📚 Subsidy: IB & CIE Economics Key Points | 补贴：IB & CIE 经济考点精讲
This article provides a comprehensive review of subsidies, a central topic in both IB and CIE Economics syllabuses. We examine the definition, diagrammatic analysis, welfare effects, incidence, and evaluation of subsidies, equipping you with all the key points needed for exam success.
本文全面梳理IB与CIE经济课程中的补贴考点，涵盖定义、图示分析、福利效应、归宿及评估，帮助考生系统掌握高分要点。
1. What is a Subsidy? | 什么是补贴？
A subsidy is a financial grant provided by the government to producers or consumers, usually calculated as a fixed amount per unit of output or consumption. Its primary purpose is to lower production costs, increase consumption of merit goods, support particular industries, or correct market failures such as positive externalities.
补贴是政府向生产者或消费者提供的资金支付，通常按每单位产出或消费的固定金额计算。其主要目的在于降低生产成本、增加有益品的消费、扶持特定产业，或纠正正外部性等市场失灵。
In supply–demand analysis, a per-unit subsidy to producers is represented by a vertical downward (rightward) shift of the supply curve by the exact amount of the subsidy. This shift reflects a reduction in the marginal cost of production.
在供需分析中，给予生产者的每单位补贴表现为供给曲线垂直向下（向右）平移，平移量等于补贴金额。这一移动反映了生产边际成本的降低。
2. Effects on Market Equilibrium | 补贴对市场均衡的影响
When a per-unit subsidy s is introduced for producers, the supply curve shifts from S to S + subsidy. The new equilibrium occurs at a lower price for consumers (Pc) and a higher quantity (Q₂). Producers receive a price Pp that equals Pc + s, because the government pays the difference.
当对生产者给予每单位补贴 s 时，供给曲线从 S 移动到 S + 补贴。新的均衡点对应更低的消费者支付价格 Pc 和更大的产量 Q₂。生产者获得的价格 Pp = Pc + s，因为差价由政府支付。
The original equilibrium price P* and quantity Q* are replaced by Pc and Q₂. The vertical distance between the two supply curves at Q₂ equals the per-unit subsidy, and the market price received by producers rises while the out-of-pocket price for consumers falls.
原有的均衡价格 P* 和数量 Q* 被 Pc 和 Q₂ 取代。在 Q₂ 处两条供给曲线的垂直距离等于每单位补贴，生产者实际到手价格上升，而消费者直接支付的价格下降。
3. Impact on Consumers and Producers | 对消费者和生产者影响
Consumers gain because they pay a lower market price for each unit. This increases their consumer surplus, measured by the area between the demand curve and the price line Pc. Producers benefit from receiving an effective price Pp that is higher than the original P*, raising their producer surplus.
消费者因每单位支付的市场价格更低而受益，消费者剩余增加，为需求曲线与价格线 Pc 之间的面积。生产者因有效价格 Pp 高于原价格 P* 而受益，生产者剩余上升。
The benefit of the subsidy is shared between consumers and producers, but not necessarily equally. How the subsidy wedge (Pp – Pc) is split depends on the price elasticities of demand and supply, with the more inelastic side gaining a larger share of the benefit per unit.
补贴的利益在消费者与生产者之间分配，但不一定均等。补贴楔子（Pp – Pc）如何分割取决于需求与供给的价格弹性：弹性较小的一方获得更大的单位收益份额。
4. Government Expenditure on Subsidies | 政府补贴支出
The total cost borne by the government is the per-unit subsidy multiplied by the post-subsidy quantity traded. This is the area of a rectangle with width Q₂ and height s.
政府承担的总成本为每单位补贴额乘以补贴后的交易量 Q₂。这是宽为 Q₂、高为 s 的矩形面积。
Government Expenditure = s × Q₂
This spending must be funded through taxation, borrowing, or reductions in other public services. Therefore, every subsidy carries an opportunity cost, and its use of scarce government funds should be justified by the welfare gain it generates.
这笔支出必须通过税收、举债或削减其他公共服务来筹集，因此任何补贴都伴随机会成本，使用稀缺的政府资金必须以其带来的福利增益作为理由。
5. Welfare Analysis: Consumer & Producer Surplus | 福利分析：消费者与生产者剩余
Before the subsidy, consumer surplus is the area below the demand curve and above P*, while producer surplus is the area above the supply curve and below P*. After the subsidy shifts the supply curve, consumer surplus increases to the area between demand and Pc, and producer surplus increases to the area above the original supply curve below Pp.
补贴前，消费者剩余为需求曲线下方、P* 上方的面积，生产者剩余为供给曲线上方、P* 下方的面积。补贴使供给曲线移动后，消费者剩余扩大至需求曲线与 Pc 之间的区域，生产者剩余变为原供给曲线上方、Pp 下方的区域。
The sum of the gains in consumer and producer surplus is, however, less than the government’s total expenditure rectangle. The difference represents a net welfare loss to society – the deadweight loss – because resources are being used to produce units whose marginal social cost exceeds their marginal social benefit.
然而，消费者与生产者剩余的增长之和小于政府支出的矩形面积。两者之差构成了社会净福利损失——无谓损失，因为资源被用于生产边际社会成本超过边际社会收益的产品。
6. Deadweight Loss from Subsidies | 补贴造成的无谓损失
The deadweight loss (DWL) arises because the subsidy encourages output beyond the allocatively efficient level Q*. For units between Q* and Q₂, the true marginal cost (including the cost of the subsidy to taxpayers) is greater than the marginal benefit consumers place on them. This overproduction leads to a net welfare loss.
无谓损失的产生是因为补贴激励产出超过分配效率水平 Q*。对介于 Q* 与 Q₂ 之间的单位，真实边际成本（包括纳税人承担的补贴成本）大于消费者赋予的边际收益。这种过度生产导致净福利损失。
The magnitude of the deadweight loss can be approximated by a triangle on the subsidy diagram. Its value is given by:
无谓损失的大小可由补贴图示中的一个三角形表示，其值计算为：
DWL = ½ × s × (Q₂ – Q*)
The more elastic the demand and supply, the larger the deadweight loss for a given subsidy, because the quantity response (Q₂ – Q*) is greater, making the overproduction distortion more severe.
对于给定的补贴，需求与供给越富有弹性，无谓损失越大，因为数量反应（Q₂ – Q*）更大，过度生产所造成的扭曲更严重。
7. Price Elasticity and Incidence of Subsidy | 价格弹性与补贴分担
The incidence of a subsidy – who benefits more – is determined by the relative elasticities. If demand is price inelastic while supply is elastic, the consumer price falls significantly and consumers capture most of the benefit. Conversely, if supply is inelastic and demand elastic, producers enjoy a larger increase in the price they receive, reaping the greater share.
补贴的归宿——谁获益更多——取决于相对弹性。若需求缺乏弹性而供给富有弹性，消费者价格大幅下降，消费者获得大部分利益。反之，若供给缺乏弹性而需求富有弹性，生产者得到的价格上涨更多，获取较大份额。
This is the mirror image of tax incidence
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