the welfare loss resulting from consumption and production externalities

Private Costs and Benefits

When you buy a video game, the private cost is the price you pay, and the private benefit is the fun you get. These are the costs and benefits that only you, the consumer, feel.

Example: Buying a Soda

• Private cost: £1.50 (price at the shop)
• Private benefit: Refreshment and enjoyment

Externalities

Externalities happen when the actions of one person affect others who are not part of the transaction. They can be positive (good for others) or negative (bad for others).

Positive Externality Example 🚗

• Someone installs a solar panel on their roof.
• Neighbors benefit from cleaner air and lower electricity prices.

Negative Externality Example 🌱

• A factory emits smoke.
• Residents nearby suffer from health problems.

Social Costs and Benefits

Social costs/benefits combine private and external effects.

Type	Private	External	Social
Cost	$MC$	$E_C$	$MC_{social}=MC+E_C$
Benefit	$MB$	$E_B$	$MB_{social}=MB+E_B$

Welfare Loss (Deadweight Loss)

When externalities exist, the market equilibrium (where private marginal cost equals private marginal benefit) is not socially optimal. The area between the social and private curves represents the welfare loss.

Mathematically:

$$ DW = \frac{1}{2} (P_{social} - P_{private}) (Q_{social} - Q_{private}) $$

Think of it like a pizza slice that you eat but your neighbour also gets a bite because you accidentally dropped it. The slice you didn't eat is the lost welfare.

Illustration of Deadweight Loss

Imagine a graph where:

• $MC$ is the upward‑sloping private marginal cost.
• $MB$ is the downward‑sloping private marginal benefit.
• $MC_{social}$ is higher (or lower) than $MC$ because of external costs (or benefits).
• $MB_{social}$ is lower (or higher) than $MB$ for the same reason.

The shaded triangle between the two curves shows the welfare loss.

Case Study: Factory Pollution

Suppose a factory produces widgets. The private marginal cost of production is £10 per widget. However, each widget also causes £3 of pollution damage to the local community.

• Private MC: $MC = £10$
• External cost: $E_C = £3$
• Social MC: $MC_{social} = £13$

If the market price is £12, the quantity produced is 100 widgets. Socially, the optimal price would be £13, leading to 90 widgets. The deadweight loss is the area between the two quantities:

$$ DW = \frac{1}{2} (13-12)(100-90) = \frac{1}{2} \times 1 \times 10 = £5 $$

So the community loses £5 in welfare because of the factory’s pollution.

Policy Solutions

1. Tax equal to the external cost (£3 per widget).
2. Regulation limiting emissions.
3. Tradable permits for pollution.

Quick Review Checklist

• Identify private vs. social costs/benefits.
• Spot positive and negative externalities.
• Draw the social and private supply/demand curves.
• Calculate deadweight loss using the triangle formula.
• Know policy tools: taxes, subsidies, regulation, tradable permits.