different cafes, styles and locations instead of one identical product, and many economists treat the small inefficiency as the cost of that variety.

Product differentiation and advertising

Because products differ, firms compete on more than price. This non-price competition includes advertising, branding, packaging, location, quality and service.

Advertising has two goals. It tries to shift the firm's demand curve right, and it tries to make demand less elastic by convincing buyers that rivals are not close substitutes. Both raise the price the firm can charge. Advertising also costs money, so it shifts ATC up.

Free entry still applies. If a campaign creates profit, rivals copy it or new firms enter, and long-run profit returns to zero. A firm that advertises heavily ends up at a new tangency, often with higher costs and a higher price than before.

How it compares with the other structures

The monopoly vs monopolistic competition comparison lines up the two structures that share a downward-sloping demand curve.

How it shows up on the AP exam

Monopolistic competition is topic 4.4 in AP Microeconomics. Questions usually ask you to:

• Draw a correctly labeled long-run graph with demand tangent to ATC, and mark P and Q.
• Shade short-run profit or loss on the firm's graph.
• Explain what happens in the long run when firms earn profit, and show the demand curve shifting left.
• Say whether the firm is allocatively or productively efficient, and explain why.
• Identify excess capacity, or explain the effect of advertising on demand.

Common mistakes:

• Drawing demand crossing ATC in a long-run graph. That shows profit or loss. Long run means tangency.
• Putting the tangency at minimum ATC. That is perfect competition. The tangency sits on the falling part of ATC.
• Saying the long-run price equals MC. Price equals ATC, and it stays above MC.
• Shifting a supply curve to show entry. The firm's graph has no supply curve. Entry shows up as the firm's demand and MR shifting left.
• Placing the tangency anywhere except directly above the MR = MC quantity.

For more practice, work through the monopolistic competition module and the topic page for AP Micro 4.4.