Topic 6.4 The Effects of Government Intervention in Different Market Structures: analyze antitrust policy, the regulation of a natural monopoly through marginal-cost and average-cost pricing, and how intervention can reduce deadweight loss when a market failure exists.

What this topic is asking

Topic 6.4 turns to government remedies for the market power failure of Unit 4. The College Board wants you to analyze antitrust policy, the regulation of a natural monopoly through marginal-cost and average-cost (fair-return) pricing, and explain how well-targeted intervention can reduce deadweight loss when a genuine market failure exists, in contrast to the surplus-reducing interventions of a competitive market in Topic 2.8.

Antitrust policy

Antitrust is the right tool where a market could be competitive but is being monopolised or coordinated. By increasing the number of effective competitors or banning collusion, antitrust pushes price down toward marginal cost and output up toward the efficient level, shrinking the deadweight loss of market power. It links directly to the oligopoly analysis of Topic 4.5, where the incentive to collude, and its illegality, were central.

Regulating a natural monopoly

For a natural monopoly (Topic 4.2), breaking the firm up would raise average cost, because economies of scale mean one firm is the cheapest supplier. So instead of antitrust, governments regulate the price. Two benchmark rules trade off efficiency against the firm's viability.

The choice is a classic trade-off: marginal-cost pricing reaches full efficiency but needs a subsidy; average-cost pricing is self-financing but leaves a little inefficiency. Both beat the unregulated monopoly, which charges the highest price and restricts output the most.

When intervention helps

This is the unifying judgment of Unit 6. Where a market already fails (market power, externalities, public goods), well-targeted policy can move output toward $MSB = MSC$ and raise total surplus. Where the market is efficient, intervention moves output away from $MSB = MSC$ and lowers surplus. The same tool can help or harm depending on the starting point.

Try this

Q1. State what marginal-cost pricing achieves for a natural monopoly and the problem it creates. [2 points]

• Cue. It reaches the allocatively efficient quantity ($P = MC$), but because average cost exceeds marginal cost the firm makes a loss and needs a subsidy.

Q2. Explain when government intervention reduces deadweight loss rather than creating it. [2 points]

• Cue. When it corrects an existing market failure (such as monopoly underproduction), moving output toward $MSB = MSC$; intervening in an already-efficient market instead creates deadweight loss.