经济学 1970

「次品车」市场：质量的不确定性与市场机制

乔治·阿克洛夫

卖家比买家更懂行时，次品会把好货逐出市场。

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In depth · the introduction

为什么一辆车，在你刚把它开出车行的那一刻，就立刻贬值好几千——哪怕它什么都没变？

核心想法

有时候，卖家知道一些买家无从知晓的事——比方说，一辆二手车的真实状况。如果买家分不清一辆好车和一辆暗藏毛病的车（美国人把次品车叫做「柠檬」），他们就只肯出一个不上不下的、平均的价钱。可这个价钱，对于手里真有一辆好车的人来说简直是侮辱，于是那些车主索性不把车拿出来卖。

如今，留着待售的就只剩下较差的车了——这意味着真实的平均质量又往下掉了一截，于是买家出价更低，这又把次一等的好车吓退，如此循环。好的，就这样被坏的逐出市场。在阿克洛夫最锋利的那个例子里，这个螺旋会一路下坠：市场可以萎缩到一无所有，尽管每一个买家本来都乐意为一辆好车付出公道的价钱。这里的反派，不是贪婪，也不是垄断——而是「谁知道什么」之间的那道鸿沟。

它是如何诞生的

乔治·阿克洛夫写下这篇短论文时，还是个年轻的经济学家，拿到博士学位才几年，时值二十世纪六十年代末。他选用二手车，并非因为它最重要，而是因为这个例子鲜活、好懂；真正瞄准的目标要大得多——为什么穷国里信贷如此难求、为什么保险这么棘手、为什么有些市场几乎不存在。

这个想法太简单了，又与当时那套整整齐齐的供求模型大相径庭，以至于好几家期刊都退了稿——据说有一家说，倘若它当真成立，经济学就得改写了；另一家则说它太琐碎。《经济学季刊》终于在 1970 年把它刊了出来。三十年后，它帮阿克洛夫分得了一份诺贝尔奖。

它为何重要

两百年来，经济学大多假定买卖双方都清楚自己在交易什么。阿克洛夫却揭示：当他们并不清楚——当信息一边倒时——哪怕是一个竞争充分、本意诚实的市场，也会失灵，让所有人都变得更糟。这撬开了一整片崭新的领域。它也终于解释了那些我们早已建立、却未必说得清缘由的日常制度：保修、品牌、专业执照，以及——为什么一个陌生人的承诺，总不如一位熟客的承诺值钱。

一个可以想象的画面

想象一个水果摊，每个苹果都裹着一层锡箔。你看不出哪个脆甜、哪个早已碰伤，于是你只肯付一个「平均苹果」的价钱。可任何手里捧着一个完美苹果的人，都不会按这个价钱卖——他把自己的苹果带回了家。于是摊子上渐渐堆满了碰伤的苹果，锡箔把一切都遮住了，平均质量越沉越低。很快，你连那个「平均价」都不愿付了。把锡箔揭开——让买家看清质量——摊子便又一下子活了过来。那层锡箔，就是信息不对称。

它的位置

亚当·斯密的看不见的手（也在本馆中）说，出于自利的交易会悄然造福每一个人——前提是双方都看得清自己换到了什么。阿克洛夫在这幅图景里找到了一道裂缝：把质量藏起来，那只手就可能踉跄。他的论文，与迈克尔·斯彭斯的信号理论、约瑟夫·斯蒂格利茨的甄别理论一道，构筑起了信息经济学这三足之鼎，并分享了 2001 年的诺贝尔奖。每一次你在网上查看一个卖家的评分，你用的正是从这个想法里生长出来的制度。

The original document

Original source text

一、导言

George A. Akerlof · The Quarterly Journal of Economics 84(3) (1970): 488–500

This paper relates quality and uncertainty. The existence of goods of many grades poses interesting and important problems for the theory of markets. On the one hand, the interaction of quality differences and uncertainty may explain important institutions of the labor market.

There are many markets in which buyers use some market statistic to judge the quality of prospective purchases. In this case there is incentive for sellers to market poor quality merchandise, since the returns for good quality accrue mainly to the entire group whose statistic is affected rather than to the individual seller. As a result there tends to be a reduction in the average quality of goods and also in the size of the market.

二·甲、汽车市场

II. The Model with Automobiles as an Example · A. The Automobiles Market

The example of used cars captures the essence of the problem. … We offer a different explanation. Suppose (for the sake of clarity rather than reality) that there are just four kinds of cars. There are new cars and used cars. There are good cars and bad cars (which in America are known as “lemons”). A new car may be a good car or a lemon, and of course the same is true of used cars.

After owning a specific car, however, for a length of time, the car owner can form a good idea of the quality of this machine … An asymmetry in available information has developed: for the sellers now have more knowledge about the quality of a car than the buyers. But good cars and bad cars must still sell at the same price — since it is impossible for a buyer to tell the difference between a good car and a bad car.

Gresham's law has made a modified reappearance. For most cars traded will be the “lemons,” and good cars may not be traded at all. The “bad” cars tend to drive out the good (in much the same way that bad money drives out the good).

But the analogy with Gresham's law is not quite complete: bad cars drive out the good because they sell at the same price as good cars … only the seller knows. In Gresham's law, however, presumably both buyer and seller can tell the difference between good and bad money. So the analogy is instructive, but not complete.

三、不诚实的代价

III. Examples and Applications · The cost of dishonesty

The purchaser's problem, of course, is to identify quality. The presence of people in the market who are willing to offer inferior goods tends to drive the market out of existence — as in the case of our automobile “lemons.”

It is this possibility that represents the major costs of dishonesty — for dishonest dealings tend to drive honest dealings out of the market. … The cost of dishonesty, therefore, lies not only in the amount by which the purchaser is cheated; the cost also must include the loss incurred from driving legitimate business out of existence.

四至五、反制的制度；结论

IV. Counteracting Institutions · V. Conclusion

Numerous institutions arise to counteract the effects of quality uncertainty. One obvious institution is guarantees. … A second example … is the brand-name good. … Licensing practices also reduce quality uncertainty. … The high school diploma, the baccalaureate degree, the Ph.D., even the Nobel Prize, to some degree, serve this function of certification.

We have been discussing economic models in which “trust” is important. Informal unwritten guarantees are preconditions for trade and production. Where these guarantees are indefinite, business will suffer — as indicated by our generalized Gresham's law.

University of California, Berkeley · Indian Statistical Institute, New Delhi · 1970