Market Failure and the Role of Government · ⁨ความล้มเหลวของตลาดและบทบาทของรัฐบาล⁩

A market outcome is socially efficient 社会有效率 when it maximizes total surplus – when the marginal social benefit equals the marginal social cost of the last unit. Perfect competition reaches this on its own. But sometimes markets fail to, producing too much or too little. This is market failure 市场失灵, and it is the main economic justification for government action.

An externality 外部性 is a cost or benefit that falls on a third party not involved in the transaction – so the private decision-makers ignore it.

• A negative externality 负外部性 (pollution) means marginal social cost > marginal private cost. The market over-produces; the deadweight loss is a triangle. Fixes: a Pigovian tax 庇古税 or regulation to make producers "internalize" the cost (shifting private cost up to social cost).
• A positive externality 正外部性 (vaccination, education) means marginal social benefit > marginal private benefit. The market under-produces. Fixes: a subsidy to consumers or producers.
• Coase theorem 科斯定理: if property rights are clear and bargaining is costless, private parties can solve an externality themselves – but that is rare with many people involved.

Worked example. A factory's private marginal cost of a chemical is $\$8$ per unit, but each unit also dumps $\$3$ of pollution on neighbors, so the marginal social cost is $\$11$. Ignoring the $\$3$, the market over-produces. A Pigovian tax of exactly $\$3$ per unit raises the firm's private cost to $\$11$ – now equal to the social cost – so the firm cuts output to the efficient quantity and the externality is "internalized."

Exam skill: identify whether an externality is positive or negative, show on a graph that private and social curves diverge, mark the deadweight loss, and name the correcting tax or subsidy.

Goods differ along two traits: rivalry 竞争性 (does one person's use reduce another's?) and excludability 排他性 (can non-payers be kept out?).

• A private good 私人物品 is rival and excludable (a sandwich) – markets handle these well.
• A public good 公共物品 is non-rival and non-excludable (national defense, a lighthouse). Because non-payers cannot be excluded, people free-ride 搭便车 – enjoy it without paying – so markets under-provide it, and government usually supplies it.
• Common resources 公共资源 are rival but non-excludable (ocean fish); over-use ruins them – the tragedy of the commons 公地悲剧.

Government tools change outcomes differently by structure:

• Regulating a monopoly's price toward $P=MC$ can increase efficiency (unlike a price control in a competitive market, which creates shortage/surplus).
• Antitrust laws 反垄断法 break up or block anti-competitive mergers and collusion.
• Taxes and subsidies shift output toward the socially efficient quantity when externalities are present.

The key insight: the same policy can help or hurt efficiency depending on whether the market was already efficient (competitive) or already failing (monopoly, externality).

Per-unit vs lump-sum. A per-unit tax 从量税 adds to marginal cost, so it shifts supply up, raises price, cuts quantity, and creates deadweight loss. A lump-sum tax 定额税 (a fixed charge like a licence fee) does not change marginal cost or marginal benefit, so in the short run the profit-maximising price and quantity are unchanged – only the firm's profit falls; in the long run it changes entry and exit. For a natural monopoly, regulating price down to $P=MC$ (allocatively efficient) forces a loss, so a lump-sum subsidy 定额补贴 is needed to keep it producing that efficient quantity.

Markets reward productivity, but the resulting income distribution 收入分配 can be very unequal. Economists measure it with the Lorenz curve 洛伦兹曲线 (share of income against share of population; the further it bows from the 45° line, the more unequal) and the Gini coefficient 基尼系数 (0 = perfect equality, 1 = perfect inequality).

Worked example. Suppose the poorest $20\%$ of households earn just $5\%$ of income, while the richest $20\%$ earn $50\%$. Under perfect equality each $20\%$ would earn exactly $20\%$, so the Lorenz curve would be the $45°$ line. Because the bottom fifth is well below $20\%$ and the top fifth well above, the curve bows far beneath the diagonal – a large gap that means a high Gini coefficient and substantial inequality.

Sources of inequality include differences in human capital 人力资本 (skills, education), inheritance, and discrimination. Governments redistribute through progressive taxes 累进税 (higher rates on higher incomes) and transfer payments 转移支付 (welfare, subsidies). There is a debated efficiency–equity trade-off 效率与公平权衡: redistribution can reduce work incentives, so societies balance fairness against the size of the total pie.

• A negative externality makes the market over-produce; correct with a Pigovian tax equal to the external cost. A positive externality under-produces; correct with a subsidy.
• A public good is non-rival and non-excludable, so free-riding leads markets to under-provide it.
• Classify goods by rivalry and excludability; common resources (rival, non-excludable) suffer the tragedy of the commons.
• The same policy can help or hurt efficiency depending on whether the market was already efficient or failing.
• Measure inequality with the Lorenz curve and Gini coefficient (0 = equality, 1 = inequality).