Analyse negative and positive externalities using marginal social and private cost and benefit curves and identify the welfare loss

What this dot point is asking

SEAB wants you to analyse negative and positive externalities using marginal social and private cost and benefit curves, to show why the market over-produces or under-produces, and to identify the resulting welfare loss. The central insight is that an externality drives a wedge between private and social value, so the market, which acts on private value, produces the wrong quantity.

The answer

What an externality is

Externalities can occur in production (affecting the cost side) or in consumption (affecting the benefit side). The key relationships:

• Marginal social cost: $MSC = MPC + \text{marginal external cost}$.
• Marginal social benefit: $MSB = MPB + \text{marginal external benefit}$.

Negative externality (over-production)

Take a negative externality in production, such as pollution. The marginal external cost makes $MSC$ lie above $MPC$. The market produces where private benefit meets private cost, $MPB = MPC$, at $Q_m$. The social optimum is where $MSB = MSC$, at $Q^*$. Because the external cost is ignored, $Q_m > Q^*$: the market over-produces.

For the units between $Q^*$ and $Q_m$, $MSC > MSB$: each costs society more than it is worth. The triangle between the $MSC$ and $MSB$ curves over this range is the deadweight welfare loss.

Positive externality (under-production)

Take a positive externality in consumption, such as vaccination, which protects others by reducing disease spread. The marginal external benefit makes $MSB$ lie above $MPB$. The market produces where $MPB = MPC$, at $Q_m$, but the optimum is where $MSB = MSC$, at $Q^*$. Because individuals ignore the benefit to others, $Q_m < Q^*$: the market under-produces.

For the units between $Q_m$ and $Q^*$, $MSB > MSC$: society would value them more than they cost. The triangle between $MSB$ and $MSC$ over this range is the deadweight welfare loss from under-consumption.

Examples in context

Example 1. Road congestion and Electronic Road Pricing. Each driver entering a congested road imposes delay and pollution costs on others (a negative externality), so the private cost of driving is below the social cost and roads are over-used. Singapore's Electronic Road Pricing charges drivers for the externality, raising the private cost toward the social cost and cutting use toward the efficient level.

Example 2. Education's spillover benefits. Education raises not only the individual's earnings (private benefit) but also productivity, civic participation and innovation for society (external benefit), so $MSB$ exceeds $MPB$. Left to the market, education would be under-consumed, which is why governments subsidise and in part compel it, pushing consumption toward the social optimum.

Try this

Q1. Define a negative externality. [2 marks]

• Cue. An external cost from a transaction that falls on a third party not involved in it, which the market price does not reflect, such as pollution.

Q2. Explain why a positive externality leads to under-production. [3 marks]

• Cue. The external benefit makes $MSB$ exceed $MPB$; the market produces where $MPB = MPC$, which is below the optimum where $MSB = MSC$, so too little is produced.

Q3. Where is the welfare loss from a negative production externality shown? [2 marks]

• Cue. It is the triangle between the $MSC$ and demand ($MSB$) curves, over the over-produced units from the social optimum $Q^*$ to the market output $Q_m$.