Chapter 4 · Government intervention in microeconomics
Indirect tax
Explain an indirect tax using demand and supply. Show consumer and producer tax burdens, government revenue and deadweight welfare loss.
The economic idea
Governments use indirect taxes to raise revenue or discourage the production or consumption of a good. Start by identifying the kind of tax: this diagram shows a specific indirect tax, a fixed amount charged per unit sold.
An indirect tax is a tax imposed on expenditure on goods and services or on their production. It is paid to government by a firm but may be passed partly or wholly to consumers through a higher market price. A specific indirect tax is a fixed amount per unit, whereas an ad valorem tax is charged as a percentage of the product’s value.
A specific indirect tax raises the marginal cost of supplying every unit by the same amount. The market supply curve therefore shifts vertically upward by the tax per unit. This creates a wedge between the price consumers pay and the price producers receive after tax.
Tax incidence describes how the economic burden is divided. The relatively less price-elastic side of the market bears the larger share because it changes behaviour less readily. Statutory incidence—who sends the payment to government—does not determine economic incidence.
In a competitive market without externalities, the tax reduces mutually beneficial exchange and creates deadweight loss. If the taxed good creates an external cost, however, a well-calibrated corrective tax may improve allocative efficiency; that conclusion requires a social-cost diagram rather than the private-market diagram alone.
Reading the market with the current tax
“Price” is measured vertically and “Quantity” horizontally.
“D” records consumers’ willingness and ability to buy, while “S₁” records firms’ original marginal private cost of supply.
“S₂ = S₁ + tax” is the post-tax supply curve. The vertical distance from “S₁” is the specific indirect tax shown by “tax”.
Starting point: E₁
“E₁” is where “D” intersects “S₁” before the tax. It establishes the original equilibrium price “P₁” and quantity “Q₁”. Consumers pay the same price that producers receive because no tax wedge yet separates them.
The policy change: S₁ to S₂ = S₁ + tax
For every unit sold, the firm must transfer the specific tax to government. To retain the amount previously required to supply any quantity, it must now receive that original supply price plus the tax. This is why “S₂ = S₁ + tax” is vertically above “S₁” when a positive tax is present.
“S₂ = S₁ + tax” lies above “S₁” by the current per-unit tax. The consumer price is above the original price, the amount producers retain is below it, and quantity traded is lower than in the untaxed market.
New outcome: E₂
“D” intersects “S₂ = S₁ + tax” at “E₂”. Consumers pay “P꜀”, producers retain “Pₚ” after tax, and the new quantity traded is “Q₂”. The distance between “P꜀” and “Pₚ” equals the tax per unit.
Read the burden and welfare areas
The rise from “P₁” to “P꜀” is the consumer burden per unit. The fall from “P₁” to “Pₚ” is the producer burden per unit. Together they add up to the tax. The relatively less price-elastic side bears the larger share because it has fewer opportunities to change its purchases or sales.
After the tax, consumer surplus is below “D” and above “P꜀”, up to “Q₂”. Producer surplus is above the original “S₁” and below the net price “Pₚ”, up to “Q₂”. Use the original supply curve because it shows the cost of production before the tax payment.
The revenue rectangle has height “P꜀” minus “Pₚ” and width “Q₂”. In the model without externalities, deadweight loss is the triangle between “D” and the original “S₁”, from “Q₂” to “Q₁”: buyers value these lost units more than they cost to produce.
How the market adjusts after the tax
Begin at “E₁”, where the untaxed market clears at “P₁” and “Q₁”.
The tax raises firms’ marginal costs and shifts supply from “S₁” to “S₂ = S₁ + tax”. At “P₁”, planned supply is now below quantity demanded, creating excess demand.
The consumer price rises, so consumers reduce quantity demanded along the unchanged “D” curve. At the same time, the lower price producers retain after tax reduces quantity supplied along the original supply curve.
Adjustment ends at “E₂”. Quantity traded falls to “Q₂”, while “P꜀” and “Pₚ” reveal how the tax burden is shared.
What the current state establishes
Government revenue is the per-unit wedge between “P꜀” and “Pₚ” multiplied by “Q₂”. It is a transfer to government, whereas the deadweight-loss triangle represents trades between “Q₂” and “Q₁” that no longer occur.
Common mistakes to avoid
- The tax does not shift “D”. Quantity demanded decreases along the unchanged demand curve because the consumer price rises.
- The whole rise from “P₁” to “P꜀” is not automatically the tax. The tax is the wedge between “P꜀” and “Pₚ”.
- Government revenue is not itself deadweight loss. Revenue is a transfer; deadweight loss is the net benefit of exchange that disappears.
- The side legally responsible for remitting the tax does not necessarily bear most of its economic burden.
- A tax that raises substantial revenue need not greatly reduce consumption. Relatively inelastic demand limits the quantity response, so link your judgement to the government’s objective.
How to draw this diagram
- Title the diagram “Indirect tax”. Label the horizontal axis “Quantity” and the vertical axis “Price”.
- Draw D first and label it clearly.
- Draw S₁ next and label it clearly.
- Draw S₂ = S₁ + tax next and label it clearly.
- Mark each relevant intersection and distinguish the initial, market and socially optimum or policy outcomes where applicable.
- Add the relevant guide lines and shade the economic area only after its boundaries are established.
- Check that every symbol used in the explanation appears on the diagram and add arrows showing the direction of change.
Assumptions and limitations
- The diagram assumes competitive price-taking behaviour and a specific per-unit tax. An ad valorem tax need not create a parallel supply shift.
- It does not reveal the size of any external cost, the government’s motive, administration and compliance costs, tax avoidance or how revenue is spent.
- The standard welfare conclusion assumes no pre-existing market failure. A corrective-tax judgement requires marginal social cost and marginal social benefit.
- Elasticities, market definition and the time period influence the size and distribution of the observed effects.