[转帖] 经济学史：那些让人困惑的模型

showcraft

http://article.yeeyan.org/view/117075/304165

经济学史：那些让人困惑的模型

回到2008年10月，就在雷曼兄弟投资银行破产不久，国际货币基金组织公布了它对2009年的增长预测。国际货币基金组织（IMF）是各国政府的全球债权人；它的经济声明备受重视。那么它预测出什么呢？美国将在2009年增长0.1%，欧元区内的国家是0.2%，而全球增长率将会有2.6%。而三者实际结算时分别下降了3.5%，4.2%和2.6%。
这样的可悲的无远见的预测并不少见。英女皇曾经有这样的著名评价：经济学家已经多次没能预测到经济衰退，而且会完全被随之来的金融危机给难住。
这一职业的不足之处已经是老新闻了。一路追溯到1994年，保罗.奥默罗德写了一本叫《经济学之死》的书，哀叹其没能预测到日本的经济衰退或者汇率机制的崩溃（英国在1992年弃用该机制）。奥默罗德写道“正统经济学在全面理解经济运行方面的能力显然是很低的（某些人甚至会说根本没有）”。
对经济学家公平点的来说，他们的预测之所以经常不准确有两个原因。第一，人类并非死物。我们会改变我们的行为而且我们会看新闻。如果每一个经济学家都预测2013年会出现经济衰退的话，同时该语言又被广泛宣传，企业将会取消他们的投资计划，消费者因害怕可能会丢掉工作而不再消费开始储蓄。经济衰退立马就会发生，而不是在下一年。
第二，经济是一套包含很多运行部分的复杂机制。经济学家不能以科学家那样的方式进行实时实验；先运作一个高利率版本的经济体，而后再运行另一个低利率的，像一个药剂师能给一个病人新药而给另一个病人安慰剂一样。我们没有办法把各种影响经济增长的因素孤立开来。
然而对于该话题的本质除了经济学家不能做出准确预测之外，还存在着一些更为根本的问题。经济学家有准确的人类动机模型吗？或者说他们是否假设我们所有的动机都是唯利是图的呢？
在乔纳森.沙立夫精彩的著作《经济学家的假设》中，他试图回归到最初的一些原则上。他写道，经济学家“对我们的世界做了一些简单的假设，并在那些假设上建立起想象中的经济体——也被称作模型——然后用他们来吸取实际的教训。”他承认这是不可避免的；经济太复杂了，没办法以任何其它方式运作。精确地使用这些简便的模型可以为经济问题给出答案。然而我们很容易因这些模型的精确而得意忘形，忘记了在做出这些简单的假设时我们选择所选择走的是捷径。
即使是最基本的经济学假设原来都是有例外的。举一个大部分人都能理解的定律做例——供给和需求。当供给相对于需求上升时，价格下跌；而当需求相对于供给上升时，则价格上升。然而这并不适用于房地产；当价格上升时，因为有更多的人想要成为有房者，于是需求也上升。而且它也不适用于所谓的凡勃伦产品（炫耀性产品），像名牌服装这样的奢侈品，它们的吸引力源于它们高昂的价格。
现代经济学家经常被指责“嫉妒物理学”，在他们的论文上充斥着复杂的方程式，使之看起来像“真的”科学一样。不过在某种意义上，这两者有相似的地方；他们都能一分为二。存在亚原子物理学来论述组成物质的微小颗粒，然后还有经典力学来论述像重力这些作用力在物体上的效果。把这两者结合并不容易。类似的，存在微观经济学来处理个体和公司的行为；而宏观经济学曾处理的是总体层面上的经济。
划分现代经济学思想的方式多种多样，其中一种划分是微观和宏观。芝加哥/新古典主义学派趋向从微观的层面上建立模型，关注理性个体都会对激励做出反应这样的方式。凯恩斯学派则认为理性个体积聚起来的反应可能会出现反常的结果，因为在节俭悖论中，这需要政府对此做出行动回应。两方都背负着积聚起来的政治包袱，芝加哥学派不愿意看到政府没干好的，而凯恩斯学派则不愿意承认政府干预的效果可能存在一定缺陷。
在个体都在寻求“最大化他们的效用”的思想中，个体都寻求购买到尽量满足我们的产品组合，在我们的一生中都能平衡我们的消费，以休闲平衡我们的工作等等。这导致芝加哥学派人士坚持认为工人和企业将能看穿政府政策的效果并据此调整他们的行为；比如今天巨大地财政赤字不可避免的导致未来几年里的高赋税。
这种“理性预期”的方法在把人作为有思想的个体而不是简单的小经济微粒反面有一定的优点。然而理性预期模型要求普通的公民去做复杂的计算，貌似除了经济学家之外的任何人都会认为这是不理性的。
正如沙尔夫写道，这些模型“说就业升上或者是下降是因为经济参与者选择在生产率高的时候工作更多而生产率低的时候则相反。这想法简直是发疯的。”如克鲁格曼调侃说，这一方法合乎逻辑的结论是工人因为享受了更多的假期而造成了大萧条。这一超凡脱俗的心态不禁让人想起凯瑟琳大帝关于政治思想的名言“它只在文件上行得通，而文件能承认所有没有困难和易被说服的东西，并且不会对你的幻想和下笔有任何的阻碍。”
另一个相关的批评是，经济学家常常认为大部分的问题通过设计准确的激励措施都能产生出来。如果出现血荒，给献血者支付报酬可能看起来是一个很棒的办法。然而研究表明大部分捐赠者的动机都是出于公民义务，而金钱上的奖励事实上可能会逐渐削弱他们的无私观念。举个相关的例子，给予一名参与者100美元然后要求他和另一名成员分享；规矩是这样的，如果受赠者拒绝分给另一成员，两人都将一无所获。一个理性的受赠者（经济学家术语）将很高兴能得到1美元，因为总比没有好；然而，从公平的角度上就是受赠者希望得到的是接近50-50的分配。如果赠与者太吝啬，受赠者会很乐意通过当场拒绝分配来惩罚他。
简而言之，很多经济学家似乎忽略了非经济因素动机的重要性，尽管被称为“行为经济学”的学派正试图对其做修正。人们对自身该怎么做有自己的想法，它来源于社会规范，而那可能不太符合效益最大化模型。正因如此，雇主在经济衰退时可能不太愿意削减他们员工的工资，尤其是工人会对削减产生怨恨并因此失去工作动力。雇员对没有通胀情况下削减5%的工资要比存在5%通胀时维持工资不变怨恨来得更多，尽管对于他们的生活标准来说效果是一样的。
经济学的历史可以看做是《花生漫画》中连续的镜头，在里面每当查理.布朗试图去踢那个足球时露丝总是拿走它。当经济学家刚刚达成意见一致，现实世界中所发生的事件证明他们是错误的。古典经济学家采取的是把问题留给经济本身，它会自己找到方法回归平衡；在低迷时期，工资将会下调然后工人也会调整自身工价以（有机会）返回工作岗位。于是大萧条来临了，这一话题被约翰.麦纳德.凯恩斯不可逆转地改变了。
在上世纪30年代，焦虑的民众决定把他们的收入储蓄起来而不再消费；这引起的需求下降导致工人失去工作，又引致了更进一步的谨慎。出于理性思考做出的储蓄决定将会让整个经济规模变得更小，这就是“节约悖论”。凯恩斯坚持认为，在这样的环境下，政府（能比个体以更低成本贷款）应该增加支出以支撑需求。
1945年之后，凯恩斯学派认为通过对政府政策的小心优化，经济就能被成功地管理好。如果存在失业，那么政府就应该增加投资；如果存在通胀，那就应该刹车。后来到了70年代，当通胀和失业同时高企时，经济思想又发生改变了。
在70年代，凯恩斯人的共识被米尔顿.费里德曼推翻了，他宣称失业和通胀间表面上的平衡是一种假象。工人通过要求更高工资回应凯恩斯人的政策，于是最终结果是高通胀的同时失业却未见减少。因此，政府应该把重点放在可以防止通胀上扬的货币供应的控制上。然而芝加哥学派对凯恩斯主义的攻击并不局限于依附于它的货币主义者；事实上，它演变出货币的性质很难被清楚解释的思想，随后因为采纳了他们意见货币目标在长达十年的时间内很大程度上被弃用。
芝加哥学派的雄心来源于其对强势政府的厌恶以及对通胀的看法；凯恩斯人的政策导致国家在经济中扮演一个比二战前大得多的角色。工业被国有化了；福利支出猛增。税收节节攀升；60年代盛行的高费率中的95%甚至使得披头士在歌曲Taxman中的抨击震撼人心（“5%显得很少吗，感激我并未全部收走”）。芝加哥学派人士坚持认为国家是无效率的资源配置器；高赋税和福利支出对于动机有抑制效果。
这些理论赢得支持者的时机到了。70年代是盛行罢工、电力断区和高油价的危机时代。政治风向标转向了；在加利福尼亚，一次公民投票，第13号提案的出现，宣布了人们对高财产税的反抗。罗纳德.里根在美国上台不久，撒切尔夫人也上台了。那是一个“文明加强”的年代——放松管制，自由化，私有化以及至关紧要的大缓和，一个稳步增长的时期，通胀下跌和低失业率是90年代和新世纪头十年的标志。
古典主义学派的思想得以回归，而自由市场看似也履行了它们的承诺。2003年在给美国经济学协会做总统演讲时，罗伯特.E.卢卡斯（一名芝加哥学派元老）是那么说的，“宏观经济学就其原意来说已经获得了成功：它关于防止萧条的核心问题已经被解决了，对于所有实际的目标实际上都已经被解决了数十年。”仅仅5年过后，自上世纪30年代以来最严重的经济下滑出现了；全世界现在仍在努力地摆脱它。
为了应付财政危机，各国政府和央行已经尝试了所有的办法；减税，把利息降到历史最低水平；用新印的钱（这一过程被称为量化宽松QE）购买资产；增加政府支出而后又削减它。这些政策哪些有效那些无效呢？没有人真正知道。
那并未让经济学家停止对问题或者相互间提出非常深刻的观点。比起那些总说“一方面这样，而另一方面那样”的摇摆不定的顾问，哈里.杜鲁门总统更需要一位有缺点的经济学家，只不过那样的时代一去不复返了。布拉德.德隆在他的博客上对他的对手写道，他们是“一群相当懒惰的意识形态拥护者，从来没有将来也不会做好他们的工作，他们只会胡吹胡扯”。而他的对手约翰.科伦克则回应道“这完全就是废话”。
长期忍受着的公众是这些辩论的茫然的观察者，期望着有英明的政客会能想出有说服力的方案。对历史学家来说，一切都是周期的问题。金融领域变得放纵，然后被管制；政府干预热情高涨，然后退却；消费者，公司和政府举债，然后又削减。我们选举出政客，然后由他们指派央行的银行家，并希望他们能控制好这些周期可他们却做不到，就算是停止萧条的进程也不行。
经济学最大的好处是我们知道我们不应该做什么；从非洲的贪官污吏到极权的北朝鲜我们有许多的现代的例子。一个功能正常的现代经济体需要尊重财产权利；政府能够收税并提供社会安全网络；银行允许支付系统运行；市场允许企业募集资金等等。一旦那些基本要素准备就绪，真实的税率是40%还是50%，真实利率是1%还是5%很大程度上只不过是反复试验，从失败中吸取经验而已，是政治上可接受范围内的。
在英国政府的紧缩财政和法国的支持增长方式之间，英国的“盎格鲁-撒克逊”模式和法国的计划调控方式之间有着很大的区别。不过先不管这些虚夸的言辞，根据国际货币基金组织（IMF）数据，2011年英国的人均国内生产总值为36090美元，而法国则是35156美元，两者几乎一样。英国计划在2017年前平衡预算，而法国也是。
尽管结果有点区别，经济学家将继续激烈地争论不同竞争机制之间的优劣，就好比宗教改革运动中的神职人员讨论圣餐变体和圣体共在论的区别一样。沙尔夫先生的书比这篇博客摘记（尽管已经很长）能阐述的要更深刻和丰富，是对长期的争辩很有价值的补充。

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http://www.economist.com/blogs/b ... 07/economic-history
Economic history
Muddled models
Jul 20th 2012, 9:47 by Buttonwood
BACK in October 2008, just after the investment bank Lehman Brothers collapsed, the International Monetary Fund unveiled its forecasts for growth in 2009. The IMF is the global lender to national governments; its economic pronouncements are highly respected. So what did it predict? The US would grow 0.1% in 2009, countries in the euro zone 0.2% and the world as a whole 2.6%. The actual outturns were declines of 3.5%, 4.2% and 2.6% respectively.

This lamentable short-sightedness was not unique. Economists have regularly failed to predict recessions and were completely caught out by the recent financial crisis, as the Queen famously noticed.

The shortfalls of the profession are old news. All the way back in 1994, Paul Ormerod wrote a book called The Death of Economics, lamenting the failure to forecast the Japanese recession or the collapse of the Exchange Rate Mechanism, from which Britain was turfed out in 1992. “The ability of orthodox economics to understand the workings of the economy at the overall level is manifestly weak (some would say it was entirely non-existent)” Ormerod wrote.

To be fair to economists, there are two reasons why their forecasts are often likely to be wrong. The first is that humans are not inanimate objects; we change our behaviour and we watch the news. If every economist forecast a recession for 2013 and the predictions were widely publicised, businesses would cancel their investment programmes and consumers would start saving, not spending, for fear of losing their jobs. The recession would occur now, not next year.

Second, the economy is a complex mechanism with many working parts. Economists cannot run real-time experiments in the same way as scientists; operating one version of the economy with high interest rates and another with low rates, as a pharmacologist can offer one patient a new drug and another a placebo. There is no way of isolating the various factors that affect growth.

But there are more fundamental questions about the nature of the subject beyond the failure of economists to make accurate forecasts. Do economists have an accurate model of human motivation? Or do they assume that our motives are entirely mercenary?

In his excellent book, “The Assumptions Economists Make” Jonathan Schlefer tries to go back to first principles. Economists, he writes, “make simplified assumptions about our world, build imaginary economies based on those assumptions – otherwise known as models – and use them to draw practical lessons.” This is, as he admits, inevitable; the economy is too complex for any other approach to work. Simplified models can be manipulated mathematically to produce answers to economic problems. But it is easy to get carried away by the elegance of the model, and to forget the short cuts that were taken when the simplified assumptions were made.

Even the most basic assumptions of economics turn out to have exceptions. Take one law that most people can grasp – supply and demand. As supply rises, relative to demand, the price falls; while if demand rises, relative to supply, the price rises. But this is not true for housing; when prices are rising, demand increases as more people want to become homeowners. And it is not true of so-called Veblen goods, luxury items such as designer clothes whose appeal is driven by their higher price.  

Modern economists are often accused of “physics envy”, filling their papers with complex equations to make them look like “real” science. But in one important sense, the subjects are similar; they can be divided into two. There is sub-atomic physics which deals with the tiny particles that make up matter, and then there is classical mechanics, which deals with the effect on bodies of forces like gravity.  Marrying the two has not always been easy. Similarly, there is micro-economics which deals with how individuals and companies behave; and macroeconomics, which deals with the overall economy.

There are various ways of dividing modern economic thought, but one divide is the way they marry the micro and the macro. The Chicago/neoclassical school tends to build up from the micro level, looking at the way that rational individuals will respond to incentives. The Keynesian school sees that the aggregated response of rational individuals might have perverse outcomes, as in the paradox of thrift, so calls on the government to take action in response. The two sides have also accumulated political baggage with the Chicago school reluctant to see that governments can do nay good and the Keynesians reluctant to acknowledge that there may be a limit to the effectiveness of government intervention.  

Take the idea that individuals seek to “maximise their utility”, to buy the combination of goods that best satisfies us, to smooth our consumption over our lifetimes, to balance work with leisure and so on. This led the Chicagoans to argue that workers and businesses would see through the effect of government policies and adjust their behaviour accordingly; for example, that a big budget deficit today would inevitably lead to higher taxes in a few years’ time.

This “rational expectations” approach has the benefit of treating people as thinking individuals rather than elementary particles. But the rational expectations model requires the average citizen to make complex calculations that seem, well, irrational for anyone but an economist to expect.

As Schlefer writes, these models “say that employment rises or falls because actors choose to work more when productivity is high and less when it’s low. This idea is nuts.” As Paul Krugman has quipped, the logical conclusion of this approach is that workers created the Great Depression by taking an extended holiday. This other-wordly mentality is reminiscent of Catherine the Great’s aphorism about political ideas which “work only on paper, which accepts anything, is smooth and flexible and offers no obstacles either to your imagination or your pen.”

A related criticism is that economists tend to think that most problems can be created by designing the right incentives. If there is a shortage of blood, making payments to blood donors might seem a brilliant idea. But studies show that most donors are motivated by an idea of civic duty and that a monetary reward might actually undermine their sense of altruism. A related example involves giving $100 to a participant and asking him to share it with another member of the group; the catch being that, if the donee rejects the deal, neither person gets anything. A rational donee (in economists’ terms) would be happy to accept $1, since it is better than nothing; however, a sense of fairness means that donees expect something much closer to a 50-50 split. If the donor is too mean, the donee is happy to punish him by rejecting the ideal outright.

In short, many economists seem to neglect the importance of non-financial motivation, although the school known as “behavioural economics” is starting to remedy this. People have an idea about how they should behave, derived from social norms, which may not fit the profit-maximising model. Thus employers may be reluctant to slash the wages of their employees in a recession, not least because workers will resent such cuts and lose motivation. Workers will resent a 5% wage cut at a time of zero inflation far more than a wage freeze at a time of 5% inflation, even though the effect on their standard of living will be the same.

The history of economics can be viewed rather like the regular sequence in the Peanuts cartoon strip, whereby Lucy snatches the football away every time that Charlie Brown tries to kick it. Just when economists have reached a consensus, events in the real world proved them wrong. Classical economists assumed that, left to itself, an economy would find its way back to balance; in a downturn, wages would fall and workers would price themselves back into jobs. Then came the Great Depression and the subject was changed irrevocably by John Maynard Keynes.

In the 1930s, worried citizens decided to save, not spend, their income; the resulting fall in demand caused workers to lose their jobs, causing an even greater level of caution. The “paradox of thrift” was that the rational decision to save could make the entire economy smaller. Keynes argued that, in such circumstances, governments (which can borrow more cheaply than individuals) should spend money and support demand.  

After 1945, the Keynesian school assumed that with careful tweaking of government policies, the economy could be managed successfully. If there was unemployment, then the government could put its foot on the accelerator; if there was inflation, then it as time to touch the brakes. Then came the 1970s, when both inflation and unemployment were high, and economic thinking changed again.

The Keynesian consensus was overturned in the 1970s by Milton Friedman, who asserted that the apparent trade-off between joblessness and inflation was an illusion. Workers responded to Keynesian policies by demanding higher wages, so the net result was higher inflation with no reduction in unemployment. Instead, governments should focus on by controlling the money supply, which would prevent inflation from rising. But the Chicago school’s attack on Keynesianism was much broader than the monetarist label which became attached to it; indeed, it turned out that the nature of money was very hard to define and monetary targets were largely abandoned within a decade or so of their adoption.

The Chicago school was inspired as much by its dislike of big government as by its views on inflation; Keynesian policies had caused the state to play a much bigger role in the economy than it did before the Second World War. Industries were nationalised; welfare spending soared. Taxes crept higher and higher; the 95% top rate prevailing in the 1960s even inspiring an attack by the Beatles in the song Taxman (“Should 5 per cent appear too small, be thankful I don’t take it all”). The Chicagoans argued that the state was an inefficient allocator of resources; that high taxes and welfare spending had a deadening effect on incentives.

The time was right for these theories to gain adherents. The 1970s was a period of crisis; of strikes, power cuts and higher oil prices. The political tide turned; in California, a referendum, Proposition 13, heralded a revolt against high property taxes. In Britain, Mrs Thatcher came to power followed shortly after by Ronald Regan in America. It was the age of the “ations” – deregulation, liberalisation, privatisation and crucially the Great Moderation, a period of steady growth, falling inflation and lower unemployment that marked the 1990s and early 2000s.  

It was a return to the thinking of the classical school and free markets appeared to deliver on their promise. In his Presidential address to the American Economic Association of 2003, Robert E Lucas (a doyen of the Chicago School) said that ““macroeconomics in this original sense has succeeded: Its central problem of depression prevention has been solved, for all practical purposes, and has in fact been solved for many decades.” Just five years later, the worst economic downturn since the 1930s occurred; a downturn from which the world is still struggling to emerge.

In response to the financial crisis, governments and central banks have tried all manner of policies; cutting taxes, slashing interest rates to historic lows; buying assets with newly-created money (the process known as quantitative easing or QE); raising government spending and then cutting it again. Which of these policies have worked and which have failed? No-one really knows.

That doesn’t stop economists from having very strong opinions on the subject and about each other. Gone are the days when President Harry Truman demanded a one-armed economist, rather than his wishy-washy advisers who said “On the one hand, this” but “On the other hand, that”. Frequent blogger Brad DeLong, writes of his opponents that they are “a bunch of rather lazy ideologues who haven't done and won't do their homework talking bullshit and trash”. John Cochrane, of the opposing school, writes that the other side is talking “absolute hogwash”.

The long-suffering public is a puzzled observer to these disputes, hoping that some bright politician will come up with a winning formula. To the historian, it is all a matter of cycles. The financial sector gets liberated, then constrained; enthusiasm for government intervention waxes, then wanes; consumers, companies and governments take on debts, then cut back. We elect politicians and appoint central bankers and expect them to manage these cycles but they cannot, any more than they can halt the progress of the seasons.

The best we can say about economics is that we know what not to do; we have plenty of modern examples from African kleptocrats to totalitarian North Korea. A functioning modern economy needs respect for property rights; a government that is able to collect taxes and offer a social safety net; banks that allow the payment system to function; markets that allow businesses to raise capital and so on. Once those essentials are in place, whether the right top tax rate is 40% or 50%, the right interest rate is 1% or 5% is largely a matter of trial and error, and of political acceptability.  

Much is made of the difference between Britain’s “Anglo-Saxon” model and of France’s dirigiste approach, between the British government’s austerity drive, and France’s pro-growth approach. But for all the rhetoric, Britain’s GDP per head in 2011 was $36,090, according to the IMF, while France’s was $35,156, almost identical. Britain plans to balance its budget by 2017, and so does France.

Despite the small differences in outcome, economists will continue to debate the merits of the competing systems as vigorously as Reformation clerics debated the difference between transubstantiation and consubstantiation. Mr Schlefer’s book, which is deeper and richer than this (already long) blog note can elucidate, is a very valuable addition to the debate.

showcraft

献血的例子，类似子贡赎人。

出处

　　《子贡赎人》，出自《吕氏春秋·察微篇》。
编辑本段
原文

　　鲁国之法，鲁人为人臣妾於诸侯，有能赎之者，取金于府。子贡赎鲁人于诸侯而让其金。孔子曰：“赐失之矣！夫圣人之举事，可以移风易俗，而教导可施于百姓，非独适己之行也。今鲁国富者寡而贫者多，取其金则无损于行，不取其金，则不复赎人矣。”
　　子路拯溺者其人拜之以牛子路受之孔子喜鲁人必多拯溺者。
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翻译

　　鲁国有一道法律，如果鲁国人在外国见到同胞（遭遇不幸），沦落为奴隶，只要能够把这些人赎回来（帮助他们恢复自由），就可以从国家获得金钱（的补偿和奖励）。 （孔子的学生）子贡，把鲁国人从外国赎回来，但不（向国家）领取金钱。孔子说：：“赐（端木赐，即子贡），你错了！圣人做的事，可用来改变民风世俗，教导可以传授给百姓，不仅仅是有利于自己的行为。 现在鲁国富的人少穷人多，（向国家）领取补偿金，（对你）没有任何损失；但不领取补偿金，鲁国（就没有）人再去赎回自己（遇难）的同胞了。”
　　子路救起一名溺水者，那人感谢他送了一头牛，子路收下了。孔子高兴地说：“鲁国人从此一定会勇于救落水者了。”