Factor Markets and Market Failure — AP Microeconomics
1. Labour Markets: MRP and Profit-Maximizing Hiring ★★☆☆☆ ⏱ 5 min
Demand for labour is a derived demand, meaning it depends entirely on consumer demand for the final good or service the labour produces. To determine how many workers to hire, firms use the concept of marginal revenue product (MRP), the additional revenue a firm earns from hiring one extra unit of labour.
MRP = MPL \times MR
For firms operating in a perfectly competitive output market, price equals marginal revenue ($P = MR$), so the formula simplifies to:
MRP = MPL \times P
The MRP curve is the firm’s downward-sloping labour demand curve, due to the law of diminishing marginal returns: as more workers are hired, each additional worker contributes less extra output, so their MRP falls. In a perfectly competitive labour market, firms are wage takers, so the marginal cost of hiring one more worker equals the market wage ($W$). Firms maximize profit by hiring workers up to the point where $MRP = W$.
Exam tip: Examiners frequently test that labour demand shifts only when MRP changes, i.e. when MPL changes (e.g. new training for workers) or output price changes (e.g. higher demand for stickers raises $P$).
2. Monopsony in Labour Markets ★★★☆☆ ⏱ 6 min
A monopsony is a labour market with only one buyer of labour, common in small, isolated markets where a single employer (e.g. a rural hospital, a regional mining company) is the only source of jobs for local workers. Unlike perfectly competitive labour markets, monopsonists are not wage takers: to hire more workers, they must raise wages for all existing workers, not just the new hire. This means the marginal factor cost (MFC) of hiring an extra worker is higher than the market wage.
For an upward-sloping linear labour supply curve, the MFC curve has the same y-intercept as the supply curve and twice the slope, so it always lies above the supply curve. A monopsonist maximizes profit by hiring labour up to the point where $MRP = MFC$, then sets the wage at the lowest level that will attract that number of workers (read off the labour supply curve). The result: monopsonies hire fewer workers and pay lower wages than a perfectly competitive labour market, creating deadweight loss.
Exam tip: A common question asks how a minimum wage affects monopsony outcomes: a minimum wage set equal to the competitive wage will raise employment to the efficient level, eliminating deadweight loss.
3. Externalities: Positive and Negative ★★★☆☆ ⏱ 6 min
An externality is a cost or benefit of a market activity that falls on a third party not involved in the original transaction. Externalities cause market failure because private buyers and sellers do not account for these third-party effects when making decisions.
For negative externalities, where an activity imposes an uncompensated cost on third parties (e.g. factory air pollution increasing local asthma rates), social marginal cost equals private marginal cost plus marginal external cost:
SMC = PMC + MEC
Unregulated markets produce where $PMC = PMB$ (private marginal benefit), which is a higher quantity than the socially optimal quantity where $SMC = SMB$, creating deadweight loss from overproduction. For positive externalities, where an activity creates uncompensated benefits for third parties (e.g. a homeowner's solar panels reducing local carbon emissions), social marginal benefit equals private marginal benefit plus marginal external benefit:
SMB = PMB + MEB
Unregulated markets produce where $PMC = PMB$, which is a lower quantity than the socially optimal quantity, creating deadweight loss from underproduction.
Exam tip: Always draw deadweight loss triangles pointing to the socially optimal quantity, like an arrow indicating where the market should be operating.
4. Public Goods and the Free-Rider Problem ★★☆☆☆ ⏱ 4 min
To classify goods, economists use two core characteristics that define private goods (the most common type of good traded in markets):
- **Excludable**: You can prevent people who do not pay for the good from using it.
- **Rival in consumption**: One person using the good reduces the amount available for others.
Non-excludability creates the free-rider problem: people can consume the good without paying for it, so they have no incentive to contribute to its production cost. Private firms cannot earn sufficient revenue from producing public goods, so they will provide zero or less than the socially optimal quantity, leading to market failure.
5. Government Intervention for Market Failure ★★☆☆☆ ⏱ 3 min
Governments use a range of targeted policy tools to correct market failure and move outcomes closer to the socially optimal quantity:
- **Negative externalities**: Impose a Pigouvian tax equal to the MEC, which shifts the PMC curve up to equal SMC, leading to the optimal quantity. Cap-and-trade systems are an alternative that set a maximum total emission level and let firms trade permits to reduce compliance costs.
- **Positive externalities**: Provide a Pigouvian subsidy equal to the MEB, which shifts the PMB curve up to equal SMB, leading to the optimal quantity (e.g. government subsidies for childhood vaccines).
- **Public goods**: Directly provide the public good using tax revenue, since the private market will not provide it. Governments use cost-benefit analysis to decide which public goods to fund.
- **Monopsony**: Set a minimum wage equal to the perfectly competitive wage, which raises wages and increases employment to the efficient level, eliminating deadweight loss.
Common Pitfalls
Why: Students mix up physical output units and the monetary revenue of additional workers
Why: Students incorrectly assume all labour markets are perfectly competitive, where firms are wage takers
Why: Students mix up overproduction (negative externalities) and underproduction (positive externalities)
Why: Students confuse the common meaning of 'public' with the economic definition of public goods
Why: Students only apply perfectly competitive labour market logic to all labour markets