Externalities, social cost and the Coase theorem

Detail

1. What an externality is

• Externality: a benefit or harm that one person's action causes to other people. The person who causes it is not paid for the benefit and is not penalised for the harm.
• The market price ignores it. Buyers and sellers look only at their own costs and gains.

• The people who feel the effect are third parties. They are outside the deal.

• There are two kinds:

• Negative externality: a spillover cost.

• Positive externality: a spillover benefit.

• How this fits the topic: under perfect competition (Class 12, Theory of the Firm), the market outcome is efficient only if price reflects all costs. An externality breaks this, so it is a market failure (a case where the free market, left alone, gives a wasteful result).

2. Negative externality: a spillover cost

• Definition: a cost that falls on third parties who are not paid for it.
• Result: the market produces too much of the good compared with the social optimum (the best level for society as a whole).
• Examples:
• River pollution by factories. Class 12 (National Income Accounting) notes that GDP counts the factory's output but does not subtract the harm. So GDP overstates welfare.
• Stubble burning in Punjab and Haryana:
  • farmers burn crop leftovers because it is the cheapest way to clear 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.
• Road congestion. Each extra car slows every other car, but the driver counts only their own time and fuel.
• Single-use plastics. Class 7 (Understanding Markets) says making them pollutes and poses health risks, so the government brings in strict rules.

3. Positive externality: a spillover benefit

• Definition: a benefit that goes to third parties who do not pay for it.
• Result: the market produces too little of the good.
• Examples:
• Vaccination leads to herd immunity. When enough people are vaccinated, the disease cannot spread easily, so even people who are not vaccinated are protected.
• Education gives a more productive and better-informed citizenry.
• R&D (research and development) spreads knowledge that other firms can copy and build on.

4. Social cost and social benefit: formulas

• 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.
• Social cost = Private cost + External cost
• Social benefit = Private benefit + External benefit
• MC means marginal cost, the extra cost of producing one more unit.
• With a negative externality:
• the market sets price = private MC;
• but society needs price = social MC;

• since social MC > private MC, market output is higher than the social optimum.

• With a positive externality:

• social benefit > private benefit;
• buyers pay only for their own benefit, so output is too low.

Worked example: negative externality (numbers are for illustration only)

• Demand for a dye: P = 100 − Q (₹ per unit).
• Private MC of the factory = ₹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 (the net loss to society from those extra units) = ½ × 10 × 10 = ₹50.

Worked example: positive externality

• A vaccine dose costs ₹400.
• The person who takes it gains ₹300 (private benefit). Their neighbours gain ₹200 in lower infection risk (external benefit).
• Social benefit = ₹500, which is more than the ₹400 cost, so society wants the dose taken.
• The person compares only ₹300 with ₹400, so they refuse. This is why output ends up too low.

5. 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 signals their value, and polluters use them as if they were free.
• This links to public goods and free riding (Class 9, The Price Puzzle):
• clean air is non-excludable, meaning no one can be stopped from enjoying it;
• so people wait for others to pay for it. That is free riding.

6. Remedies

(a) Command-and-control

• Definition: the government directly orders what may or may not be done, through bans, standards and emission norms.
• Indian example: Plastic Waste Management (Amendment) Rules 2021.
• Notified by the Ministry of Environment, Forest and Climate Change on 12 August 2021 [2].
• Banned the manufacture, import, stocking, distribution, sale and use of identified single-use plastic items from 1 July 2022. These items have low usefulness and a high chance of being littered [2][3].
• Banned items include: 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: bags thinner than 75 micron were banned from 30 September 2021, and bags thinner than 120 micron from 31 December 2022 [2].
• Enforcement: CPCB (Central Pollution Control Board), the State Pollution Control Boards and local bodies have run enforcement drives across India since July 2022 [3].

• This matches the "strict regulations" in Class 7.

• Weakness: every firm must meet the same norm, even when some firms could cut pollution far more cheaply than others. So the total cost to society is higher than it needs to be.

(b) Pigouvian tax

• Source: A.C. Pigou, The Economics of Welfare (1920).
• Definition: a tax on each unit of a harmful activity, equal to the marginal external cost (the extra harm done to others by one more unit).
• It "internalises" the externality, meaning it makes the polluter feel the harm as a cost in their own accounts.
• In the worked example above: a tax of ₹10 per unit →
• the firm's MC rises from ₹40 to ₹50 →
• output falls from 60 to 50 →

• output is now at the social optimum.

• Examples: the tobacco and alcohol taxes that Class 9 mentions, and the coal cess (see the taxation topic).

(c) Pigouvian subsidy

• Definition: a payment to an activity with a positive externality, equal to the external benefit. It pushes output up to the social optimum.
• In the vaccine example: a ₹200 subsidy lowers the person's cost to ₹200. That is below their ₹300 private benefit, so they take the dose.
• Indian examples:
• free vaccines;
• support for buying electric vehicles (FAME / PM E-DRIVE), because EVs mean less tailpipe pollution;
• rooftop solar subsidies (PM Surya Ghar).

(d) Tradable permits (cap-and-trade)

• Definition: the government fixes a cap (a limit) on total emissions and hands out or sells permits that add up to that cap. Firms can buy and sell permits among themselves.
• Why it works: firms that can cut emissions cheaply cut more and sell their spare permits. Firms that find cutting costly buy permits instead. So the total cut happens where it is cheapest.

Worked example: cap-and-trade (numbers are for illustration only)

• The target is to cut 10 tonnes of emissions in total.
• Firm A can cut at ₹100 per tonne. Firm B can cut at ₹300 per tonne.
• Command-and-control (each firm told to cut 5 tonnes): 5 × 100 + 5 × 300 = ₹2,000.
• Trading:
• A cuts all 10 tonnes, at a cost of ₹1,000;

• A sells its 5 spare permits to B at any price between ₹100 and ₹300.

• The same cut costs society ₹1,000 less.

7. The Coase theorem

• Source: Ronald Coase, "The Problem of Social Cost" (1960).
• About Coase: a British-born American economist who won the Nobel Prize in Economics in 1991 [4]. This 1960 paper was his most influential work [5].
• Statement: if property rights are clear and transaction costs are low, private bargaining reaches the efficient outcome. This is true whoever holds the right, the polluter or the victim.
• Property right: a legal right to use something, or to stop others from harming it.

• Who holds the right changes who pays whom. It does not change the efficient amount of pollution.

• Britannica's wording: when information and transaction costs are low, the market produces an efficient answer to "nuisances" (harms to neighbours) no matter where the law places the liability (the legal duty to pay for the harm) [5].

• Why it matters: Coase moved the debate away from relying only on government action (Pigou's taxes and subsidies). He showed that private negotiation can also solve externalities [4].

Transaction costs

• Definition: the costs of making an exchange, over and above the price itself. They include the costs of:
• searching for the other side;
• negotiating;
• drawing up contracts;

• enforcing the deal.

• Coase's core point: with no transaction costs, the exact law does not matter, because people can always bargain to reach the best result [4].

Worked example: the Coase bargain (numbers are for illustration only)

• A factory's waste causes ₹600 of damage to fishermen downstream. A filter that stops the waste costs ₹400.
• Efficient outcome: install the filter, because ₹400 < ₹600.
• Case 1: the fishermen hold the right to a clean river.
• The factory must either pay ₹600 in compensation or buy the ₹400 filter.

• It installs the filter, and the factory pays.

• Case 2: the factory holds the right to pollute.

• The fishermen offer the factory between ₹400 and ₹600 to install the filter.

• The filter is installed, and the fishermen pay.

• Same result (the filter is installed). Different payer. This is the heart of the theorem.

8. Why the Coase theorem often fails in practice

• Many parties, high bargaining costs:
• Delhi's smog has lakhs of polluters (farmers, vehicles, industry) and crores of victims →
• bringing them all to one table is impossibly costly →

• so no bargain happens.

• Free riding among victims: each victim hopes the others will pay for the deal. So too little money is collected.

• Unclear rights and weak courts:
• if nobody knows who owns "clean air", nobody can bargain over it;

• slow courts make any deal hard to enforce.

• So the state steps in, often by creating markets. It defines rights (permits) and lowers transaction costs (trading platforms). That is how cap-and-trade puts Coase's idea into practice.

9. Property-rights markets in India

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).
• It covers textile and dyeing units.
• About 300 firms in Surat were involved in the trial [6]:
• some were placed in the trading scheme (the treatment group);
• the rest stayed under old command-and-control rules (the control group).

• This allowed a proper comparison of the two methods.

• The World Bank describes the results of this evaluation as "truly quite dramatic" (ABCDE conference, 2025) [6].

• The scaffold's figure is that a J-PAL evaluation reported pollution falling about 20-30% at lower abatement cost (abatement cost is the cost of cutting pollution). This exact figure was not confirmed from whitelisted sources, so verify it.
• 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 a carbon credit trading scheme [8][9].
• Notification: S.O. 2825(E), dated 28 June 2023, 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 set GHG emission-intensity reduction norms in each compliance cycle. Emission intensity means emissions per unit of output, not total emissions.

• Offset mechanism: entities that are not obligated can join voluntarily and earn credits for projects that cut emissions.

• Nine energy-intensive sectors are covered [8]:

• aluminium, cement, steel, paper and chlor-alkali;

• fertiliser, refinery, petrochemical and textile.

• MRV framework: BEE's Detailed Procedure for Compliance Mechanism sets rules for Measurement, Reporting and Verification (MRV). It covers target setting, monitoring, reporting and verification, and the issue and trading of carbon credit certificates [8].

• Coverage is growing: the government has since notified GHG emission-intensity targets for 208 more carbon-intensive industries [11].
• How it differs from pure cap-and-trade:
• CCTS caps intensity, not total emissions;
• a firm that beats its target earns certificates it can sell;

• a firm that misses its target must buy certificates.

• See the environment topic for more.

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: clear property rights + low transaction costs → an efficient bargain, whoever holds the right. The right changes only who pays, not the efficient level of pollution. From "The Problem of Social Cost" (1960). Coase won the Nobel in 1991 [4][5].
• Trap: the Coase theorem does not say that the allocation of rights "does not matter" for fairness. It says it does not matter for efficiency, and only when transaction costs are low.
• Single-use plastic ban: PWM (Amendment) Rules notified 12 August 2021. The ban started on 1 July 2022. Carry bags thinner than 75 micron were banned from 30 September 2021, and thinner than 120 micron from 31 December 2022 [2][3].
• 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 9 sectors [8][9].
• Surat ETS (2019, Gujarat Pollution Control Board) is the world's first market for particulate matter, not carbon. It covered about 300 firms in a randomised trial [6].
• 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 (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 [6].

• Market tools need strong MRV (measurement, reporting and verification) and enforcement capacity [8].

• Stubble burning shows why Coase fails:

• there are too many parties, rights are unclear and bargaining costs are high, so private deals cannot solve it;

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

• CCTS and India's climate goals:

• intensity-based targets let industry keep growing while cutting emissions per unit of output;
• the risks are weak caps, low certificate prices and double counting with offsets;

• widening coverage (208 more industries) deepens the market [11].

• Welfare measurement (GS-III, growth and development): GDP does not subtract externality costs, so it overstates welfare. This supports "green GDP" (GDP adjusted for environmental damage) and natural-capital accounting.