Chapter 5 · Market failure and socially undesirable outcomes I: Common pool resources and negative externalities
Negative consumption externality
Compare marginal private and social benefits when consumption harms third parties. Explain overconsumption and the resulting welfare loss.
The economic idea
A negative consumption externality exists when consuming a good or service imposes an uncompensated cost on third parties. The consumer and seller take part in the market exchange, while those experiencing the external harm are outside it.
Marginal private benefit is the additional benefit received by the consumer from one more unit. Marginal external cost of consumption is the additional harm imposed on others. Marginal social benefit is the private benefit net of that external cost, so for a negative consumption externality MSB is below MPB.
Because the model locates the externality on the consumption side, it assumes no separate production externality and shows MSC equal to MPC. The private market reaches equilibrium where MPB equals MPC, but allocative efficiency requires MSB to equal MSC.
A demerit good is considered harmful to the consumer and may be overconsumed because consumers lack information, discount future costs or experience addiction. A demerit good can also generate a negative consumption externality, but the concepts are not identical: harm to the consumer is private, while harm to third parties is external.
When MPB overstates the benefit to society, the market quantity exceeds the social optimum. Welfare loss arises on the excess units because their marginal social cost is greater than their marginal social benefit.
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 perceived and received by consumers. It determines their willingness to pay.
“MSB” deducts the marginal external cost imposed on third parties and is therefore the benefit relevant to society.
“MSC = MPC” means that the diagram assumes marginal social cost equals marginal private cost; the market failure is on the benefit side.
What consumers choose: Eₘ
“Eₘ” is where “MPB” intersects “MSC = MPC”. Consumers and producers generate the market price “Pₘ” and quantity “Qₘ” without accounting for harm to third parties.
What society would choose: E*
“E*” is where “MSB” intersects “MSC = MPC”. It gives the social price “P*” and quantity “Q*”, where the full marginal benefit to society equals marginal social cost.
The consumption gap: Qₘ and Q*
“Qₘ” is to the right of “Q*”. Consumers choose more than the socially efficient quantity because “MPB” exceeds “MSB”: part of the apparent private benefit is offset by harm imposed on third parties.
Read the harm on the excess units
From “Q*” to “Qₘ”, “MSC = MPC” is above “MSB”. The area between them is the welfare loss: the excess units cost society more than the social benefit they provide. Do not use the private-benefit curve as the top of this welfare-loss area.
Separate harm to the consumer from harm to others. For example, a smoker’s own health cost is private, while smoke affecting another person is external. This distinction explains why a demerit-good argument and an externality argument need different evidence.
How private choice produces overconsumption
Consumers make choices from “MPB”, which includes the benefit they receive but omits external harm borne by others.
The market clears at “Eₘ” and “Qₘ”, where private benefit equals private cost.
Subtracting marginal external cost places “MSB” below “MPB”. Society therefore values additional consumption less than the consumer does.
Applying MSB = MSC gives “E*” and “Q*”. “Qₘ” is to the right of “Q*”. Consumers choose more than the socially efficient quantity because “MPB” exceeds “MSB”: part of the apparent private benefit is offset by harm imposed on third parties.
What the current state establishes
Between “Q*” and “Qₘ”, “MSC = MPC” exceeds “MSB”. The welfare-loss area is the net social loss from those excess units.
Common mistakes to avoid
- The gap between “MPB” and “MSB” is a benefit-side externality. It does not mean production costs have shifted.
- Private harm to the consumer is not an externality. A third party must be affected for the cost to be external.
- Demerit goods and negative consumption externalities can overlap, but information failure or self-harm alone is not a third-party external cost.
- The social optimum is not where “MPB” meets cost. It is where “MSB” equals “MSC = MPC”.
How to draw this diagram
- Title the diagram “Negative 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 model represents marginal external cost as a stable gap between benefit curves. Actual harm may vary by user, location, intensity and cumulative exposure.
- Observed overconsumption may reflect addiction, present bias, imperfect information or social norms as well as an externality. These require different policy reasoning.
- Elasticities determine how strongly consumption responds to price or information and therefore affect policy effectiveness.
- Equity matters because taxes or restrictions may burden low-income or dependent consumers disproportionately.
- The diagram identifies a social optimum only if marginal social benefit and marginal social cost can be estimated reliably.