陶哲轩赞成凯恩斯主义zz

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收藏 2013-03-04

Terence Tao - 2011-8-18 - Buzz
- 公开

In a perfect free market, the only economic transactions conducted are those that benefit all parties involved, and so the self-interest of each individual leads to a benefit (in the sense of increased efficiency) of the economy as a whole, a phenomenon popularly known as the "invisible hand".

There is however a major caveat to this phenomenon, though, and that is when transactions carry externalities - costs that are unwillingly borne by third parties (or benefits unwillingly granted to third parties) as a consequence of an economic decision. A typical example is that of pollution; in the absence of laws or other mechanisms to penalise an individual from polluting, self-interest would induce individuals or corporations to inflict costs on the wider community in exchange for a self-benefit. Generally speaking, government intervention (e.g. through regulation) can improve upon the outcome of a free market when there are significant externalities, but when externalities are negligible, a mature and healthy free market is most efficient (in the sense of coming closest to Pareto-optimality) with minimal government intervention (beyond such basic governmental functions such as preventing fraud and theft, of course). [EDIT: this does not preclude the need for other, non-market, interventions to improve efficiency, or to obtain other societal objectives such as increased equality or increased allocative efficiency (which is stronger than Pareto efficiency). In some cases, it is worth reducing the efficiency of an externality-free market (e.g through taxation) in order to improve the efficiency of an externality-laden one, or to improve equality through redistributive policies.]

I was recently puzzled, though, by how this reasoning would apply to Keynesian economics, particularly with regard to the need for government intervention to fight recessions. At first glance, there was no obvious externalities in a recession that would prevent a free market from correcting a recession automatically. I did finally figure out that there was in fact such an externality, though, which explains the need for government intervention.

To oversimplify, the economic activity of an individual person or company in a free market can be broadly divided into three categories:

* Production. Using labour (or other inputs) to produce goods and services, which one exchanges for money (either directly through a market, or indirectly through a salaried contract).

* Spending. Using money to exchange for goods and services produced by others.

* Saving. This is the difference between production and spending; it is positive if one earns more than one spends, and is negative if one spends more than on earns.

A self-interested individual would, generally speaking, prefer to work (i.e. produce) as little as possible, spend as much as possible, and save as much as possible. Of course, these three objectives are mutually inconsistent, due to the constraint

Saving = Production - Spending (*)

and so one has to optimise one's production, spending, and saving subject to the constraint (*).

Each individual (or corporation) would have a different preferred allocation of production, spending, and saving. For instance, individuals typically plan to have positive savings through their working years, and negative saving through retirement. Conversely, companies often have negative savings when starting out and positive savings once they become profitable. One of the main reasons we use money at all is in order to allow individual savings to be positive or negative as the need requires. However, there is a fundamental collective constraint on savings, caused by the equation

Total production = Total spending (**)

or equivalently

Total saving = 0 (***).

This simply reflects the fact that in order for one individual to buy a good or service, some other individual has to produce that good or service. (This assumes that one is in a closed economy; the situation is more complicated once one allows imports and exports, but let me ignore this aspect for simplicity.)

In view of (***), we see that the economy is in equilibrium when the participants in the economy who are trying to have positive saving are balanced out by the participants who have negative saving. However, for a variety of reasons (e.g. external economic shocks), the economy can end out of equilibrium, and in particular in a situation in which there is a net desire for saving: roughly speaking, more participants in the economy are trying to save money than to spend it.

Thanks to (*), there are only two ways to increase savings: either increase production, or decrease spending. In many cases (e.g. with salaried workers), the former option is not available or is insufficient to meet one's savings goals, and so what happens is that many individuals choose to cut spending. But here is where the externality occurs: due to (**), every time an individual reduces his or her spending, somebody somewhere loses the business (and hence, the profit) that would have otherwise been obtained from that spending. That individual, in order to maintain his or her own savings goals, must now either make up that lost production, or else also cut spending. Again, a significant fraction of the time, spending will again be cut, and the cycle continues. When this process becomes widespread, a recession occurs. Ironically, in the early stages of a recession, the free market often makes the problem worse; once it becomes apparent that the economy is in recession, it is natural for individuals to save even more to ride out the recession, which only serves to exacerbate the problem (the "paradox of thrift").

The usual regulatory methods of dealing with externalities do not work here; one can penalise someone for polluting, but in a capitalist system, one cannot penalise someone for failing to buy something from someone else. Instead, there are two major types of government intervention to deal with recessions:

* Fiscal policy. By stimulating demand through countercyclical spending (or tax reductions), one can allow individuals to meet their savings goals through increased production, rather than reduced spending; if the stimulus is large enough, this can arrest the recession cycle. (To balance things out, of course, the government should also be engaged in deficit reduction (and tax increases) during boom times, when the private sector has an excess of spending rather than of saving.)

* Monetary policy. By reducing interest rates, one can make saving less attractive (and spending more attractive, thanks to the more favourable terms of credit), returning individual savings preferences closer to balance (***). (In an open economy, devaluation of currency can also be effective, though at a global level this method suffers from a similar externality problem, as it basically exports the recession to other countries.) Another option is to increase the money supply (e.g. by quantitative easing) to meet the demand for savings, although the effectiveness of this method is still in dispute.

What happens in the absence of government intervention? Interest rates still exist in a free market, so in some cases a recession can correct itself through a free-market fall in interest rates. However, there are circumstances in which interest rates will not fall on their own, for instance if the currency is pegged to an external reference point (e.g. the euro, or gold). Also if interest rates are already close to zero, then this self-correcting mechanism becomes unavailable (this is arguably the situation that much of the West is currently in). In that case, the free-market will still self-correct, but in a much more painful way: by reducing both total production and total spending in real terms until individuals are forced to abandon their excess savings goals because they cannot afford them any more (and must instead use all their earned income for subsistence spending). This process is often also accompanied by deflation, as individuals and companies cut prices and wages in an attempt to boost demand, but ironically deflation can exacerbate the situation by making saving more attractive (and, in the case of debtors, more urgent).

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2011wi

2013-3-4 09:49:52

这人是干嘛的？不认识...

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zds999

2013-3-4 13:57:13

数学天才啊

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jdzz

2013-3-4 19:17:17

陶哲轩也关注过经济学？请问这是出自哪里的资料？我有两点疑问，希望交流。一，如文中所说，某些参与者因某种原因减少支出，导致支出品的供给者产出和收入下降，通过乘数的链式反应使最终产出较大幅度下降。疑惑是，在这过程中，难道市场不会自动调节？本文没有给出导致市场不会自动调节的假设过原因，教科书的说法是该理论的前提假设是价格水平不变——这就外生地废除了市价自动调节机制，整个逻辑是“因为A所以A”。市场因各种原因发生各种供求变化是正常的，怎能说一批客户突然消失了，就会因外部性导致整个市场产出大减？如果一些产品市场需求萎缩，那就降价呗！再不就减少产出规模，倒闭一些公司，生产者可转向其他产业，这是正常的市场调节啊！只要竞争发挥作用，就没外部性什么事！二，可能会反驳，若所有产业的总需求都下降呢？这是个值得探讨的问题。原文只把它看成一个外部冲击，无缘由地原本的开支变为储蓄。这种假设是经不起推敲的。凯恩斯认为，随着收入增加，投资、消费各种支出会因回报率下降而出现减少的倾向，从而经济发展一段就会萧条。我没看过凯恩斯的原著，不知这个观点的理由是什么。长期看，资本的边际产出下降，但这真是近几百年来历次经济萧条的基本原因？我认为，比较正常的分析应是这样：任何经济参与者获得收入后，通过市场交易增进收益，一部分购买食物等各类消费品，生产劳动能力增益；一部分购买股票、债券、银行存款等，也是一种增值；当然还有可能直接购买厂房机器，获得增值。经济人会比较各类收益而选择支出渠道。这样看，正常情况下，是没有什么需求不足的。若真是如凯恩斯所说，各类投资的边际产出均无一例外下降，那就是经济该萎缩。设想一下，如果人类到了一定年龄就会得厌食症，经济不该萎缩消失吗？所以说，边际下降的假设总是让人感觉牵强。然而，现实中确实能观察到，有些时期会出现投资消费减少的问题，即人们只愿持有不增值的货币或收藏品贵金属等，减少消费和投资等各类其他开支。我认为，这应归结为有限理性，即人们做决策面临较高的信息费用，对投资前景缺乏明确预期，导致人们选择最安全但无收益的方式。然而，完全竞争情况下，市价的指引会减少决策者的信息费用，信息费用的增加只能说明，价格机制受到干扰。在宏观经济层面，干扰价格机制的可能是货币政策，行政干预，政治因素等等。所以，总支出的减少只能是决策者面临不确定性的条件下，规避风险的理性选择，可看成是一种保险消费。只有在这种情况下，ZF干预经济才具有合理性，而ZF干预的有效措施应是帮助恢复市价指引的机制，帮助公众恢复信心，随便增加ZF购买未必有效，反而可能是政客纯个人利益上的考虑。

资本边际收益下降只可能是长期经济进入稳态的原因，短期的萧条可解释为市场机制受干扰，公众决策的信息费用上升。在市场充分调节的情况下，某一个产业或市场需求受冲击不会导致整个经济持续萧条。
希望您仔细分析以上逻辑。

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remlus

2013-3-5 15:47:03

他的理由是流动性陷阱，这几年美国宏观经济学家对于零名义利率约束的兴趣从零跃迁到无穷大。宏观经济学是很应景的。

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