一年前，大多数分析师都同意：美元兑欧元与其它主要货币只会下跌。美国已无法支撑其巨额经常账户赤字，美元需要大幅贬值。一年后，事实证明这些分析师都错了。美元兑欧元上升了近15%。出了什么问题？我说一个有关比利时巧克力的故事来回答这个问题。
几周前，布鲁塞尔食品展上进行了一个有趣的实验。在这个一年一度的食品展上，摊位里摆满了各种你能想到的美食，而美食爱好者则在许多摊位之间来回穿梭。其中一间搭好的摊位在销售盒装比利时巧克力。第一天，巧克力的售价定在每盒9欧元，生意不错。第二天，价格上涨到了每盒15欧元。假如你饱读经济理论，可能会认为需求现在会下降了。错。需求翻番了。第3天，价格降至每盒2欧元，对巧克力的需求一落千丈。需求法则出了什么问题？
心理学家对此给出了解释。消费者即便有可能，也很难在店里仅凭巧克力的外表来判断它们的质量。当消费者对事物的内在价值如此不确定时，就会用他们了解的、简单的经验法则。心理学家称之为“探索法”。在上面的例子里，巧克力的价格就是经验法则。
大多消费者都有某些经验，会让他们把高价格与高品质联系起来。事情并非总是如此，但一般而言可能如此。因此，当看到每盒售价15欧元的巧克力时，消费者推断这个高价反映了高质量，然后就买下了巧克力。当看到每盒标价2欧元的巧克力时，消费者推断这些巧克力的质量不能让人放心，所以他们没有去买。需求法则完全颠倒了。
现在让我们来看看美元。大部分在外汇市场进行交易的人，对美元的基本价值（美元的“质量”）没什么头绪。专家和教授们也不知道，至少在美元兑欧元的大区间里是如此，比如在1.0美元到1.3美元之间。每当有专家告诉我们，美元对欧元的基本价值接近1.3美元，总会有另一个专家断言，基本价值接近1.0美元。而两者间的任何数字都会有人选择。面对这样的不确定性，美元与欧元的交易商都无法得知确切情况，因此，他们就会使用他们所了解的简单指导——经验法则。美元的市场价格就是这样的经验法则。当美元出于某个技术原因上涨(或偶然上涨)时，不断上涨的美元就是给人们的信号，说明必然有某种隐藏的经济力量在推动美元。所以他们买进美元，就像比利时巧克力的爱好者在高价时买巧克力一样。相反，当美元出于这样那样的原因走低时，人们把美元的下跌解读为反映了美元的基本弱势，因此他们抛售美元。
作为这一机制的结果，美元在上下限越来越大的区间里浮动，这是我们对美元的基本价值缺乏了解所造成的。这些浮动可能是由影响美元基本价值的变量（比如经常账户）变化造成的，也可能不是。
但比利时巧克力和美元之间也有不同之处。高价购买巧克力的比利时巧克力爱好者，可以马上尝一口巧克力以检验品质。而美元买家却不行，但他需要证明，他买得很好，于是分析师就派上用场了。
对于美元的基本价值，分析师并不比毫无戒心的买家了解得更多，所以他们就编故事。因此，当美元上涨时，分析师就去寻找走向正确的变量，以及可以和美元上涨联系在一起的变量，并谨慎地从分析中去掉所有走向错误的其它基本变量。所以我们被告知，去年美元的强势是由于利差有利于美元。在上一次的分析中，进一步扩大的经常账户赤字是关注的中心，而在新的分析中却被小心翼翼地剔除掉了。
我们的确不知道去年美元为何上涨的真相，但我们就是不想承认我们不知道。我们打心眼里痛恨茫然无知。那就是分析师的服务不断有需求的原因。他们满足了人们了解真相的心理需要。如今，每当美元上涨或下跌，汇率经济学就会编一个新故事以满足这种需求。
关于新的一年，它给我们的启示是什么？你现在可能推断说，这个专栏的怀疑论调，没给我留下多少空间来说些有用的东西。然而我确实认为，美国日益增长的经常账户赤字和债务的累积力量将是无法抵抗的，它会拖低美元，但别问我是什么时候。
原文：The Belgian chocolate theory of the dollar
A year ago, most analysts agreed: the dollar could only go down against the euro and the other main currencies. The huge US current account deficits had become unsustainable and called for a big decline of the dollar. A year later these analysts proved to be wrong. The dollar went up by close to 15 per cent against the euro. What went wrong? Let me answer the question by telling a story about Belgian chocolates.
A few weeks ago an interesting experiment was undertaken at the Brussels food fair, a yearly affair where food lovers wander around among the many stalls stuffed with all imaginable delicacies. A shop was put up selling boxes of Belgian chocolates. The first day the price was set at EUR9 for each box. Sales went well. The next day the price was raised to EUR15 per box. Steeped in economic theory, you might think that demand now declined. Wrong. Demand doubled. On the third day the price was lowered to EUR2 for each box. Demand for chocolates collapsed. What went wrong with the law of demand?
The explanation is given by psychologists. It is very difficult, if not impossible, for the consumer to find out the quality of chocolates by just looking at their appearance in the shop. When confronted with such uncertainty about the intrinsic value of things, consumers use simple rules of thumb that they understand. Psychologists call these “heuristics”. In this case, the price of the chocolates provides the rule of thumb.
Most consumers have some experience that allows them to associate high price with high quality. It is not always like that, but on average it probably is. Thus when looking at the EUR15 box the consumers infer that the high price reflects high quality and they buy the chocolates. Consumers who see the boxes priced at EUR2 infer that the quality of these chocolates is not to be trusted, and they do not buy them. The law of demand is turned upside down.
Let us now return to the dollar. Most people dealing in the foreign exchange market have no clue about the fundamental value of the dollar (the "quality" of the dollar). Specialists and professors do not know either, at least within a broad range of dollar-euro rates between say, $1.0 and $1.3. For every specialist telling us that the fundamental value of the dollar is close to $1.3 to the euro there will be another one affirming that $1.0 is near the fundamental value. Anything in between is fair game. Faced with such uncertainty, traders in dollars and euros have no way of knowing. They will therefore use a rule of thumb, an easy guide that they understand. The market price of the dollar provides such a rule of thumb. Thus when the dollar goes up for some technical reason, or just by chance, the increasing dollar is a signal for people that there must be some hidden economic strength driving the dollar. And they buy dollars; like the Belgian chocolate lovers buy chocolates when they are highly priced. Conversely, when the dollar moves down for whatever reason, people interpret the decline of the dollar as reflecting a fundamental weakness of the dollar, and they sell dollars.
As a result of this mechanism, the dollar moves up and down within upper and lower bounds that are determined by our lack of knowledge of the fundamental value of the currency. These movements may or may not be caused by changes in the variables (such as the current account) that influence the fundamental value of the dollar.
There is a difference, though, between Belgian chocolates and the dollar. The Belgian chocolate lover buying high-priced chocolates can immediately check the quality by tasting the chocolates. The buyer of dollars cannot. But he is in need of a justification of his buy as a good one. And here the analyst comes in handy.
The analyst, who does not know more about the fundamental value of the dollar than the unsuspecting buyer, invents stories. Thus when the dollar goes up, the analyst goes on a search for variables that move in the right direction and that can be linked to the rising dollar, carefully eliminating from the analysis all the other fundamental variables that move in the wrong direction. And so we are told that the strength of the dollar last year was due to interest rate differentials favouring the dollar. The further widening of the current account deficit, which in a previous analysis got centre stage, is carefully dropped from the new analysis.
The honest story of why the dollar increased last year is that we simply do not know. But we do not like to admit that we do not know. Our psyche abhors the darkness of ignorance. That is why analysts' services continue to be demanded. They fulfil a psychological need to understand. Exchange rate economics these days satisfies this need by telling a new story each time the dollar goes up or down.
What does that tell us about the coming year? You may now be concluding that the sceptical tone of this column does not leave me much scope to say something useful. Yet I do think that the cumulated force of increasing US current account deficits and debts will be overwhelming, bringing down the dollar. But do not ask me when this will happen.
(The writer is professor of economics at the University of Leuven)