Which statement best defines an externality?

• A. They Involve Only Private Costs and Benefits
• B. They Are Fully Reflected in Market Prices
• C. They Do Not Affect Third Parties
• D. They Involve Costs or Benefits to Third Parties Not Captured by Market Prices

Answer: (D)

Externalities are spillover effects from a decision or activity that impact people who are not directly involved in the transaction, and these effects aren’t reflected in the market price. For example, pollution from a factory imposes costs on nearby residents and the environment that the factory doesn’t pay for, so the market price understates the true social cost. On the flip side, benefits from activities like vaccination or education spill over to others, who don’t pay for them in the market. Because these third-party costs or benefits aren’t captured by market prices, externalities occur. That’s why the statement that best defines an externality is: they involve costs or benefits to third parties not captured by market prices. The other ideas would ignore the role of third parties, the imperfect pricing, or the impact on others.