Difficulty: Intermediate | Prerequisites: Unit 3 (Production Markets, cost and revenue concepts)
Tags: marginal revenue product, MRP, marginal resource cost, MRC, least cost rule, profit maximisation for resources, externalities, MSB, MSC, market failure, government intervention, AP Microeconomics, ECON 101, Units 4 and 5
Unit 4 flips the perspective from the output market to the input market. Instead of asking "how much should the firm produce," the question becomes "how many workers (or units of capital) should the firm hire?" The logic is parallel: hire where the additional revenue from one more unit of the resource equals its additional cost.
Unit 5 then asks what happens when markets fail. Externalities occur when a transaction affects third parties who had no say in it. When social costs or benefits diverge from private ones, the market produces too much or too little, and government intervention (taxes, subsidies, regulations) can move the outcome closer to efficiency.
Firms hire resources where marginal revenue product equals marginal resource cost (MRP = MRC). The least-cost rule and the profit-maximisation rule for multiple resources extend this to choosing between inputs. Meanwhile, externalities represent cases where markets fail because costs or benefits spill over onto third parties, and the government corrects them with taxes, subsidies, or regulation to align private incentives with social outcomes.
Marginal revenue product (MRP)
The change in total revenue from employing one additional unit of a resource. Think of it as: how much extra revenue does hiring one more worker bring in?
Marginal resource cost (MRC)
The change in total resource cost from employing one additional unit of a resource. Also called marginal factor cost. Think of it as: how much does it cost to hire one more worker?
Least-cost rule
The condition for producing a given level of output at the lowest possible cost. The ratio of marginal product to price must be equal across all inputs.
Profit maximisation rule for multiple resources
The extension of MRP = MRC to multiple inputs. The ratio of marginal product to input price must be equal across all inputs and must equal 1.
Wage taker
A firm in a perfectly competitive labour market that accepts the market wage as given. Its MRC equals the market wage.
Wage maker (monopsony)
A firm that is the sole (or dominant) employer in its labour market. It faces an upward-sloping labour supply curve and its MRC lies above that supply curve.
Market equilibrium (in the context of externalities)
The point where marginal private cost equals marginal private benefit (MPC = MPB). This is where the market settles without intervention.
Marginal social benefit (MSB)
The total benefit to society from one additional unit, including both private benefit and any external benefit.
Marginal social cost (MSC)
The total cost to society from one additional unit, including both private cost and any external cost.
Externality
A cost or benefit that falls on someone who is not party to the transaction. In simple terms, this means a side effect of economic activity that affects bystanders.
Negative production externality
The firm's production imposes costs on third parties (e.g. pollution). Social cost exceeds private cost. The market overproduces.
Positive production externality
The firm's production creates benefits for third parties (e.g. technology spillovers). Social cost is less than private cost. The market underproduces.
Negative consumption externality
Consumption imposes costs on third parties (e.g. second-hand smoke, noise from gambling). Social benefit is less than private benefit. The market overproduces.
Positive consumption externality
Consumption creates benefits for third parties (e.g. vaccinations, education). Social benefit exceeds private benefit. The market underproduces.
Hiring decisions
A firm hires resources (labour, capital) where MRP = MRC
MRP = Change in TR / Change in quantity of resource
MRC = Change in total resource cost / Change in quantity of resource
This is the resource-market equivalent of MR = MC in the output market
Perfect competition in resource markets
Market demand for labour = sum of all individual firms' MRP curves (D = sum of MRPs)
MRP = product price x marginal product of the resource
Market wage = individual firm's MRC (the firm is a wage taker)
The firm hires workers until MRP falls to the level of the wage
Imperfect competition in resource markets
MRP = (product price x marginal product) minus the price change on previously sold units
Because the firm must lower its output price to sell more, each additional worker adds less to revenue than in perfect competition
In a monopsony (single buyer of labour), the wage is set where MRP = MRC, read off the labour supply curve
MRC lies above the supply curve because hiring one more worker raises the wage for all workers, not just the new one
Least-cost rule
Marginal product of labour / Price of labour = Marginal product of capital / Price of capital
This finds the input combination that produces a given output at minimum cost
It does not by itself maximise profit; it only minimises cost
Profit maximisation rule for multiple resources
Marginal product of labour / Price of labour = Marginal product of capital / Price of capital = 1
The "= 1" is the difference: the last unit of money spent on each input must produce exactly one unit of money's worth of output
The efficient outcome
Social efficiency requires MSB = MSC
Without externalities, MPC = MPB delivers this result automatically
With externalities, private decisions do not align with social welfare
Four types of externality
Negative production externality (overallocation)
Social cost > Private cost
Example: a factory polluting a river
Fix: taxes on the producer, regulation
Positive production externality (underallocation)
Social cost < Private cost (equivalently, there are external benefits from production)
Example: R&D that other firms can learn from
Fix: subsidies, regulations
Negative consumption externality (overallocation)
Social benefit < Private benefit
Examples: cigarettes, alcohol, gambling
Fix: taxes on the consumer, regulation
Positive consumption externality (underallocation)
Social benefit > Private benefit
Examples: education, vaccines, smoke alarms
Fix: subsidies, tax breaks, or regulation
\text{MRP} = \frac{\Delta \text{TR}}{\Delta Q_{\text{resource}}}\text{MRC} = \frac{\Delta \text{TC}_{\text{resource}}}{\Delta Q_{\text{resource}}}Profit maximisation (single resource): MRP = MRC
\frac{\text{MP}_{L}}{P_{L}} = \frac{\text{MP}_{K}}{P_{K}} \quad \text{(Least-cost rule)}\frac{\text{MP}_{L}}{P_{L}} = \frac{\text{MP}_{K}}{P_{K}} = 1 \quad \text{(Profit maximisation, multiple resources)}Market equilibrium (no externalities): MPC = MPB
Social efficiency: MSB = MSC
The MRP = MRC rule is how firms decide staffing levels. A restaurant hires waiters until the revenue from serving one more table no longer exceeds the cost of paying one more waiter. A tech company hires engineers until the expected revenue from one more engineer's output falls to the level of their salary.
Externalities are everywhere. Carbon emissions are the textbook negative production externality: the factory does not pay for the environmental damage it causes, so it produces more than is socially optimal. Vaccination is the textbook positive consumption externality: the vaccinated person benefits, but so does everyone around them who is now less likely to catch the disease. Governments tax carbon and subsidise vaccines for exactly this reason.
Students confuse the least-cost rule with the profit-maximisation rule. The least-cost rule sets the MP/P ratios equal to each other. The profit-maximisation rule sets them equal to each other and to 1. The least-cost rule finds the cheapest way to produce; the profit-maximisation rule also tells you the right amount to produce.
In a monopsony, the wage is read from the supply curve, not from the MRC curve. MRC is above the supply curve. Students often set the wage at the MRC level, which is too high.
Negative externality does not mean "bad product." Cigarettes have a negative consumption externality, but they also have a private benefit to the smoker. The externality is specifically the cost imposed on others.
Students frequently mix up overallocation and underallocation. Negative externalities cause overproduction (the market produces too much). Positive externalities cause underproduction (the market produces too little).
⚠️ MRP = MRC is the resource-market profit maximisation rule. You will see it on the exam. Know how to calculate MRP from a data table and identify the optimal hiring level.
⚠️ The distinction between the least-cost rule and the profit-maximisation rule for multiple resources is a common exam question. Remember: least cost sets the ratios equal; profit max sets them equal to 1.
⚠️ For externalities, you must be able to identify all four types (negative/positive, production/consumption), state whether the market overallocates or underallocates, and name the appropriate government fix.
⚠️ Monopsony is a frequent free-response topic. Be able to draw the graph: labour supply curve, MRC above it, MRP curve, and show how the wage is set below the competitive level.
True or false: the least-cost rule requires MP/P ratios to equal 1.
Fill in the blank: in a monopsony, MRC lies ______ the labour supply curve.
True or false: a positive production externality leads to overallocation by the market.
Fill in the blank: the government typically corrects a negative externality with a ______ or regulation.
True or false: in perfect competition for labour, a single firm's MRC equals the market wage.
Answers: 1. False (that is the profit-maximisation rule; least cost just requires the ratios to be equal to each other). 2. Above. 3. False (underallocation; the market produces too little). 4. Tax. 5. True.
Q: A firm in a perfectly competitive labour market can hire workers at a wage of 20 per hour. The marginal product of the 5th worker is 3 units and the product sells for 8 per unit. Should the firm hire the 5th worker?
A: MRP of the 5th worker = product price x MP = 8 x 3 = 24. MRC = 20 (the wage). Since MRP (24) > MRC (20), yes, the firm should hire the 5th worker.
Q: A firm uses labour and capital. MP of labour is 10, price of labour is 5. MP of capital is 20, price of capital is 8. Is the firm minimising costs?
A: MP(L)/P(L) = 10/5 = 2. MP(K)/P(K) = 20/8 = 2.5. The ratios are not equal, so the firm is not minimising costs. It should use more capital and less labour (capital gives more output per unit of money spent).
Q: A factory emits pollution that harms nearby residents. Is this a negative production or negative consumption externality, and does it cause overallocation or underallocation?
A: Negative production externality. The production process (not the consumption of the product) creates the external cost. It causes overallocation: the market produces more than the socially optimal quantity because the firm does not bear the full cost.
Q: The government subsidises flu vaccinations. Which type of externality is it correcting, and why?
A: Positive consumption externality. Vaccination benefits not only the person vaccinated but also the people around them (herd immunity). Without the subsidy, the market underallocates: fewer people get vaccinated than is socially optimal.
Q: In a monopsony labour market, why is MRC above the labour supply curve?
A: Because the monopsonist is the only employer, it faces the entire upward-sloping supply curve. To hire one more worker, it must raise the wage for all workers, not just the marginal one. The cost of hiring one more is the new wage plus the extra paid to every existing worker, which exceeds the wage itself.
The MRP calculation depends directly on marginal product (from Unit 3) and on the product's price (from the output market in Unit 3). If you are unsure how MP works, review the product section of the Unit 3 notes.
Externalities connect back to elasticity (Unit 2): the burden of a corrective tax (a Pigouvian tax) depends on the relative elasticities of supply and demand, just as with any tax.
The entire externalities framework is the economic rationale for environmental regulation, public health policy, and education funding. It is also the foundation for more advanced topics in welfare economics and public finance if you continue to intermediate micro.
Marginal revenue product, MRP, marginal resource cost, MRC, marginal factor cost, derived demand, least-cost rule, profit maximisation rule for resources, wage taker, wage maker, monopsony, labour market, factor market, externality, negative externality, positive externality, production externality, consumption externality, overallocation, underallocation, MSB, MSC, MPC, MPB, Pigouvian tax, corrective tax, subsidy, market failure, government intervention, social efficiency, AP Micro Unit 4, AP Micro Unit 5, resource markets formulas, externalities formulas