Chapter 6 · Market failure and socially undesirable outcomes II: Positive externalities, public goods, asymmetric information and inability to achieve equity
Subsidy: merit-good consumption
Explain how a subsidy can increase consumption of a good with external benefits. Compare private and social benefits and the remaining welfare loss.
The economic idea
A consumption subsidy is a payment or price reduction linked to the purchase or use of a good. It lowers the effective price faced by consumers or raises the amount received by providers.
With a positive consumption externality, marginal social benefit exceeds marginal private benefit because consumption benefits third parties. The unassisted market therefore consumes less than the quantity where MSB equals MSC.
The subsidy lowers the consumer-facing supply schedule. At the lower effective price, quantity demanded expands along the unchanged MPB curve, while providers receive the market payment plus the subsidy.
In the constant-gap model, a subsidy equal to marginal external benefit can move consumption to the social optimum. A smaller subsidy leaves underconsumption; an excessive subsidy can push consumption beyond Q*.
A merit-good argument may additionally involve underestimated private benefit, affordability or equity. Those reasons can support intervention, but they are conceptually distinct from the external benefit represented by the MPB–MSB gap.
Reading a subsidy that leaves beneficial consumption too low
“Price” is measured vertically and “Quantity” horizontally.
“MPB” represents marginal private benefit and “MSB” marginal social benefit.
“MSC = MPC” represents marginal private cost and “MSC = MPC” marginal social cost.
“MPC − subsidy” shows the intervention currently applied. The vertical distance between the original and subsidised supply schedules is the subsidy per unit.
Before intervention: Eₘ
“Eₘ” is the private-market equilibrium, where “MPB” meets “MSC = MPC”, giving “Pₘ” and “Qₘ”.
Social benchmark: E*
“E*” is where “MSB” meets “MSC = MPC” and gives “Q*”.
Post-intervention outcome: E₂
The subsidy lowers the price consumers pay, increasing quantity demanded along the unchanged MPB curve. The resulting outcome is “E₂” at “P꜀” and “Q₂”. “Q₂” is above “Qₘ” but remains below “Q*”. The intervention reduces underprovision without eliminating it.
What the intervention changes, and what it does not show
This subsidy makes the good cheaper for consumers through “MPC − subsidy”, so they increase quantity demanded along “MPB”. It does not raise “MSB”: the benefit to third parties was already part of that social-benefit curve.
Government pays the subsidy on all eligible units at “Q₂”, not only the increase from “Qₘ”. The welfare gain is the net social benefit of the extra beneficial consumption, not the whole government-spending rectangle.
How the consumption subsidy changes the outcome
The market initially reaches “Eₘ” and “Qₘ” because private decision makers follow “MPB” and “MSC = MPC”.
The social comparison uses “MSB” and “MSC = MPC”, producing the benchmark “E*” and “Q*”.
The subsidy lowers the price consumers pay, increasing quantity demanded along the unchanged MPB curve.
The new intersection is “E₂” and “Q₂”. “Q₂” is above “Qₘ” but remains below “Q*”. The intervention reduces underprovision without eliminating it.
What the current state establishes
“Q₂” is above “Qₘ” but remains below “Q*”. The intervention reduces underprovision without eliminating it.
A rise in activity is not enough to prove success; the remaining gap to Q* represents beneficial units still forgone.
Common mistakes to avoid
- The subsidy does not shift MPB or MSB in this model. It changes the effective price and the quantity chosen along private demand.
- The intervention is not automatically optimal because “Q₂” exceeds “Qₘ”. It must be compared with “Q*” and evaluated against its costs.
- Government expenditure is not automatically deadweight loss, but it has an opportunity cost and may require distortionary taxation.
- The welfare conclusion depends on the current position of “Q₂” relative to “Q*”, not on the policy label alone.
How to draw this diagram
- Title the diagram “Subsidy: merit-good 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 − subsidy 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
- A price subsidy may have little effect when distrust, inconvenience or missing information is the main constraint.
- Universal subsidies can create large fiscal costs and deadweight spending on consumers who would have purchased without support.
- Supply bottlenecks may raise provider prices and limit the increase in real consumption.
- The external benefit varies across people and epidemiological conditions, making a uniform subsidy imprecise.
- Financing and distributional effects are not captured by the simple welfare diagram.