AP Microeconomics: Externalities (Positive vs Negative, Coase Theorem, Pigouvian Taxes/Subsidies)

AP Microeconomics – Externalities (Positive vs Negative, Coase Theorem, Pigouvian Taxes/Subsidies)

What This Is

Externalities are costs or benefits that affect third?parties who are not directly involved in a market transaction. They cause markets to produce too much (negative externality) or too little (positive externality) relative to the socially optimal level. On the AP?Micro exam you’ll be asked to identify the type of externality, draw the dead?weight?loss (DWL) graph, and evaluate policy tools such as Pigouvian taxes/subsidies or the Coase theorem. Real?world example: A factory emits sulfur dioxide that harms nearby residents’ health (negative externality); the government may impose a per?ton tax on emissions to internalize the cost.

Key Terms & Formulas

• Negative Externality – A market activity that imposes uncompensated costs on third parties (e.g., air pollution).
• Positive Externality – A market activity that confers uncompensated benefits on third parties (e.g., a homeowner’s well?kept garden raising neighborhood property values).
• Social Marginal Cost (SMC) – SMC = Private MC + External Cost; the true cost to society of producing one more unit.
• Social Marginal Benefit (SMB) – SMB = Private MB + External Benefit; the true benefit to society of consuming one more unit.
• Pigouvian Tax – A per?unit tax set equal to the marginal external cost (MEC) so that Private MC + Tax = SMC; shifts the private MC curve upward to the SMC curve.
• Pigouvian Subsidy – A per?unit subsidy equal to the marginal external benefit (MEB) so that Private MB + Subsidy = SMB; shifts the private MB curve upward to the SMB curve.
• Coase Theorem – If property rights are well?defined and transaction costs are negligible, private bargaining will lead to the socially optimal outcome regardless of the initial allocation of rights.
• Dead?Weight Loss (DWL) from Externality – The triangular area between the private MC (or MB) curve and the social MC (or MB) curve, bounded by the market quantity and the socially optimal quantity.
• External Cost (EC) / Marginal External Cost (MEC) – The additional cost imposed on third parties by one more unit of production.
• External Benefit (EB) / Marginal External Benefit (MEB) – The additional benefit enjoyed by third parties from one more unit of consumption.
• Supply Curve with Negative Externality (Private MC) – Vertical axis: Price (P); Horizontal axis: Quantity (Q). The curve represents the firm’s marginal cost excluding the external cost.
• Supply Curve with Positive Externality (Private MB) – Same axes; the curve represents consumers’ marginal benefit excluding the external benefit.

Step?by?Step / Process Flow

1. Identify the externality type – read the prompt for “uncompensated cost” (negative) or “uncompensated benefit” (positive).
2. Draw the basic market graph – label P (price) on the vertical axis and Q on the horizontal axis. Plot the Private MC (Supply) and Private MB (Demand) curves; mark the market equilibrium (E?) where they intersect.
3. Add the social curve –
4. For a negative externality, shift the Private MC upward by the MEC to get the Social MC (SMC) curve.
5. For a positive externality, shift the Private MB upward by the MEB to get the Social MB (SMB) curve.
6. Locate the socially optimal quantity – Intersection of SMC with Private MB (negative) or Private MC with SMB (positive). Mark this point (E?).
7. Calculate DWL – The triangle between the private and social curves from Q? to Q?. (You may be asked to compute its area: ½?×?base?×?height.)
8. Apply the policy tool –
9. For a Pigouvian tax, draw a vertical distance equal to MEC between the private and social MC curves; the tax moves the private MC up to SMC, eliminating DWL.
10. For a Pigouvian subsidy, draw a vertical distance equal to MEB between the private and social MB curves; the subsidy moves the private MB up to SMB.
11. If the question mentions bargaining, invoke the Coase theorem and explain why the outcome can be efficient if transaction costs are low.

Common Mistakes

• Mistake: Confusing a shift of the supply curve with a movement along the curve.
  Correction: A tax or external cost shifts the supply curve (SMC) upward; a change in the market price causes a movement along the original private MC curve.

• Mistake: Using the tax amount as the new equilibrium price instead of adding it to the private MC.
  Correction: The Pigouvian tax raises the firm’s marginal cost; the equilibrium price paid by consumers is where the new (tax?adjusted) supply curve meets demand.

• Mistake: Forgetting that a positive externality requires a subsidy (not a tax) to raise the marginal benefit curve.
  Correction: Positive externalities create under?consumption; a subsidy shifts the demand/MB curve upward to the social level.

• Mistake: Assuming the Coase theorem always works regardless of transaction costs.
  Correction: The theorem holds only when transaction costs are negligible and property rights are clearly defined; otherwise government intervention may be needed.

• Mistake: Calculating DWL using the wrong base (using price difference instead of quantity difference).
  Correction: DWL base is the difference in quantity (Q–?Q?); height is the vertical gap between private and social curves at that quantity.

AP Exam Insights

1. Graph?only FRQs – You’ll often be asked to draw the market, add the social curve, label the DWL triangle, and show the effect of a tax/subsidy. Remember to label every axis, curve, and equilibrium point.
2. Multiple?choice traps – Look for answer choices that mix up “tax” vs. “subsidy” or that treat the external cost as a shift of the demand curve instead of the supply curve.
3. Coase theorem vs. government policy – FRQs may ask you to compare the efficiency of a private bargaining solution with a Pigouvian tax; be ready to discuss transaction costs and the feasibility of assigning property rights.
4. Quantitative DWL – Occasionally a question will give the MEC (or MEB) and the difference between Q? and Q?; you’ll need to compute DWL = ½?×?(MEC or MEB)?×?|Q–?Q?|.

Quick Check Questions

1. MCQ: A city experiences heavy traffic congestion, which imposes $5 per driver in time costs on other drivers. Which policy will most efficiently eliminate the dead?weight loss?
2. Answer: Impose a $5 per?vehicle Pigouvian tax.

3. Explanation: The tax equals the marginal external cost, shifting private MC up to the social MC and restoring the optimal traffic level.

4. FRQ?style: A beekeeper’s hives increase nearby orchard yields by $2 per pound of apples sold. The market price of apples is $10 per pound. What policy should the government use, and what is the new equilibrium price to consumers?

5. Answer: Provide a $2 per?pound subsidy to apple growers (or a $2 per?pound tax credit to beekeepers). The consumer price remains $10; producers receive $12, internalizing the external benefit.

6. Explanation: The subsidy shifts the demand (MB) curve up by $2, moving the equilibrium to the socially optimal quantity without changing the consumer price.

7. MCQ: Two neighboring farms suffer from pesticide drift from a third farm. If transaction costs are low, what does the Coase theorem predict?

8. Answer: The farms can negotiate a mutually beneficial agreement that eliminates the drift, achieving the socially optimal outcome.
9. Explanation: With well?defined rights and negligible transaction costs, private bargaining internalizes the externality.

Last?Minute Cram Sheet (10 one?liners)

1. Negative externality-SMC = Private MC + MEC (vertical shift of supply).
2. Positive externality-SMB = Private MB + MEB (vertical shift of demand).
3. Pigouvian tax = MEC; set tax so Private MC + Tax = SMC.
4. Pigouvian subsidy = MEB; set subsidy so Private MB + Subsidy = SMB.
5. DWL triangle area = ½?×?|Q–?Q?|?×?|Vertical gap between private & social curves|.
6. Coase theorem works only if transaction costs-0 and property rights are clear.
7. Supply curve shift vs. movement: A tax shifts the supply curve; a price change moves along the original curve.
8. In a negative externality graph, the socially optimal quantity is left of the market quantity.
9. In a positive externality graph, the socially optimal quantity is right of the market quantity.
10. When asked for “the effect of a tax,” always draw the tax as a vertical distance between the private and social MC curves, not as a new demand curve.