7.4 Private Costs and Benefits, Externalities and Social Costs and Benefits

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Define and calculate social costs (SC) as the sum of private costs and external costs, including their marginal forms (MSC, MPC, MEC)
  2. Define and calculate social benefits (SB) as the sum of private benefits and external benefits, including their marginal forms (MSB, MPB, MEB)
  3. Define positive externality and negative externality
  4. Identify and explain positive and negative externalities from both consumption and production
  5. Explain deadweight welfare losses that arise from externalities
  6. Explain asymmetric information and moral hazard
  7. Use costs and benefits in analysing decisions

Main Content

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Section 1: Private Costs and Social Costs

What is a Private Cost?

A private cost is any cost that is paid directly by the person or business responsible for an economic activity. In simple terms, it is the cost that you bear yourself.

  • For a producer (a firm or business), private costs include things like wages paid to workers, the cost of raw materials, electricity bills, and rent for a factory.
  • For a consumer (a person buying or using something), private costs include the price paid for a product, such as the fare paid to travel by car or the price of a cigarette.

Example: A factory that produces steel pays for iron ore, labour, and energy. These are all private costs — the factory pays them directly out of its own pocket.

What is an External Cost?

Sometimes, an economic activity causes harm or costs to people who are not directly involved in that activity. These people are called third parties — they are neither the buyer nor the seller, yet they are affected.

An external cost (also called a negative externality) is a cost imposed on third parties, for which no payment or compensation is made.

Example: A factory produces steel, but in doing so, it also releases pollution into a nearby river. Local fishermen lose their fish and farmers lose clean water. These losses are external costs — the factory does not pay the fishermen or farmers for the damage it causes.

What is Social Cost?

Social cost (SC) is the total cost of an economic activity to society as a whole. It includes both what the individual or business pays (private cost) and the costs imposed on others (external costs).

Formula:

Social Cost (SC) = Private Cost (PC) + External Cost (EC)

Marginal Forms

In economics, the word "marginal" means "the extra amount from producing or consuming one more unit." So:

  • Marginal Private Cost (MPC) — the extra private cost of producing or consuming one more unit. This is what firms use when making production decisions.
  • Marginal External Cost (MEC) — the extra cost imposed on third parties when one more unit is produced or consumed.
  • Marginal Social Cost (MSC) — the extra cost to society as a whole of producing or consuming one more unit.

Formula:

MSC = MPC + MEC

Example: A car journey costs the driver 5 dollars in petrol (MPC). The exhaust fumes cause pollution that harms nearby residents — economists estimate this harm equals 3 dollars per journey (MEC). Therefore, MSC = 5 + 3 = 8 dollars.

Key point: When there are no external costs (i.e., MEC = 0), MSC = MPC. This is the ideal situation — when a market causes no harm to outside parties.

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