Key Idea: Most firms are not tiny sellers of identical wheat. A phone maker, a rail network, two ferry companies on the same route: each can choose its price, at least a little. This topic asks how much power a firm has over its price, where that power comes from, and what it costs everyone else. The whole topic runs on one rule. A firm keeps producing while the next unit adds more revenue than cost, so it stops where MR = MC. Then the number of rivals and the height of the barriers decide whether its profit lasts. Where you meet it:
Paper 1
- Essays.
- Explain [10]: why abnormal profit lasts in a monopoly but not under perfect or monopolistic competition.
- Evaluate / Discuss [15]: is market power always undesirable, and does government intervention work?
Paper 2
- Data response.
- Define [2]: monopoly, profit.
- Explain [4] with a diagram: a monopolist making a loss, or two economies of scale a small producer cannot reach.
Paper 3
- Calculations and policy.
- Calculate [1 or 2]: marginal cost, marginal revenue and profit from a cost and revenue table.
- Sketch [2] a monopoly's welfare loss; Recommend [10] a policy against abuse of market power.
Profit and the MR = MC rule
Profit = total revenue − total cost, and total revenue is price × quantity. Marginal revenue (MR) is the extra revenue from selling one more unit; marginal cost (MC) is the extra cost of making it. A firm that wants the biggest profit compares the two, unit by unit.
Reading any firm diagram
- Find MR = MC — While MR is above MC the next unit adds to profit, so make it. Once MC is above MR it subtracts. Stop where they cross: the profit-maximising output.
- Drop down for the quantity — From the crossing, go straight down to the quantity axis. That is how much the firm makes.
- Go up to demand for the price — From that quantity go up to the demand (AR) curve, not to MR. Demand tells you what buyers will pay for that quantity. A price taker has P = AR = MR on one horizontal line; a price maker has MR below demand, because selling one more unit means cutting the price on every unit.
- Compare price with ATC — Price above ATC: abnormal profit, the rectangle (P − ATC) × Q. Price equal to ATC: normal profit. Price below ATC: a loss.
MR = MC → down for Q → up to demand for P → compare P with ATC
Normal profit is the return just big enough to keep the owner in this business rather than another. It is counted inside costs, so a firm with price = ATC is doing fine, not failing. Abnormal (supernormal) profit is anything above that. It is what attracts new firms in, wherever they can get in.
Costs: the short run and the long run
In the short run at least one input is fixed, usually the factory. In the long run the firm can change everything, including its size. Costs behave differently in the two, and questions test whether you can tell them apart.
Short run: one input is stuck: **Fixed costs** (rent) do not change with output; **variable costs** (materials, wages) do. TC = FC + VC.. **ATC = TC ÷ Q.** **MC** = the change in TC ÷ the change in Q.. Adding more workers to a fixed factory eventually adds less output per worker: the **law of diminishing returns**. That is why MC and ATC turn upwards.. **MC cuts ATC at its minimum**: below the average it pulls the average down, above it pushes it up..
Long run: choose your scale: All inputs can change, so there are no fixed costs.. **Economies of scale**: a bigger firm has a lower average cost. Bulk buying, bigger machines, specialist managers, cheaper loans.. **Diseconomies of scale**: grow too big, and communication and coordination problems push average cost up again.. **Minimum efficient scale**: the smallest output at which average cost stops falling. When it is huge compared with the whole market, one firm serves it most cheaply: a **natural monopoly** (a water network, a rail network)..
Important: Asked why costs rise as output grows, check the time frame. Short run → diminishing returns: more workers in the same factory. Long run → diseconomies of scale: the whole firm got too big. A firm can face both at once: crowded in today's factory, yet cheaper per unit if it built a bigger one.
Four market structures side by side
Three things decide how much power a firm has: how many firms there are, whether the products are identical or different, and how high the barriers to entry stand. Change those three and you move along the line from perfect competition to monopoly.
| Structure | Firms and product | Barriers to entry | Demand curve and long-run profit |
|---|---|---|---|
| Perfect competition | very many firms, an identical product (wheat) | none: free entry and exit | horizontal, so P = AR = MR; normal profit only in the long run |
| Monopolistic competition | many firms, slightly different products (restaurants, hairdressers) | low | downward sloping and fairly elastic; normal profit only in the long run |
| Oligopoly | a few large firms, identical or branded products (airlines, phone makers) | high | depends on what rivals do; abnormal profit can last |
| Monopoly | one firm, no close substitute (a rail network) | very high: patents, scale, a key resource | the whole market demand curve, MR below it; abnormal profit lasts |
Market power is a firm's ability to raise its price above marginal cost without losing all its customers. A price taker has none; a price maker has some. Being big is evidence of power, not the definition. A large wheat farm is still a price taker.
⏳ Abnormal profit: does it last?
In the short run any firm can earn abnormal profit if price is above ATC at its MR = MC output. Whether it keeps that profit depends on one thing: can new firms get in?
No barriers: profit is competed away: Abnormal profit attracts new firms.. **Perfect competition**: market supply rises, the market price falls, and each firm's horizontal demand line drops until P = minimum ATC. Normal profit only.. **Monopolistic competition**: new rivals take customers, so each firm's demand curve shifts **left and becomes more elastic**, until it just **touches** ATC. Normal profit only, on the falling part of ATC: **excess capacity**.. Losses work the other way round: firms leave, the price rises, and the losses close..
High barriers: profit stays: **Monopoly**: nobody can enter, so the profit rectangle stays year after year. The firm faces the whole market demand curve, with MR below it.. **Oligopoly**: high barriers keep the few firms few, so abnormal profit can persist, especially if they collude.. A monopolist can still make a **loss**: if ATC sits above the demand curve at every output, MR = MC only picks the smallest loss.. **Shutdown rules**: in the short run keep producing while P ≥ AVC; in the long run leave the industry when P < ATC..
The profit or loss rectangle sits between price and ATC, across to the MR = MC quantity. Read the price off demand, never off MR. In the long-run diagram for perfect or monopolistic competition the demand curve touches ATC at the chosen output: no rectangle, normal profit.
Oligopoly: a few firms watching each other
In an oligopoly every decision depends on how rivals will react: that is interdependence. Firms can collude, openly as a cartel or quietly by following a price leader, or compete, usually on advertising and quality rather than price. A payoff matrix shows why collusion is tempting and why it breaks.
| Profit (€m): A's choice ↓, B's choice → | B keeps the price high | B cuts the price |
|---|---|---|
| A keeps the price high | A 10, B 10: the joint best, what a cartel aims for | A 4, B 14: B gains customers, A loses them |
| A cuts the price | A 14, B 4: A gains customers, B loses them | A 6, B 6: a price war, the Nash equilibrium |
Important: Whatever B does, A earns more by cutting: 14 beats 10, and 6 beats 4. Cutting is A's dominant strategy, and B's too. Both cut, both end on 6, though both would rather have 10. That is the prisoner's dilemma, and it is why cartels are unstable. While collusion holds, the firms act like one monopoly: a higher price, less output, P above MC, a welfare loss. That is why price fixing is illegal and fined. Prices are also sticky: a rise is not followed by rivals, so customers leave; a cut is matched, so few customers are gained. The explorer's kinked demand curve shows it.
Efficiency, and when the government steps in
Efficiency has two tests at a point in time and one over time. Perfect competition passes both static tests in the long run. Every structure with market power fails them, and that is the case for stepping in.
| Efficiency | The test | Who achieves it |
|---|---|---|
| Allocative | P = MC: the last unit is worth to buyers exactly what it cost to make. No welfare loss. | perfect competition in the long run; nothing with market power |
| Productive | output at minimum ATC: made as cheaply as possible | perfect competition in the long run; monopolistic competition misses it through excess capacity |
| Dynamic | products and methods keep improving over time, paid for by research | more likely in monopoly and oligopoly, whose abnormal profit funds it |
| X-inefficiency | costs above the minimum because nobody is forcing them down | the risk wherever competition is weak |
Competition law: fine cartels, block mergers that would cut competition, and in extreme cases break a firm up. Regulate a natural monopoly's price (often at P = AC, so normal profit only) instead of breaking it up, because two water networks cost more than one. Open the market to imports, so foreign firms provide the competition. The case for market power: economies of scale can make one big firm cheaper than many small ones, and abnormal profit can fund research a competitive firm could not afford. So the verdict is it depends: on how large the scale gains are, whether the firm really innovates, and whether the regulator knows enough to act.
Worked examples
Three shapes this topic is asked in, with new scenarios. Try each before opening the plan. The first one uses this table.
| Output (000 units a week) | Price (€) | Total revenue (€000) | Total cost (€000) |
|---|---|---|---|
| 1 | 30 | 30 | 20 |
| 2 | 28 | 56 | 34 |
| 3 | 26 | 78 | 50 |
| 4 | 24 | 96 | 70 |
| 5 | 22 | 110 | 96 |
A firm's weekly output, price, total revenue and total cost are shown in the table. Calculate the profit earned at the profit-maximising level of output.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Now the Paper 1 explain.
Using diagrams, explain why a monopoly can earn abnormal profit in the long run while a firm in monopolistic competition cannot.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
And the 15-marker.
Using real-world examples, evaluate the view that firms with significant market power are always undesirable.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Quick self-check
Tap each card to check yourself.
At its current output a firm has MR = 18 and MC = 20. What should it do? Make less. The last unit cost 20 and brought in 18, so it cut profit. Reduce output until MR = MC.
Where do you read the monopoly price? Off the demand (AR) curve, straight above the MR = MC output. Reading it off MR is the classic slip.
Why does MR lie below demand for a price maker? To sell one more unit the firm must lower the price on every unit, so the extra revenue is less than the new price. On a straight-line demand curve MR falls twice as steeply.
Why does MC cut ATC at its minimum? While the next unit costs less than the average it pulls the average down; once it costs more it pushes the average up. They cross exactly where the average stops falling.
A factory adds a night shift and each extra worker adds less output. Diminishing returns or diseconomies of scale? Diminishing returns. The factory is fixed, so this is the short run. Diseconomies of scale need the whole firm to grow.
What is a natural monopoly, and what should a government do about it? One firm can supply the whole market at a lower average cost than two could, because fixed costs are huge (a water network). Regulate the price, do not break it up: two networks cost more than one.
In the long run a firm in monopolistic competition earns normal profit. Why is it still inefficient? Demand touches ATC on its falling part, so output is below minimum ATC: excess capacity. And price is above MC, so it is not allocatively efficient either.
In a payoff matrix, what is a dominant strategy? The choice that is best whatever the rival does. When both firms follow theirs, the result is the Nash equilibrium, even if both would prefer the collusive outcome.
Exam tips
- MR = MC finds the output. Then go up to demand for the price, never to MR.
- Profit or loss is the rectangle between price and ATC at that output. Normal profit is a cost, so price = ATC is a firm doing fine.
- Short run: diminishing returns. Long run: economies and diseconomies of scale. Check the time frame before you explain why costs rise.
- Whether abnormal profit lasts is a question about barriers to entry, not size. Say so in every long-run answer.
- In a calculation, MR and MC are changes between two outputs, so name the two outputs. Show the subtraction and write the units.
- Oligopoly: use the word interdependence early, then explain the payoff matrix in words as well as numbers.
- In a 15-marker on market power give the case for as well as against: economies of scale and research funding are real. End with it depends on and say on what.