📚 Income Determination | 国民收入决定
In macroeconomics, income determination explains why an economy settles at a particular level of national income, output and employment in the short run. The central question is how aggregate demand and aggregate supply interact to set the equilibrium level of real GDP. Cambridge A-Level Economics focuses especially on the Keynesian cross, the multiplier and the distinction between equilibrium income and full-employment income.
在宏观经济学中，收入决定解释了一个经济体为何在短期内会稳定在某一国民收入、产出和就业水平。核心问题是总需求与总供给如何相互作用，从而决定实际 GDP 的均衡水平。剑桥 A-Level 经济学特别关注凯恩斯交叉、乘数以及均衡收入与充分就业收入之间的区别。
1. The Circular Flow of Income | 收入的循环流动
The circular flow model shows how income flows between households and firms. Households supply factors of production and receive factor incomes, which they spend on goods and services produced by firms. In a closed economy with no government, withdrawals equal saving (S), while injections equal investment (I).
循环流动模型展示了收入如何在家庭和企业之间流动。家庭提供生产要素并获得要素收入，这些收入又被用于购买企业生产的商品和服务。在一个没有政府的封闭经济中，漏出等于储蓄（S），注入等于投资（I）。
Equilibrium requires that total injections equal total withdrawals. If injections exceed withdrawals, income expands; if withdrawals exceed injections, income contracts.
均衡要求总注入等于总漏出。如果注入大于漏出，收入扩张；如果漏出大于注入，收入收缩。
2. Aggregate Demand and Its Components | 总需求及其构成
Aggregate demand (AD) is the total planned spending on domestic goods and services at a given price level. Its components are consumption (C), investment (I), government spending (G) and net exports (X − M).
总需求（AD）是在给定价格水平下，对国内商品和服务的计划总支出。其构成包括消费（C）、投资（I）、政府支出（G）和净出口（X − M）。
AD = C + I + G + (X − M)
Consumption is the largest component in most economies and depends heavily on disposable income. The consumption function can be written as C = a + bY, where a is autonomous consumption and b is the marginal propensity to consume.
在大多数经济体中，消费是最大的组成部分，并且严重依赖于可支配收入。消费函数可以写为 C = a + bY，其中 a 是自主消费，b 是边际消费倾向。
3. Aggregate Supply: Short Run and Long Run | 总供给：短期与长期
Aggregate supply (AS) is the total output firms are willing and able to produce at each price level. In the short run, the AS curve slopes upward because input prices such as wages are sticky, so a higher price level raises profit margins and encourages more output.
总供给（AS）是企业在每个价格水平上愿意并且能够生产的总产出。在短期内，AS 曲线向上倾斜，因为工资等投入价格具有粘性，因此较高的价格水平会提高利润率并鼓励更多产出。
In the long run, the classical AS curve is vertical at the full-employment level of output, because factor prices adjust fully and output is determined by resources and technology rather than by the price level.
在长期，古典 AS 曲线在充分就业产出水平上是垂直的，因为要素价格会充分调整，产出由资源和技术决定，而不是由价格水平决定。
4. Equilibrium National Income: The AD-AS Model | 均衡国民收入：AD-AS 模型
Macroeconomic equilibrium occurs where aggregate demand equals aggregate supply. In the short run, this intersection determines the equilibrium price level and equilibrium real output. If AD exceeds AS at the current price level, firms run down inventories and raise output; if AD is below AS, inventories build up and firms cut output.
宏观经济均衡出现在总需求等于总供给之处。在短期内，这一交点决定了均衡价格水平和均衡实际产出。如果当前价格水平下 AD 大于 AS，企业会减少库存并增加产出；如果 AD 低于 AS，库存增加，企业会削减产出。
Equilibrium income is therefore the level of real GDP at which planned spending equals actual output. This is a useful starting point, but it does not guarantee full employment.
因此，均衡收入是计划支出等于实际产出时的实际 GDP 水平。这是一个有用的起点，但它并不保证充分就业。
5. The Keynesian Cross and the 45-Degree Diagram | 凯恩斯交叉与45度线图
The Keynesian cross uses a 45-degree line to represent all points where planned expenditure equals income. The aggregate expenditure (AE) schedule is upward sloping, but its slope is less than one because part of any extra income leaks into saving, taxes and imports.
凯恩斯交叉使用 45 度线来表示计划支出等于收入的所有点。总支出（AE）曲线向上倾斜，但其斜率小于 1，因为任何额外收入都会有一部分漏出到储蓄、税收和进口中。
Equilibrium occurs at the intersection of the AE schedule and the 45-degree line. At this point, unplanned inventory investment is zero. If output is above equilibrium, unplanned inventories rise, so firms reduce production; if output is below equilibrium, inventories fall and firms raise production.
均衡出现在 AE 曲线与 45 度线的交点。此时，非计划存货投资为零。如果产出高于均衡水平，非计划存货增加，企业会减少生产；如果产出低于均衡水平，存货减少，企业会增加生产。
Equilibrium: Y = AE
6. The Withdrawals-Injections Approach | 漏出-注入法
The same equilibrium can be shown using withdrawals and injections. Withdrawals (W) are income flows that leave the domestic circular flow: saving (S), taxation (T) and imports (M). Injections (J) are spending flows entering the circular flow: investment (I), government spending (G) and exports (X).
同样的均衡也可以用漏出和注入来表示。漏出（W）是离开国内循环流动的收入流：储蓄（S）、税收（T）和进口（M）。注入（J）是进入循环流动的支出流：投资（I）、政府支出（G）和出口（X）。
Equilibrium: S + T + M = I + G + X
When W exceeds J,
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