A lump-sum tax is a fixed tax that doesn't change with how much a firm produces, so it affects only fixed costs (shifting ATC up) and leaves marginal cost untouched. The firm's profit-maximizing price and quantity stay the same; only profit shrinks (EK POL-4.A.2).
A lump-sum tax is a flat fee the government charges a firm regardless of output. Produce 10 units or 10,000 units, the tax bill is identical. Think of it like an annual license fee. Because the tax doesn't change with quantity, it's a fixed cost, and fixed costs never show up in marginal cost.
That's the whole trick. Firms maximize profit where MR = MC, and a lump-sum tax doesn't move MR or MC, so the profit-maximizing quantity and price don't move either. On a graph, the ATC curve shifts up while the MC curve stays put. The only casualty is profit, which falls by exactly the amount of the tax. This holds in any market structure (perfect competition, monopoly, you name it), which is exactly what EK POL-4.A.2 says: lump-sum taxes and subsidies change neither marginal cost nor marginal benefit, only fixed costs.
Lump-sum taxes live in Topic 6.4 (The Effects of Government Intervention in Different Market Structures) in Unit 6, under learning objectives 6.4.A, 6.4.B, and 6.4.C. You're expected to define the intervention, show its effect on graphs, and calculate the new profit. The reason the CED cares is the contrast it sets up. Per-unit taxes shift MC and change equilibrium quantity, price, and deadweight loss. Lump-sum taxes don't. A lump-sum tax on a monopoly earning positive profit just transfers some of that profit to the government without creating any new inefficiency. That makes it the classic example of a policy that raises revenue without distorting output, and it's a favorite MCQ trap because the intuitive answer (a tax must raise price) is wrong.
Keep studying AP® Microeconomics Unit 6
Per-Unit Tax (Unit 6)
This is the direct contrast the exam builds questions around. A per-unit tax scales with output, so it shifts MC and the MR = MC point moves, changing quantity and price. A lump-sum tax is output-blind, so nothing moves except profit. If you can graph both side by side, you've basically mastered Topic 6.4's tax material.
Average Total Cost (ATC) (Unit 3)
A lump-sum tax is just an increase in total fixed cost, and Unit 3 already told you what that does. ATC and AFC shift up, MC and AVC stay put. Lump-sum tax questions are really Unit 3 cost-curve questions wearing a government-policy costume.
Deadweight Loss (Units 2 & 6)
Because a lump-sum tax doesn't change quantity, it creates no new deadweight loss. Compare that to a per-unit tax in a competitive market, which shrinks quantity below equilibrium and burns surplus. This is why economists call lump-sum taxes 'non-distortionary.'
Profit Maximization (MR = MC) (Unit 3 & 4)
The entire lump-sum result rests on the MR = MC rule. Since the tax touches neither curve, the profit-maximizing output is unchanged whether the firm is a perfect competitor or a monopolist. One caveat for the long run: if the tax pushes profit negative, firms exit.
Lump-sum taxes are mostly an MCQ concept, and the questions follow predictable patterns. One stem asks which cost component the tax alters (answer: fixed cost, and therefore ATC and AFC). Another describes a 'fixed annual fee that does not vary with output' and asks you to classify it, so recognize that phrasing as a lump-sum tax even when the word 'tax' isn't used. The most common setup compares a per-unit tax versus a lump-sum tax on a profitable monopoly in the short run. The correct answer is always some version of 'per-unit tax changes Q and P; lump-sum tax changes neither, only profit falls.' On FRQs, this shows up as a graphing or calculation task under 6.4.B and 6.4.C, like shifting ATC up on a monopoly graph and computing the new (smaller) profit rectangle while leaving quantity and price labeled exactly where they were.
A per-unit tax is charged on every unit produced, so it's a variable cost that shifts MC (and ATC) upward, which lowers the profit-maximizing quantity and raises price. A lump-sum tax is one flat payment no matter the output, so it's a fixed cost that shifts only ATC. Quantity and price don't budge; profit just falls by the tax amount. Quick test: if the tax bill grows when output grows, it's per-unit. If the bill is the same at any output, it's lump-sum.
A lump-sum tax is a fixed fee that doesn't vary with output, so it's treated as a fixed cost.
It shifts ATC (and AFC) upward but leaves MC, AVC, MR, and demand completely unchanged.
Because MR = MC is undisturbed, the firm's profit-maximizing quantity and price stay the same; only profit falls by the amount of the tax.
A lump-sum tax creates no deadweight loss, unlike a per-unit tax, which changes quantity and distorts the market outcome.
This result holds in both perfectly competitive and monopoly markets, which is why exam questions love applying it to a profitable monopoly.
In the long run, if a lump-sum tax turns profit into a loss, firms in a competitive market will exit.
It's a fixed tax amount that doesn't change with how much a firm produces or sells, like a flat annual license fee. Per EK POL-4.A.2, it affects only fixed costs, not marginal cost or marginal benefit.
No. Since the tax doesn't touch MR or MC, the monopoly's profit-maximizing quantity and price stay exactly the same. The firm just earns less profit. This is one of the most-tested results in Topic 6.4.
A per-unit tax is charged on each unit, so it shifts MC up, reduces quantity, and raises price. A lump-sum tax is one flat payment, so it shifts only ATC, leaving quantity and price unchanged. If the tax bill grows with output, it's per-unit; if it's constant, it's lump-sum.
Only ATC and AFC shift upward. MC, AVC, MR, and demand all stay where they were, which is why output doesn't change. On a graph, you redraw ATC higher and show a smaller profit rectangle.
No. Because quantity produced doesn't change, no mutually beneficial trades are lost, so there's no deadweight loss. That's the key contrast with a per-unit tax, which shrinks quantity and does create deadweight loss in a competitive market.
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