IBonomicsNegative Externality of ConsumptionSL
MicroeconomicsA diagram illustrating a negative externality of consumption, where the marginal social benefit (MSB) is lower than the marginal private benefit (MPB), leading to overconsumption and welfare loss.
Curves and elements
- MPB = Demand
- The marginal private benefit consumers receive from consuming the good.
- MSB
- Marginal social benefit — lower than MPB due to external costs imposed on society.
- Supply = MSC = MPC
- In this case supply represents both marginal private and marginal social cost.
- Price effect
- The free market price (Pm) is higher than the socially optimal price (Popt).
- Quantity effect
- The market consumes more (Qm) than the socially optimal amount (Qopt).
- Welfare Loss
- The deadweight loss, shown as the shaded triangle between MSB and the supply curve (MSC), from Qopt to Qm.
Key explanations
- 1
Negative externalities of consumption occur when consuming a good imposes external costs on third parties that are not reflected in the private benefit consumers receive.
- 2
In the free market, consumers choose to consume at Qm where marginal private benefit (MPB) equals marginal private cost (MPC), leading to price Pm.
- 3
However, because consumption imposes external costs, the marginal social benefit (MSB) is lower than MPB. The socially optimal level of consumption is Qopt, where MSB equals MSC.
- 4
Since MPB > MSB, the market overconsumes the good (Qm > Qopt), meaning too many resources are allocated toward consumption.
- 5
The shaded triangle represents the welfare loss — the deadweight loss that arises because the external cost of consumption is not accounted for in the market equilibrium.
Example exam question
Using a diagram, explain how a negative externality of consumption leads to market failure. Suggest a policy the government could use to reduce the welfare loss.
Show example answer
In the diagram, consumers base their decisions on marginal private benefit (MPB), reaching equilibrium at Qm and Pm. However, consumption imposes external costs on third parties, such as secondhand smoke or noise pollution, which lowers the marginal social benefit (MSB). The socially optimal level of consumption is Qopt at price Popt, where MSB = MSC. Because the market outcome results in overconsumption (Qm > Qopt), a welfare loss emerges, illustrated by the shaded triangle. A government could correct this market failure by imposing an indirect tax or introducing regulations that reduce consumption to the socially optimal level.
