Chapter 6 · Market failure and socially undesirable outcomes II: Positive externalities, public goods, asymmetric information and inability to achieve equity
Subsidy: beneficial production
Show how a subsidy encourages production with external benefits. Compare the new market output with the socially efficient quantity.
The economic idea
A subsidy is a payment by government to consumers or producers intended to lower costs, encourage an activity or improve access. A producer subsidy is paid per unit of output or against eligible production costs.
With a positive production externality, marginal social cost lies below marginal private cost because production creates a marginal external benefit for third parties. The free market therefore produces less than the quantity where MSB equals MSC.
A per-unit subsidy lowers the effective marginal private cost faced by firms and shifts the subsidised supply schedule downward. At each market price, producers are willing to supply more because they receive the market price plus the subsidy.
In the constant-gap model, a subsidy equal to marginal external benefit aligns the subsidised private-cost schedule with marginal social cost. A smaller subsidy leaves underproduction; a larger subsidy can create overproduction.
Government expenditure equals subsidy per unit multiplied by subsidised output. It is a fiscal transfer in this diagram, although raising public funds has opportunity costs and may create wider distortions.
Reading a subsidy that leaves beneficial production too low
“Price” is measured vertically and “Quantity” horizontally.
“MSB = MPB” represents marginal private benefit and “MSB = MPB” marginal social benefit.
“MPC” represents marginal private cost and “MSC” 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 “MSB = MPB” meets “MPC”, giving “Pₘ” and “Qₘ”.
Social benchmark: E*
“E*” is where “MSB = MPB” meets “MSC” and gives “Q*”.
Post-intervention outcome: E₂
The subsidy lowers firms’ effective marginal private cost and expands output along the unchanged benefit 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
The subsidy lowers the private cost firms face, shown by “MPC − subsidy”. It does not itself move the social-cost benchmark “MSC”: that curve already includes the external benefit of production.
The subsidy per unit multiplied by “Q₂” gives government expenditure, not just the subsidy on extra output. Consumers and producers can gain, but include that public cost before judging total welfare. The gain from correcting the externality comes from producing units whose social benefit exceeds social cost.
How the producer subsidy changes the outcome
The market initially reaches “Eₘ” and “Qₘ” because private decision makers follow “MSB = MPB” and “MPC”.
The social comparison uses “MSB = MPB” and “MSC”, producing the benchmark “E*” and “Q*”.
The subsidy lowers firms’ effective marginal private cost and expands output along the unchanged benefit 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
- Subsidy expenditure is not itself the welfare gain. The gain comes from enabling units whose marginal social benefit exceeds marginal social cost.
- 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: beneficial production”. Label the horizontal axis “Quantity” and the vertical axis “Price”.
- Draw MSB = MPB first and label it clearly.
- Draw MPC next and label it clearly.
- Draw MSC 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
- Government may not know the marginal external benefit and can set the subsidy too low or too high.
- Firms may relabel existing activity to qualify, creating deadweight fiscal expenditure without additional output.
- The policy can favour established firms or sectors and may be vulnerable to lobbying and regulatory capture.
- Subsidy finance has an opportunity cost and can create distortions elsewhere in the economy.
- The external benefit may vary between providers, making a uniform per-unit rate imprecise.