經濟學 1970

「次品車」市場：品質的不確定性與市場機制

喬治·阿克洛夫

賣家比買家更懂行時，次品會把好貨逐出市場。

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In depth · the introduction

為什麼一輛車，在你剛把它開出車行的那一刻，就立刻貶值好幾千——哪怕它什麼都沒變？

核心想法

有時候，賣家知道一些買家無從知曉的事——比方說，一輛二手車的真實狀況。如果買家分不清一輛好車和一輛暗藏毛病的車（美國人把次品車叫做「檸檬」），他們就只肯出一個不上不下的、平均的價錢。可這個價錢，對於手裡真有一輛好車的人來說簡直是侮辱，於是那些車主索性不把車拿出來賣。

如今，留著待售的就只剩下較差的車了——這意味著真實的平均品質又往下掉了一截，於是買家出價更低，這又把次一等的好車嚇退，如此循環。好的，就這樣被壞的逐出市場。在阿克洛夫最鋒利的那個例子裡，這個螺旋會一路下墜：市場可以萎縮到一無所有，儘管每一個買家本來都樂意為一輛好車付出公道的價錢。這裡的反派，不是貪婪，也不是壟斷——而是「誰知道什麼」之間的那道鴻溝。

它是如何誕生的

喬治·阿克洛夫寫下這篇短論文時，還是個年輕的經濟學家，拿到博士學位才幾年，時值二十世紀六十年代末。他選用二手車，並非因為它最重要，而是因為這個例子鮮活、好懂；真正瞄準的目標要大得多——為什麼窮國裡信貸如此難求、為什麼保險這麼棘手、為什麼有些市場幾乎不存在。

這個想法太簡單了，又與當時那套整整齊齊的供求模型大相逕庭，以至於好幾家期刊都退了稿——據說有一家說，倘若它當真成立，經濟學就得改寫了；另一家則說它太瑣碎。《經濟學季刊》終於在 1970 年把它刊了出來。三十年後，它幫阿克洛夫分得了一份諾貝爾獎。

它為何重要

兩百年來，經濟學大多假定買賣雙方都清楚自己在交易什麼。阿克洛夫卻揭示：當他們並不清楚——當資訊一邊倒時——哪怕是一個競爭充分、本意誠實的市場，也會失靈，讓所有人都變得更糟。這撬開了一整片嶄新的領域。它也終於解釋了那些我們早已建立、卻未必說得清緣由的日常制度：保固、品牌、專業執照，以及——為什麼一個陌生人的承諾，總不如一位熟客的承諾值錢。

一個可以想像的畫面

想像一個水果攤，每個蘋果都裹著一層錫箔。你看不出哪個脆甜、哪個早已碰傷，於是你只肯付一個「平均蘋果」的價錢。可任何手裡捧著一個完美蘋果的人，都不會按這個價錢賣——他把自己的蘋果帶回了家。於是攤子上漸漸堆滿了碰傷的蘋果，錫箔把一切都遮住了，平均品質越沉越低。很快，你連那個「平均價」都不願付了。把錫箔揭開——讓買家看清品質——攤子便又一下子活了過來。那層錫箔，就是資訊不對稱。

它的位置

亞當·斯密的看不見的手（也在本館中）說，出於自利的交易會悄然造福每一個人——前提是雙方都看得清自己換到了什麼。阿克洛夫在這幅圖景裡找到了一道裂縫：把品質藏起來，那隻手就可能踉蹌。他的論文，與麥可·斯彭斯的訊號理論、約瑟夫·史迪格里茲的甄別理論一道，構築起了資訊經濟學這三足之鼎，並分享了 2001 年的諾貝爾獎。每一次你在網上查看一個賣家的評分，你用的正是從這個想法裡生長出來的制度。

The original document

Original source text

一、導言

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、汽車市場

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、不誠實的代價

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；結論

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