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波斯纳《管锥编》(4):“无理性人”证据不足  

2009-04-07 20:53:59|  分类: 默认分类 |  标签: |举报 |字号 订阅

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中文博主按:“无理性人”,是希勒的一个著名论断。他那本有名的《非理性繁荣》(irrational exuberance),至今也具有启示意义。邹恒甫教授认为,这本书译成“非理性繁荣”,很难听。他说得对。我对不起邹老师,因为他的湖南口音,我没有记得他的更正。


四、“无理性人”证据不足

 

People buy common stock when stock prices are rising. They(notoriously) bought houses during the early 2000s when house prices wererising. Since almost no one can predict the ups and downs of the stock marketor the housing market, these purchases must have been motivated, Akerlof andShiller argue, by something other than a rational investment strategy. But thisis not at all obvious, or implied by Keynes's usage. Stocks have generally beena good investment, at least when held for a considerable period. And since noone is able to time market turns, no one knows when the market is overpricedand therefore when one should sell rather than buy. Indeed, the idea of sellingat the "top" of the market is incoherent, because if it were knownthat stock prices had peaked, no one would buy. Buying stock, or buying a house,is at any time a guess about the future, a venture into the unknown. Yet thatdoes not imply irrationality.

股价上涨的时候,人们购买股票。21年代初,房价上升时,他们(臭名昭著)地购买房子。因为基本上没人能够预测股市或房市的上下波动,所以,阿克洛夫和希勒认为,这样的购买,都必定是受到了“理性投资策略”之外的因素的推动。但是,这一点,却一点也不清楚,凯恩斯的用法中也根本没有暗示过。一般来说,股票一直是个不错的投资,最少,当人们持有一段较长的时期后。而且,没有能确定市场拐点的时间,所以,没人知道何时才算是市场定价过高,因而,也就没人知道何时该抛出(而不是买入)。实际上,“逃顶”的想法,就是胡说八道,因为如果人们知道了股价到顶了,就不会有人再买了。买股票或者买房子,在任何时候,都只是对未来的猜测,是对未知事物的一次冒险。然而,这种行为却不是“无理性”。

 

In the early 2000s, interest rates were very low because of amistaken decision by Alan Greenspan (but who knew?); and since a house is aproduct purchased with debt (a mortgage), houses became a more than usuallyattractive investment. The housing stock expands only slowly because it is sodurable, so the increase in demand for houses outran the increase in supply(new housing starts), causing prices to rise. Since very few economists and nogovernment officials warned of a bubble, it was not irrational for people tothink that houses were a good investment, even though house prices had risensteeply since the 1990s.

21世纪初期,因为格林斯潘采取了错误的决策(但是,天晓得是怎么回事!),利率是很低的。而且,因为房子是用借债(抵押贷款)来购买的产品,所以,房子投资的吸引力,就超过了平常时刻。因为住房是耐用品,所以,房地产股票涨得很慢,这样,住房需求的增长,就超过了住房供给(新建房屋)的增长,这使得价格上升。因为几乎没有经济学家,也没有政府官员,提出过“泡沫警告”,所以,老百姓认为房子是一个好投资,就不是无理性的了。即使自1990年代以来,房价是直线上升,也是如此。

 

They were wrong. But mistakes and ignorance are not symptoms ofirrationality. They usually are the result of limited information. BenBernanke, in October 2005, just before the housing bubble began to leak air,denied that the rise in housing prices was a bubble. Was he irrational? Thatthe errors of experts can lead to disaster is hardly a novelty, but it does notfollow that only an irrational person would heed the advice of experts.

他们犯了错。但是,错误和疏忽,并不是“无理性”的表症。那常常是信息有限的结果。本·伯南克,在200510月,就在房地产泡沫开始漏气的时候,还否认房价上涨是一个泡沫呢。他是无理性的吗?因为专家错误,而导致灾难,并不是一件新鲜事,但是,我们却不能得出结论说:只有无理性的人,才会留意到专家的建议。

 

Akerlof and Shiller rightly associate booms with "newera" thinking, but wrongly deem such thinking irrational. Stocks soared inthe late 1920s because it was a period of rapid economic growth based onrapidly rising labor productivity and new products such as the massproducedautomobile, new methods of retailing such as the chain store, and new methodsof finance such as installment buying and the purchase of common stock onmargin. There was no reason to think that existing stock prices reflected anexaggerated expectation of increased wealth in so dynamic an era. The late1990s were likewise heralded as a new era, this time on the basis ofexpectations that the computer would transform the economy. The early 2000sseemed to most people still another new era, this one based on the seeminglymagical conjunction of low interest rates with low inflation, rising assetvalues, and new financial instruments that were believed to enable greaterlending and borrowing with less risk. In all three cases the new era turnedout, at least in the short run, to be a false dawn, and an asset-price crashensued. Given the uncertainty of the economic environment, stressed by Keynes,such disappointments are not surprising, and they do not show that investorsare irrational.

阿克洛夫和希勒,将繁荣与“新时代”思想,联系在一起,这是对的;但是,认为这个思想是无理性的,这又错了。股票在1920年代后期直冲云天,那是因为那个时代,劳动生产率迅速上升,像大生产的汽车这样的新产品的迅速增加,像连锁店这样的零售新技术迅速涌现,像分期付款购买和限界购股的金融新技术迅速产生,这才造成了经济的快速增长。人们没有理由认为:在那一个动力十足的时代,当时的股票,反映了人们对财富增加具有夸大的期望。在1990年代后期,似乎也预示着一个新时代,那是一个基于“计算机会改变经济”这个预期的新时代。21时代初,对于大多数人来说,似乎是又一个新时代,这个新时代的魔幻是以下因素的并存:低利率、低通胀、资产价值上升、新的金融工具(人们相信能以较低的风险进行更大的借贷)。在这三种情形下,“新时代”,最少在短期内,结果都变成了一个误判了的曙光,紧接着的都是资产价格的崩溃。正如凯恩斯所强调的,因为经济环境存在不确定性,这样的失望,并不令人惊奇,而且,也不说明投资者是无理性的。


(待续)

 

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