Externalities arise when economic activity affects third parties outside the price system. To analyze them properly, you must compare private incentives with social incentives, identify inefficiency, and compute the correct policy intervention.
Why Externalities Create Inefficiency
For a negative production externality:
\[ MC_{social} = MC_{private} + MEC \]
Competitive equilibrium satisfies:
\[ MB = MC_{private} \]
Efficiency requires:
\[ MB = MC_{social} \]
How to Analyze Externalities Step by Step
1. Solve Competitive Equilibrium
Set marginal benefit equal to private marginal cost.
2. Solve Social Optimum
Set marginal benefit equal to social marginal cost.
3. Compute Deadweight Loss
Measure welfare loss between the two quantities.
4. Design Optimal Policy
\[ t^* = MEC(Q^*) \]
The optimal Pigouvian tax equals marginal external cost at the efficient quantity.
Numerical Example
\[ MB = 100 – Q \]
\[ MC_{private} = Q \]
\[ MEC = 20 \]
Competitive equilibrium:
\[ 100 – Q = Q \Rightarrow Q_c = 50 \]
Social optimum:
\[ 100 – Q = Q + 20 \Rightarrow Q^* = 40 \]
Common Exam Mistakes
- Using total cost instead of marginal cost.
- Forgetting to include marginal external cost.
- Confusing positive vs negative externalities.
- Stating “tax fixes it” without computing the correct level.
Why This Matters
Externalities explain pollution policy, climate regulation, vaccination programs, and many graduate microeconomic policy problems. Understanding how to compute efficient outcomes is essential for prelim exams.
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