Negative externality

Also called: External cost · Topic: Market Structures, Market Failure and Competition · NCERT: Beyond NCERT

Meaning

A negative externality (also called an external cost) is a harm that a person's or firm's production or consumption causes to third parties (people outside the deal). The people harmed are not paid for it, and the one who causes it does not pay for it. Because the market price ignores this cost, the market produces too much of the good compared with the social optimum (the best level for society as a whole). This makes it a classic market failure (a case where the free market, left alone, gives a wasteful result).

Formula: Social cost = Private cost + External cost

Explanation

How it works

• Private cost: the cost the producer actually pays, such as wages, raw materials and fuel.
• External cost: the cost pushed onto others, such as health bills or a dirty river.
• MC means marginal cost (the extra cost of producing one more unit).
• Why the market overproduces:
• the firm sets price = private MC;
• society needs price = social MC;

• social MC is greater than private MC, so market output is higher than the social optimum.

• Deadweight loss: the net loss to society from the extra units. These are units whose cost to society is greater than their value to buyers.

Worked example (numbers are for illustration only)

• Demand for a dye: P = 100 − Q (₹ per unit).
• The factory's private MC = ₹40 per unit. The external cost (river damage) = ₹10 per unit.
• Social MC = 40 + 10 = ₹50.
• Market output: 100 − Q = 40 → Q = 60 units.
• Social optimum: 100 − Q = 50 → Q* = 50 units.
• So the market overproduces by 10 units.
• Deadweight loss = ½ × 10 × 10 = ₹50.
• The fix (a Pigouvian tax):
• a tax of ₹10 per unit raises the firm's MC from ₹40 to ₹50 →
• output falls from 60 to 50 →
• output is now at the social optimum.

Root cause: missing markets

• Missing market: something has value, but there is no place to buy or sell it.
• There is no market for clean air or quiet streets. So no price shows their value, and polluters use them as if they were free.
• Clean air is non-excludable (no one can be stopped from enjoying it). So people wait for others to pay to protect it. This is free riding.
• Welfare link: GDP counts the factory's output but does not subtract the harm it causes. So GDP overstates welfare.

Remedies

• Command-and-control: the government directly orders what may or may not be done, through bans, standards and emission norms.
• It is simple and certain.

• But every firm must meet the same norm, even firms that could cut pollution much more cheaply than others. So the total cost to society is higher than it needs to be.

• Pigouvian tax (A.C. Pigou, The Economics of Welfare, 1920): a tax on each unit, equal to the marginal external cost (the extra harm to others from one more unit).

• It "internalises" the externality. This means the polluter now feels the harm as a cost in their own accounts.

• Tradable permits (cap-and-trade): the government fixes a cap (a limit) on total emissions and gives out permits that add up to that cap. Firms can buy and sell these permits.

• Firms that can cut cheaply cut more and sell their spare permits.
• So the total cut happens where it is cheapest.

• Example: 10 tonnes must be cut. Firm A cuts at ₹100 per tonne and Firm B at ₹300 per tonne.

  • If each firm is told to cut 5 tonnes, the cost is ₹2,000.
  • With trading, A cuts all 10 tonnes for ₹1,000 and sells its spare permits to B.
  • The same cut costs society ₹1,000 less.

• Coase theorem (Ronald Coase, "The Problem of Social Cost", 1960): if property rights are clear and transaction costs are low, private bargaining reaches the efficient outcome, whoever holds the right [3][4].

• Property right: a legal right to use something, or to stop others from harming it.
• Transaction costs: the costs of making a deal apart from the price itself, such as finding the other side, negotiating, writing contracts and enforcing the deal.
• Who holds the right changes only who pays whom. It does not change the efficient level of pollution.
• Example (numbers are for illustration only): a factory's waste does ₹600 of damage to fishermen, and a filter costs ₹400.
  • If the fishermen hold the right to a clean river, the factory buys the filter.
  • If the factory holds the right to pollute, the fishermen pay it between ₹400 and ₹600 to install the filter.
  • Either way, the filter is installed. Only the payer is different.

In India

• Stubble burning in Punjab and Haryana:
• burning crop leftovers is the cheapest way for farmers to clear their fields →
• the smoke drifts to Delhi →

• Delhi's winter air gets worse. Nobody pays the farmer to stop, and the farmer does not pay for the health damage.

• Why Coase fails here:

• Delhi's smog has lakhs of polluters (farmers, vehicles, industry) and crores of victims, so bargaining would cost far too much;
• each victim hopes others will pay for a deal (free riding);
• nobody clearly owns "clean air", and slow courts make deals hard to enforce.

• So the state steps in.

• Road congestion: each extra car slows every other car, but the driver counts only their own time and fuel.

• Command-and-control: the single-use plastic ban
• The Plastic Waste Management (Amendment) Rules 2021 were notified by the Ministry of Environment, Forest and Climate Change on 12 August 2021 [1].
• Identified single-use plastic items were banned from 1 July 2022 [1][2].
• Carry bags thinner than 75 micron were banned from 30 September 2021, and bags thinner than 120 micron from 31 December 2022 [1].

• CPCB (Central Pollution Control Board), the State Pollution Control Boards and local bodies have run enforcement drives since July 2022 [2].

• Pigouvian taxes: taxes on tobacco and alcohol, and the coal cess.

• Market-based tool: Surat Particulate Matter Emissions Trading Scheme
• Launched by the Gujarat Pollution Control Board in September 2019. It was the world's first market for particulate emissions (fine dust and soot), and it covered textile and dyeing units.
• About 300 firms were part of the trial. Some were placed in the trading scheme and the rest stayed under old command-and-control rules, so the two methods could be compared properly [5].

• At its ABCDE conference in 2025, the World Bank called the results "truly quite dramatic" [5].

• Carbon Credit Trading Scheme (CCTS)

• Its legal base is the Energy Conservation (Amendment) Act 2022. The scheme was notified by S.O. 2825(E) on 28 June 2023, and the Bureau of Energy Efficiency (BEE) is the nodal technical body [6][7].
• It has two mechanisms: compliance and offset [6][8].
• It covers 9 energy-intensive sectors [6].
• Targets have since been notified for 208 more carbon-intensive industries [9].

Don't confuse with

• Positive externality: a spillover benefit to third parties, such as vaccination or education. The market underproduces it, and the fix is a Pigouvian subsidy, not a tax.
• Private cost: only the cost the producer pays. Social cost = private cost + external cost. The market looks only at private cost.
• Public good: a good that is non-excludable (no one can be kept out). Clean air is a public good, and pollution is a negative externality that damages it. One is the good; the other is the harm done to it.
• Coase theorem vs Pigouvian tax: Coase relies on private bargaining once property rights are clear. Pigou relies on government taxing the harm. Coase needs low transaction costs to work.

Prelims Hooks

• Social cost = Private cost + External cost. With a negative externality, the market produces more than the social optimum.
• Pigouvian tax = marginal external cost at the optimum. It comes from A.C. Pigou, The Economics of Welfare (1920).
• Coase theorem ("The Problem of Social Cost", 1960; Coase won the Nobel in 1991): with clear property rights and low transaction costs, bargaining gives an efficient result whoever holds the right [3][4]. Trap: the allocation of rights still matters for fairness (who pays). It does not matter only for efficiency.
• Surat ETS (2019, Gujarat Pollution Control Board) is the world's first market for particulate matter, not carbon. About 300 firms took part in a randomised trial [5].
• CCTS: notified on 28 June 2023 under the Energy Conservation (Amendment) Act 2022. BEE is the nodal body, and the scheme covers 9 sectors [6][7].
• Pairs to remember: stubble burning → negative externality; tobacco tax → Pigouvian tax; GDP ignores pollution → GDP overstates welfare.

Mains Points

• Choosing the right tool:
• command-and-control (for example, the plastic ban) is simple and certain, but it costs more because every firm must cut by the same amount;
• market tools (Pigouvian tax, cap-and-trade) reach the same cut at lower cost. The Surat ETS trial of about 300 firms, compared against a command-and-control group, shows this in practice [5];

• but market tools need strong MRV (measurement, reporting and verification) and enforcement capacity [6].

• Stubble burning shows why Coase fails:

• there are too many parties, rights are unclear and bargaining costs are high, so private deals cannot fix the problem;

• this justifies state action: subsidies for machines that manage crop leftovers (a Pigouvian subsidy), penalties, and markets for crop residue.

• Welfare measurement (GS-III): GDP does not subtract external costs, so it overstates welfare. This supports "green GDP" (GDP adjusted for environmental damage) and natural-capital accounting. CCTS uses intensity-based targets (emissions per unit of output), so industry can keep growing while cutting emissions per unit. The risks are weak caps, low certificate prices and double counting with offsets [9].

Related concepts

• Externality
• Positive externality
• Social cost
• Pigouvian subsidy
• Coase theorem
• Transaction costs

Read more

• Externalities, social cost and the Coase theorem