Chapter 6 · Market failure and socially undesirable outcomes II: Positive externalities, public goods, asymmetric information and inability to achieve equity
Positive consumption externality
Explain underconsumption when a good benefits third parties. Compare marginal private and social benefits and identify the welfare loss.
The economic idea
A positive consumption externality exists when consuming a good or service creates an uncompensated benefit for third parties. The consumer takes part in the transaction, while some of the people who benefit do not.
Marginal private benefit is the additional benefit received by the consumer. Marginal external benefit of consumption is the additional benefit received by third parties. Marginal social benefit includes both, so MSB equals MPB plus marginal external benefit.
Because the model places the externality on the consumption side, it assumes no separate production externality and shows marginal social cost equal to marginal private cost. The market chooses MPB = MPC, while allocative efficiency requires MSB = MSC.
A merit good is regarded as beneficial and may be underconsumed because consumers underestimate its private benefits, lack information or cannot afford access. A merit good may also create a positive consumption externality, but private underestimation and third-party benefit are different mechanisms.
When MPB understates benefit to society, the market quantity is below the social optimum. Welfare loss represents the net social benefit forgone on units that would create more social benefit than social cost.
Reading the gap between private choice and social benefit
“Price” is measured vertically and “Quantity” horizontally. The vertical scale can be read as willingness to pay, marginal benefit or marginal cost per unit.
“MPB” shows the benefit captured by consumers and determines market demand.
“MSB” adds the marginal external benefit received by third parties and is therefore above private willingness to pay.
“MSC = MPC” means that private and social production costs coincide; the market failure shown is on the benefit side.
What consumers choose: Eₘ
“Eₘ” is where “MPB” intersects “MSC = MPC”, giving “Pₘ” and “Qₘ”. Consumers act on the benefit they receive themselves.
What society would choose: E*
“E*” is where “MSB” intersects “MSC = MPC”, giving “P*” and “Q*”. This includes the benefit received by third parties.
The consumption gap: Qₘ and Q*
“Qₘ” lies to the left of “Q*”. The free market therefore underconsumes: between these quantities, “MSB” exceeds “MSC = MPC”, so marginal social benefit is greater than marginal social cost.
Identify whose benefit is missing from the decision
The consumer considers “MPB”, while “MSB” also includes benefits to other people. For a vaccination that reduces transmission, personal protection is the private benefit; protection of others is the external benefit. A large private benefit alone is not proof of an externality.
From “Qₘ” to “Q*”, “MSB” is above “MSC = MPC”. The welfare-loss area lies between those two curves: it measures beneficial consumption that does not occur, not the entire gap between private and social benefit.
How private choice produces underconsumption
Consumers make choices from “MPB”, which excludes benefits that spill over to other people.
The market clears at “Eₘ” and “Qₘ”, where private benefit equals private cost.
Adding marginal external benefit places “MSB” above “MPB”. Society therefore values additional consumption more highly than the consumer does.
Applying MSB = MSC gives “E*” and “Q*”. “Qₘ” lies to the left of “Q*”. The free market therefore underconsumes: between these quantities, “MSB” exceeds “MSC = MPC”, so marginal social benefit is greater than marginal social cost.
What the current state establishes
Between “Qₘ” and “Q*”, “MSB” exceeds “MSC = MPC”. The welfare-loss area is the net social benefit forgone through underconsumption.
Common mistakes to avoid
- The gap between “MPB” and “MSB” is a benefit-side spillover. It does not mean production costs have shifted.
- A benefit received only by the consumer is private, not external. A third party must gain for the effect to be an externality.
- Merit goods and positive consumption externalities may overlap, but information failure or affordability can cause underconsumption even without a third-party benefit.
- The social optimum is where “MSB” equals “MSC = MPC”, not where private demand meets cost.
How to draw this diagram
- Title the diagram “Positive consumption externality”. 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.
- 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 parallel benefit curves imply a constant marginal external benefit. Actual spillovers may vary with prevalence, location and the number already consuming the good.
- Consumers may also face affordability, credit, behavioural or information constraints that the externality model does not isolate.
- The size and distribution of third-party benefits can be uncertain and may change over time.
- Private providers may capture some external benefit through reputation, networks or bundled services, narrowing the gap.
- The diagram diagnoses underconsumption but does not identify the least-cost or most equitable intervention.