A per-unit subsidy is a government payment of a fixed dollar amount for each unit of a good produced or consumed, which lowers the marginal cost (or raises the marginal benefit) per unit and shifts the curve to correct the underproduction caused by a positive externality.
A per-unit subsidy is the government paying a set amount, say $400, for every single unit of a good that gets made or bought. Because the payment is attached to each unit, it changes the marginal incentive. A subsidy to producers lowers their cost per unit, shifting the supply (marginal private cost) curve down and to the right. A subsidy to consumers raises their willingness to pay, shifting demand (marginal private benefit) up.
In AP Micro, this tool lives in Topic 6.2 Externalities. When a good creates a positive externality, like flu vaccines protecting people who never got the shot, rational agents only respond to their private benefits (EK POL-3.A.3). They ignore the external benefit, so the market produces less than the socially optimal quantity where MSB = MSC. A per-unit subsidy equal to the marginal external benefit closes that gap. It makes the private incentive line up with the social one, pushing output to the socially optimal quantity and eliminating the deadweight loss from underproduction.
Per-unit subsidies sit inside Unit 6 (Market Failure and the Role of Government) and directly support learning objective 6.2.B, which asks you to explain, using graphs, how public policies address externalities. EK POL-3.B.1 lists subsidies first among the policy fixes alongside taxes, regulation, public provision, and property rights. The big idea here is that markets with externalities don't self-correct. EK POL-3.A.1 tells you the socially optimal quantity is where marginal social benefit equals marginal social cost, and a per-unit subsidy is the standard AP tool for getting an underproducing market to that point. If you can draw a positive externality graph and show exactly how big the subsidy should be, you've mastered one of the most reliably tested skills in Unit 6.
Keep studying AP Microeconomics Unit 6
Per-Unit Tax (Unit 6)
A per-unit tax is the mirror image. The tax shrinks output to fix overproduction from a negative externality, while the subsidy expands output to fix underproduction from a positive one. Same logic, opposite direction, and the optimal size of each equals the marginal external cost or benefit.
Positive Externalities (Unit 6)
The per-unit subsidy exists to fix this exact problem. When MSB lies above MPB, the market quantity falls short of the social optimum, and a subsidy equal to the vertical gap between those curves moves the market to where MSB = MSC.
Deadweight Loss (Units 2 & 6)
In Unit 2, a subsidy in a market with no externality pushes output past the efficient quantity and creates deadweight loss. In Unit 6, the same subsidy eliminates deadweight loss because the market was underproducing to begin with. Whether a subsidy helps or hurts depends entirely on whether an externality exists.
Supply and Determinants of Supply (Unit 2)
A producer subsidy is a textbook supply shifter. It works like a negative cost of production, shifting supply rightward by exactly the subsidy amount per unit. Your Unit 2 shifting skills are the foundation for the Unit 6 externality graphs.
Per-unit subsidies show up two ways. In MCQs, you'll get stems like "a positive externality exists in the market for flu vaccinations; to reach the socially optimal quantity, the government should..." and the answer is a per-unit subsidy equal to the marginal external benefit. Some questions make you calculate it. Given demand, supply, and an MEB of $400 per solar panel, the correct per-unit subsidy is exactly $400. On FRQs, externality graphing is a recurring task. The 2022 FRQ on the guava market gave MPB, MPC, and MSB curves and asked for market versus socially optimal quantities, and the 2024 exam again featured externality and competitive-market graphs. Expect to draw the MSB curve above MPB, label the market quantity and the socially optimal quantity, identify the deadweight loss triangle, and state the dollar size of the subsidy that fixes it. The number one graded skill is knowing the subsidy must equal the per-unit external benefit, no more and no less.
A per-unit subsidy pays a fixed amount for every unit, so it changes marginal cost and shifts the supply curve, which changes the quantity produced. A lump-sum subsidy is one flat payment regardless of output, so it lowers fixed costs and average total cost but leaves marginal cost untouched. A profit-maximizing firm setting MR = MC won't change its output from a lump-sum payment. Only the per-unit version can move a market to the socially optimal quantity, which is why it's the externality fix on the AP exam.
A per-unit subsidy pays a fixed dollar amount for each unit produced or consumed, which shifts the supply curve right (producer subsidy) or the demand curve right (consumer subsidy).
The correct subsidy to fix a positive externality equals the marginal external benefit per unit, which is the vertical gap between the MSB and MPB curves.
Markets with positive externalities underproduce because rational agents respond only to private benefits (EK POL-3.A.3), and the subsidy realigns private incentives with social ones.
A well-sized per-unit subsidy moves output to the socially optimal quantity where MSB = MSC and eliminates the deadweight loss from underproduction.
A per-unit subsidy changes marginal cost and therefore output, while a lump-sum subsidy only lowers fixed costs and leaves a firm's profit-maximizing quantity unchanged.
In a market with no externality, a per-unit subsidy pushes output beyond the efficient quantity and actually creates deadweight loss.
It's a government payment of a set amount for each unit of a good produced or sold, like $400 per solar panel. It lowers producers' marginal cost (or raises consumers' marginal benefit), shifting the curve right and increasing the equilibrium quantity.
Exactly equal to the marginal external benefit per unit. If solar panels have demand P = 2,000 - Q, supply P = 200 + Q, and an MEB of $400 each, the optimal per-unit subsidy is $400, because that's the vertical gap between MSB and MPB.
No. It only increases efficiency when a positive externality exists and the market is underproducing. In a market with no externality, a subsidy pushes output past the point where MSB = MSC and creates deadweight loss instead of removing it.
A per-unit subsidy scales with output, so it lowers marginal cost and changes the quantity a firm produces. A lump-sum subsidy is one flat payment that lowers fixed costs and ATC but not MC, so a profit-maximizing firm produces the same quantity as before.
Essentially, yes. A per-unit tax equal to the marginal external cost fixes overproduction from a negative externality, while a per-unit subsidy equal to the marginal external benefit fixes underproduction from a positive one. EK POL-3.B.1 lists both as standard externality-correcting policies.