National Income and Price Determination
AP Macroeconomics Topic 3 8:50 English narration · English + 中文 subtitles burned in
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A government spends ten billion dollars on new railways. How much does the country's output rise?
政府花一百亿美元修建新铁路，一个国家的产出会增加多少？
Not ten billion. It can rise by fifty billion.
不是一百亿， 而可能增加五百亿。
Money spent by one person becomes income for someone else, who spends part of it again.
一个人花出去的钱会变成另一个人的收入， 这个人又把其中一部分再花出去。
This unit builds the model behind that number.
本单元要建立的，就是这个数字背后的模型。
Here is the plan.
本节的安排是这样的。
First, aggregate demand, and why the curve slopes down.
首先是总需求，以及这条曲线为什么向下倾斜。
Then the multiplier — the calculation this exam asks for almost every year.
然后是乘数——这门考试几乎每年都要考的计算。
Then aggregate supply, short run and long run.
接着是总供给，分短期和长期。
Then equilibrium and the two output gaps.
再然后是均衡与两种产出缺口。
And last, shocks, self-adjustment, and fiscal policy.
最后是冲击、自我修复和财政政策。
Aggregate demand is the total output buyers want at each price level.
总需求是在每个价格水平上，买家想要的总产出。
It slopes downward for three reasons — and none of them is substitution.
它向下倾斜有三个原因，而其中没有一个是替代效应。
The wealth effect: a higher price level makes money worth less.
财富效应：价格水平更高，货币就更不值钱。
The interest-rate effect: higher prices push interest rates up, so firms invest less.
利率效应：价格更高会把利率推上去，企业于是减少投资。
The exchange-rate effect: exports get dearer, so net exports fall.
汇率效应：出口变贵，净出口下降。
Aggregate demand is four kinds of spending.
总需求由四种支出组成。
Consumption is spending by households, and it is by far the biggest part.
消费是家庭的支出，也是最大的一部分。
Investment is spending by firms on capital.
投资是企业购买资本品的支出。
Government spending is the third.
政府支出是第三项。
Net exports are exports minus imports.
净出口是出口减进口。
Now the distinction the exam tests every year: the price level moves you along the curve, but a change in any of these four shifts it.
接下来是考试每年都考的区分：价格水平让你沿着曲线移动， 而这四项中任何一项变化，整条曲线都会移动。
Here is why a small injection matters.
这就是一笔小注入为什么如此重要。
The government spends one hundred dollars.
政府花了一百美元。
That is income for somebody, and if people spend sixty percent of extra income, sixty dollars is spent again.
这是某个人的收入； 如果人们把额外收入的百分之六十花掉，就有六十美元被再次花出去。
That sixty is someone else's income, and thirty-six is spent again.
这六十美元是另一个人的收入，其中三十六美元又被花掉。
Each round is smaller, but they do not stop.
每一轮都更小，但不会停下来。
In total, spending rises by two hundred and fifty.
总共，支出增加了二百五十。
Now put numbers on it, with people spending eighty percent this time.
现在给它加上数字，这次人们花掉的是百分之八十。
The blue bars are the spending in each round, each smaller than the one before.
蓝色柱子是每一轮的支出，一根比一根矮。
The orange line adds them up: it climbs, but by less and less, until it flattens.
橙色曲线把它们累加：它不断上升，但增幅越来越小，最后趋于平缓。
The formula at the top gives the finish line.
顶部的公式给出最终结果。
The spending multiplier is one divided by one minus the marginal propensity to consume.
支出乘数等于一除以「一减去边际消费倾向」。
Here it is five.
在这里它等于五。
Now the calculation.
现在来算一算。
The marginal propensity to consume is nought point eight, and the government raises spending by ten billion dollars.
边际消费倾向是零点八，政府把支出提高一百亿美元。
Pause here and try it yourself.
先暂停，自己算一遍。
The propensity to save is one minus nought point eight, which is nought point two.
边际储蓄倾向等于一减零点八，也就是零点二。
The spending multiplier is one divided by nought point two, which is five, so real GDP rises by fifty billion dollars.
支出乘数等于一除以零点二，等于五，所以实际 GDP 增加五百亿美元。
Now the same ten billion as a tax cut instead.
现在把同样的一百亿改成减税。
The tax multiplier is minus four, so GDP rises by only forty billion — part of a tax cut is saved.
税收乘数是负四，所以 GDP 只增加四百亿。 减税中有一部分被储蓄起来了。
Now the supply side: short-run aggregate supply, S R A S.
现在看供给一侧：短期总供给，S R A S。
In the short run some costs are sticky — above all nominal wages, fixed by contracts.
在短期，有些成本是粘性的—— 尤其是由合同固定的名义工资。
When the price level rises and wages do not, each unit earns more profit, so firms produce more.
当价格水平上升而工资不变时，每一单位的利润更高，企业就生产更多。
That is why short-run supply slopes upward.
这就是短期供给向上倾斜的原因。
Anything that changes production costs shifts it: higher input prices to the left, better productivity to the right.
任何改变生产成本的因素都会让它移动： 投入品价格上升让它左移，生产率提高让它右移。
Taxes and subsidies on producers move it too, and so does a supply shock.
对生产者的税收和补贴也会让它移动，供给冲击同样如此。
Long-run aggregate supply, L R A S, is next.
接下来是长期总供给，L R A S。
In the long run every price and wage is flexible, so sticky wages are gone.
在长期，所有价格和工资都是灵活的，粘性工资不再存在。
Output settles at potential output — the full-employment level.
产出稳定在潜在产出上——也就是充分就业的水平。
So long-run supply is a vertical line: the same real output at any price level.
所以长期供给是一条垂直线：在任何价格水平上，实际产出都相同。
Same idea as the production possibilities curve, the P P C: both mark the economy's maximum sustainable capacity, so an outward shift of one is the same event as an outward shift of the other.
这和生产可能性曲线（P P C）是同一个想法：两者都标示经济的最大可持续产能， 所以其中一条外移，和另一条外移，说的是同一件事。
Only real things move it: more resources, better technology.
只有实实在在的因素才能让它移动：更多资源、更好的技术。
That is economic growth — the line shifts right, and the price level falls.
这就是经济增长——这条线右移，价格水平下降。
Put the two together.
把两条曲线放在一起。
The economy settles where A D meets S R A S — where aggregate demand crosses short-run supply.
经济体停在总需求与短期供给相交的地方。
Read the price level off the vertical axis, and real output off the horizontal one.
从纵轴读出价格水平，从横轴读出实际产出。
Then the step students forget: compare that output with the vertical long-run line, full capacity.
然后是学生常忘的一步：把这个产出与那条垂直的长期线——充分产能——作比较。
On the line, the economy is at full employment. Off it, there is a gap.
在这条线上，经济处于充分就业；不在这条线上，就存在缺口。
Two gaps, and you must name them correctly.
缺口有两种，名字必须叫对。
Output below potential is a recessionary gap: spare capacity and high cyclical unemployment.
产出低于潜在水平，就是衰退缺口： 有闲置产能，周期性失业率高。
Output above potential is an inflationary gap: the economy is overheating and prices are pushed up.
产出高于潜在水平，就是通胀缺口：经济过热，价格被推高。
The size of the gap is the distance from actual output to potential.
缺口的大小，就是实际产出到潜在产出的距离。
Potential one thousand, actual nine hundred and twenty — a gap of eighty.
潜在产出一千，实际九百二十——缺口就是八十。
Now shock the model.
现在给模型一个冲击。
Spending rises: more confidence, a tax cut, more government spending.
支出增加了：信心更足、减了税、政府支出更多。
Aggregate demand shifts right along an unchanged supply curve, and the new crossing sits up and to the right.
总需求沿着没有变化的供给曲线右移，新的交点落在右上方。
The price level rises, output rises with it, unemployment falls.
价格水平上升，产出随之上升，失业率下降。
A demand shift moves prices and output the same way — demand-pull inflation.
需求移动让价格和产出朝同一个方向变化——这就是需求拉动型通货膨胀。
A supply shock is nastier.
供给冲击更麻烦。
Oil prices jump, costs rise, and short-run supply shifts left.
石油价格暴涨，成本上升，短期供给左移。
Along an unchanged demand curve, the price level rises but output falls and unemployment rises.
沿着没有变化的需求曲线来看：价格水平上升，但产出下降，失业率上升。
Prices and output move in opposite directions.
价格和产出朝相反方向变化。
That is stagflation — the hardest case, because the cure for one side makes the other worse.
这就是滞胀——最难处理的情况， 因为治好一边会让另一边更糟。
What if the government does nothing?
如果政府什么都不做呢？
Left alone, the economy heals itself — by moving short-run supply, never demand.
放着不管，经济会自我修复——靠的是移动短期供给，绝不是需求。
In a recessionary gap, high unemployment forces nominal wages down and supply shifts right, until output is back at potential.
在衰退缺口中，高失业迫使工资下降，供给右移，直到产出回到潜在水平。
In an inflationary gap, wages are bid up and supply shifts left.
在通胀缺口中，工资被抬高，供给左移。
Either way output returns to potential, but slowly.
两种情况下产出都会回到潜在水平，但很慢。
That is the argument for acting, not waiting.
这正是主张出手干预而非等待的理由。
So governments act.
于是政府出手。
Fiscal policy is the use of government spending and taxes to move aggregate demand.
财政政策就是用政府支出和税收来移动总需求。
To close a recessionary gap, use expansionary policy: spend more, or tax less.
要消除衰退缺口，就用扩张性政策：增加支出，或者减少税收。
Demand shifts right and output rises — but the price level rises too, which is the cost.
需求右移，产出上升——但价格水平也上升，这就是代价。
For an inflationary gap, do the reverse: contractionary policy — spend less or tax more.
遇到通胀缺口就反过来，用紧缩性政策：减少支出或提高税收。
Fiscal policy has limits, though: time lags, political difficulty, and crowding out — the government borrowing to spend pushes interest rates up and squeezes out private investment.
不过财政政策也有局限：时滞、政治阻力，还有挤出效应—— 政府借钱花钱会推高利率，把私人投资挤走。
Now the exam question.
现在来看真正会出的考题。
A recessionary gap of one hundred billion dollars, and a marginal propensity to consume of nought point seven five.
一千亿美元的衰退缺口，边际消费倾向零点七五。
How much must spending rise to close it?
支出要增加多少才能消除缺口？
Pause here and try it yourself.
先暂停，自己算一遍。
First the multiplier: one divided by one minus nought point seven five, which is four.
第一步求乘数：一除以「一减零点七五」，等于四。
Then the rule: the required change in spending is the gap divided by the multiplier.
然后是那条规则：所需的支出变动等于缺口除以乘数。
One hundred divided by four is twenty-five billion dollars.
一千亿除以四，等于二百五十亿美元。
And with a tax cut instead?
那如果改用减税呢？
About thirty-three billion — always larger, because a tax cut leaks into saving first.
大约需要三百三十亿——总是更大，因为减税的钱会先漏一部分进储蓄。
One last tool, and it needs no vote.
最后一个工具，而且完全不需要投票。
Automatic stabilizers are already in the law, and they work without new legislation.
自动稳定器已经写在法律里了。 它们不需要新的立法就会起作用。
In a downturn a progressive tax system collects less, and transfer payments — unemployment benefits, welfare — rise on their own, so spending is cushioned.
在经济下行时，累进税制收到的税更少， 转移支付——失业救济、福利——会自动增加，于是支出得到缓冲。
In a boom both work the other way.
在繁荣时，两者都反过来。
Nothing new has to be passed, so they act instantly, with no legislative lag.
因为不需要通过新法案，它们立刻生效，没有立法时滞。
Discretionary policy — a new bill — is precise, but slow.
相机抉择政策，也就是新法案，更精准，但很慢。
Three marks students throw away.
三个学生常丢的分。
First, decide whether it is a movement or a shift: only the price level moves you along aggregate demand.
第一，先判断是沿线移动还是整条曲线移动： 只有价格水平的变化，才是沿着总需求曲线移动。
Second, never use the spending multiplier for a tax change — the tax multiplier is smaller and negative.
第二，绝不要把支出乘数用在税收变动上——税收乘数更小，而且是负的。
Third, draw the graph properly: both axes, all three curves, the new equilibrium.
第三，把图画规范：两条坐标轴、三条曲线，以及新的均衡点。
Name the gap, choose the policy, show the direction.
说出是哪种缺口，选定政策，标出移动方向。