Chapter 5 · Market failure and socially undesirable outcomes I: Common pool resources and negative externalities
Corrective tax: harmful consumption
Use a per-unit tax to reduce overconsumption when consumption creates external costs. Compare the market outcome with socially efficient output.
The economic idea
A corrective indirect tax is imposed to internalise a negative externality by making market decision makers face more of the external cost they create. Unlike a tax designed only to raise revenue, its intended rate is linked to marginal external cost.
With a negative consumption externality, marginal private benefit exceeds marginal social benefit because each unit consumed imposes harm on third parties. Production is assumed not to create a separate externality, so marginal social cost equals marginal private cost.
The unregulated market equates MPB with MPC and therefore consumes beyond the allocatively efficient point where MSB equals MSC. A per-unit tax raises the consumer price, reducing quantity demanded along the unchanged MPB curve.
In the constant-gap model, a tax equal to marginal external cost closes the difference between private and social incentives at the efficient quantity. A smaller tax leaves residual overconsumption, whereas a larger tax suppresses consumption below the social optimum.
Tax incidence still depends on relative elasticities. Who legally remits the tax does not determine whether consumers or producers bear more of its economic burden, and that distribution question is separate from the externality correction.
Reading a tax that leaves residual overconsumption
“Price” is measured vertically and “Quantity” horizontally.
“MPB” records consumers’ private willingness to pay, while “MSB” subtracts marginal external cost from consumption.
“MSC = MPC” indicates that private and social production costs coincide in this consumption-side model.
“MPC + tax” is the supply schedule including the per-unit tax. Its vertical distance from “MSC = MPC” is the tax per unit, not the external cost unless the policy is correctly calibrated.
Unregulated market: Eₘ
“Eₘ” is where “MPB” meets “MSC = MPC”, producing “Qₘ” at “Pₘ”. Consumers respond to their private benefit and do not fully face the third-party harm represented by the gap to “MSB”.
Social benchmark: E*
“E*” is where “MSB” meets “MSC = MPC”. Its quantity “Q*” is the allocatively efficient consumption level.
Post-tax market: E₂
Consumption falls from “Qₘ” to “Q₂”, but “Q₂” remains above “Q*”. The tax partially internalises the external cost without eliminating overconsumption.
The current per-unit tax is smaller than the constant marginal external cost represented by the MPB–MSB gap.
Separate the tax payment from the welfare effect
Use “MSC = MPC” and “MSB” to assess social welfare; “MPC + tax” includes a payment to government, not an extra resource cost. Tax revenue is the tax per unit multiplied by the post-tax quantity, while welfare depends on the social value of the units no longer consumed.
The remaining welfare loss lies between “MSB” and “MSC = MPC”, from “Q*” to “Q₂”. Those units are still consumed even though social cost exceeds social benefit.
How the tax changes consumption
Begin at “Eₘ”, where “MPB” and “MSC = MPC” determine “Qₘ”.
The per-unit tax shifts the consumer-facing supply schedule upward to “MPC + tax”. At “Pₘ”, quantity demanded now exceeds quantity supplied.
The consumer price rises, reducing quantity demanded along the unchanged “MPB” curve. Producers also change quantity supplied according to the net price they receive.
The market clears at “E₂”, with price “P꜀” and quantity “Q₂”. Consumption falls from “Qₘ” to “Q₂”, but “Q₂” remains above “Q*”. The tax partially internalises the external cost without eliminating overconsumption.
What the current state establishes
Tax revenue equals the per-unit tax multiplied by “Q₂”. It is a transfer to government; the welfare gain comes from reducing consumption for which social cost exceeds social benefit.
Common mistakes to avoid
- The tax does not shift “MPB” in this model. It raises the market price and causes movement along the unchanged private-benefit curve.
- The tax is not automatically optimal merely because “Q₂” is below “Qₘ”. Compare it with “Q*”.
- Tax revenue is a transfer, not the welfare gain and not automatically deadweight loss.
- A negative consumption externality concerns harm to third parties. Harm borne only by the consumer is private and may instead support a demerit-good or information-failure argument.
How to draw this diagram
- Title the diagram “Corrective tax: harmful consumption”. Label the horizontal axis “Quantity” and the vertical axis “Price”.
- Draw MPB first and label it clearly.
- Draw MSC = MPC next and label it clearly.
- Draw MSB next and label it clearly.
- Draw MPC + 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
- Marginal external cost may differ by consumer, place and time, making a uniform tax imprecise.
- Inelastic or addictive demand can produce a large tax burden with only a small reduction in harmful consumption.
- Consumers may substitute toward untaxed but equally harmful alternatives, weakening the intended welfare gain.
- The tax may be regressive unless revenue recycling or targeted support addresses distributional effects.
- Administrative cost, evasion and political acceptability are not shown in the diagram.