Externality

Also called: Spillover effect, External effect, external effects of markets · Topic: Market Structures, Market Failure and Competition · NCERT: Class 7, Ch 12 "Understanding Markets"; Class 8, Ch 7 "Factors of Production"; Class 11, Ch 4 "Human Capital Formation in India"; Class 12, Ch 2 "National Income Accounting"

Meaning

An externality (also called a spillover effect) is a benefit or harm that one person's action causes to other people who are outside the deal. The person who causes it is not paid for the benefit and is not penalised for the harm.

• Because the market price ignores it, the free market produces the wrong amount of the good. Too much is made when there is a spillover cost, and too little when there is a spillover benefit. This makes an externality a classic market failure (a case where the free market, left alone, gives a wasteful result).
• Social cost = Private cost + External cost
• Social benefit = Private benefit + External benefit

Explanation

How it works: the market sees only private costs

• Buyers and sellers look only at their own costs and gains.
• The people who feel the effect are third parties. They have no say in the price.
• Under perfect competition, the market outcome is efficient only if the price reflects all costs. An externality breaks this condition.
• Key terms:
• Private cost: the cost the producer actually pays (wages, raw materials, fuel).
• External cost: the cost pushed onto others (health bills, a dirty river).

• MC (marginal cost): the extra cost of producing one more unit.

• The rule:

• the market sets price = private MC;
• society needs price = social MC;
• with a spillover cost, social MC > private MC, so market output is higher than the social optimum (the best level for society as a whole).

Two types

• Negative externality (spillover cost): the market produces too much.
• River pollution by factories. GDP counts the factory's output but does not subtract the harm, so GDP overstates welfare (Class 12, National Income Accounting).
• Stubble burning in Punjab and Haryana:
  • burning is the cheapest way for farmers to clear fields →
  • the smoke drifts to Delhi →
  • Delhi's winter air gets worse, and the farmer does not pay for the health damage.
• Road congestion. Each extra car slows all the others, but the driver counts only their own time and fuel.

• Single-use plastics. Making them pollutes and poses health risks (Class 7, Understanding Markets).

• Positive externality (spillover benefit): the market produces too little.

• Vaccination → herd immunity. When enough people are vaccinated, the disease cannot spread easily, so even people who are not vaccinated are protected.
• Education. It creates more productive and better-informed citizens.
• R&D (research and development). Other firms can copy the knowledge and build on it.

Worked examples (numbers are for illustration only)

Negative externality: a dye factory

• Demand: P = 100 − Q (₹ per unit).
• Private MC = ₹40. External cost (river damage) = ₹10 per unit.
• Social MC = 40 + 10 = ₹50.
• Market output: 100 − Q = 40 → Q = 60.
• Social optimum: 100 − Q = 50 → Q* = 50.
• The market overproduces by 10 units.
• Deadweight loss (the net loss to society from those extra units) = ½ × 10 × 10 = ₹50.

Positive externality: a vaccine dose

• A dose costs ₹400. The person gains ₹300 (private benefit). Neighbours gain ₹200 (external benefit).
• Social benefit = ₹500, which is more than ₹400, so society wants the dose taken.
• The person compares only ₹300 with ₹400, so they refuse. Output ends up too low.

Root cause and remedies

• Root cause: missing markets. There is no market for clean air or quiet streets. With no price, 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. This is free riding.

• Remedies:

• Command-and-control: the government orders what may or may not be done, through bans and emission norms.
  • Weakness: every firm must meet the same norm, even when some firms could cut pollution far more cheaply. So society pays more than it needs to.
• Pigouvian tax (A.C. Pigou, The Economics of Welfare, 1920): a tax per unit equal to the marginal external cost (the extra harm to others from one more unit).
  • It "internalises" the externality, so the polluter feels the harm as a cost in their own accounts.
  • In the dye example: a ₹10 tax → the firm's MC rises from ₹40 to ₹50 → output falls from 60 to 50 → the social optimum is reached.
• Pigouvian subsidy: a payment equal to the external benefit. In the vaccine example, a ₹200 subsidy cuts the person's cost to ₹200, which is below their ₹300 gain, so they take the dose.
• Tradable permits (cap-and-trade): the government fixes a cap (a limit) on total emissions and issues permits that add up to it. Firms can buy and sell the permits.
  • Example (illustration only): the target is a 10-tonne cut. Firm A can cut at ₹100 per tonne and Firm B at ₹300 per tonne.
  • Command-and-control (5 tonnes each) costs 5 × 100 + 5 × 300 = ₹2,000.
  • With trading, A cuts all 10 tonnes for ₹1,000 and sells its 5 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 [5]. (A property right is a legal right to use something, or to stop others from harming it. Transaction costs are the costs of searching, negotiating, writing contracts and enforcing a deal.) Coase won the Nobel Prize in Economics in 1991 [4].
  • Example (illustration only): factory waste does ₹600 of damage to fishermen, and a filter costs ₹400. The filter gets installed either way. If the fishermen hold the right, the factory pays. If the factory holds the right, the fishermen pay the factory ₹400–600.
  • Why it fails in practice:
  • Delhi's smog has lakhs of polluters and crores of victims, so bargaining costs are far too high;
  • each victim hopes others will pay (free riding);
  • rights over "clean air" are unclear, and slow courts make deals hard to enforce.
  • So the state steps in and creates markets. It defines rights (permits) and lowers bargaining costs (trading platforms).

In India

• Command-and-control: the single-use plastic ban
• The Ministry of Environment, Forest and Climate Change notified the Plastic Waste Management (Amendment) Rules 2021 on 12 August 2021 [2].
• Identified single-use plastic items with low usefulness and a high chance of being littered were banned from 1 July 2022 [2][3]. The list includes ear buds with plastic sticks, balloon sticks, plastic flags, candy sticks, ice-cream sticks, thermocol for decoration, plates, cups, glasses, cutlery, straws, trays, wrapping film on sweet boxes, invitation cards and cigarette packets, PVC banners under 100 micron, and stirrers [3].
• Carry bags thinner than 75 micron were banned from 30 September 2021, and bags thinner than 120 micron from 31 December 2022 [2].

• CPCB (Central Pollution Control Board), the State Pollution Control Boards and local bodies have run enforcement drives since July 2022 [3].

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

• Pigouvian subsidies:
• free vaccines;
• FAME / PM E-DRIVE support for buying electric vehicles, which cause less tailpipe pollution;

• PM Surya Ghar rooftop solar subsidies.

• Surat Particulate Matter Emissions Trading Scheme

• Launched by the Gujarat Pollution Control Board in September 2019. It is the world's first market for particulate emissions (fine dust and soot), and it covers textile and dyeing units.
• About 300 firms took part in a trial. Some were placed in the trading scheme and the rest stayed under the old command-and-control rules, so the two methods could be compared fairly [6].
• At the ABCDE conference in 2025, the World Bank called the results "truly quite dramatic" [6].

• The World Bank also supported the design of market-based emissions trading schemes to improve air quality in India (project P126356) [7].

• Carbon Credit Trading Scheme (CCTS)

• Legal base: the Energy Conservation (Amendment) Act 2022. It lets the Central Government, in consultation with the Bureau of Energy Efficiency (BEE), specify the scheme [8][9].
• Notified: S.O. 2825(E), dated 28 June 2023, and amended by S.O. 5369(E), dated 19 December 2023 [8][9].
• Two mechanisms [8][10]:
  • Compliance mechanism: "obligated entities" (firms required by law to take part) must meet GHG emission-intensity reduction norms. Emission intensity means emissions per unit of output.
  • Offset mechanism: firms that are not obligated can join voluntarily and earn credits.
• Nine sectors are covered: aluminium, cement, steel, paper, chlor-alkali, fertiliser, refinery, petrochemical and textile [8].
• MRV (Measurement, Reporting and Verification) rules come from BEE's Detailed Procedure for Compliance Mechanism. They cover the issue and trading of carbon credit certificates [8].
• Targets have since been notified for 208 more carbon-intensive industries [11].

Don't confuse with

• Public good: a good that is non-excludable and non-rival, such as clean air. An externality is a side effect of an action on third parties. The two are linked because clean air has no market, which is why pollution goes unpriced.
• Private cost: only the cost the producer pays. Social cost adds the external cost. The market works on private cost, and that gap is the source of the problem.
• Command-and-control vs Pigouvian tax: both can reach the same level of pollution. The ban or norm forces every firm to cut the same amount, which costs more. The tax lets each firm choose how much to cut, so the cut is cheaper overall.
• CCTS vs pure cap-and-trade: pure cap-and-trade caps total emissions. CCTS caps emission intensity (emissions per unit of output). A firm that beats its target sells certificates, and a firm that misses it must buy them.

Prelims Hooks

• Social cost = Private cost + External cost. With a negative externality the market overproduces. With a positive externality it underproduces.
• 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): clear property rights + low transaction costs → an efficient bargain, whoever holds the right. Coase won the Nobel in 1991 [4][5]. Trap: who holds the right does not matter for efficiency, but it does decide who pays. It does not mean the right is irrelevant for fairness.
• Surat ETS (2019, Gujarat Pollution Control Board) is the world's first market for particulate matter, not carbon. It ran as a trial with about 300 firms [6].
• CCTS: notified 28 June 2023 (S.O. 2825(E)) under the Energy Conservation (Amendment) Act 2022. The nodal technical body is BEE. It has two mechanisms (compliance and offset) and covers 9 sectors [8][9].
• Pairs to remember: vaccination → positive externality (herd immunity). Stubble burning → negative externality. PM Surya Ghar or PM E-DRIVE → Pigouvian subsidy. Tobacco tax → Pigouvian tax.

Mains Points

• Choosing the right tool: command-and-control, such as the plastic ban [2][3], 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 compared trading firms with a command-and-control group and showed this in practice [6]. Market tools need strong MRV and enforcement capacity [8].
• Stubble burning shows why Coase fails:
• there are too many parties, rights are unclear and bargaining costs are high, so no private deal happens;

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

• Welfare and climate (GS-III):

• GDP does not subtract externality costs, so it overstates welfare. This supports "green GDP" (GDP adjusted for environmental damage) and natural-capital accounting.
• CCTS intensity targets let industry grow while cutting emissions per unit of output. The risks are weak caps, low certificate prices and double counting with offsets. Adding 208 more industries makes the market deeper [11].

Related concepts

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

Read more

• Externalities, social cost and the Coase theorem