The Markets for “Lemons”:
Quality uncertainty and The Market Mechanism
Geogre A。Akerlof 阿克洛夫
一、引言
This paper relates quality and uncertainty. The existence of goods of many grades poses interesting and important problems for the theory of markets。（本文论述的是质量和不确定性问题。现实中存在大量多种档次的物品给市场理论提出了饶有趣味而十分重大的难题）On the one hand, the interaction of quality differences and uncertainty may explain important institutions of the labor market.(一方面，质量差异和不确定性的相互作用可以解释劳动力的重要机制）On the other hand，this paper presents a struggling attempt to give structure to the statement: ”Business in under-developed countries is difficult”; in particular, a structure is given for determining the economic costs of dishonesty。(另一方面，本文试图通过讨论获得这样的结论：在不发达国家，商业交易是艰难的，其中，特殊论及了欺骗性交易的经济成本）Additional applications of the theory include comments on the structure of money ma
rkets，，and on brand—name goods。（本文的理论还可以用来研究货币市场、保险可行性、耐用品的流动性和名牌商品等问题）
There are many markets in which buyers use some market statistic to judge the quality of prospective purchases.（在许多市场中，买者利用市场的统计数据来判断他们将要购买的商品的质量）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。（在这种情况下，卖者有动力提供低质量商品，因为某种商品的价格主要取决于所有同类商品质量的统计数据而非该商品的实际质量。结果，商品的平均质量将趋于下降，市场规模将不断缩小）It should also be perceived that in these markets social and private returns differ，and therefore, in some cases，governmental intervention may increase the welfare of all parties。Or private institutions may arise to take advantage of the potential increases in welfare which can accrue to all parties.（我们还可以观察到在这种市场上，对个人和社会有不同的回报，因此，某种情况下，政府的干预可以增进社会的总体福利水平。或者说，私人组织利用了整个社会福利水平潜在的增长机会而使自己获利）By nature，however，these institutions are nonatomistic，and therefore concentrations of power- with ill consequences of their own-can develop.(实际上，这些私人组织的行
动所产生的影响并非可以忽稍不计，因此，尽管集权本身有许多负面影响，但是，一定程度上的集权可以保证经济的健康发展）
The automobile market is used as a finger exercise to illustrate and develop these thoughts。It should be emphasized that this market is chosen for its concreteness and ease in understanding rather than for its importance or realism.（下面，我们用汽车市
场作为例子来阐释和进一步发展以上的思想.需要指出的是，之所以选择旧车市
场来讨论，是因为这个例子很具体，并且容易理解，而不是因为它的重要性和有何现实意义）
二、以汽车市场模型为例
(一)汽车市场
The example of used cars captures the essence of the problem。From time to time one hears either mention of or surprise at the large price difference between new cars and those which have just left the showroom. The usual lunch table justification
。（旧车市场的例子可以抓住问题的本质。人们不止一次的听说或者惊讶于新车与刚刚开出样品陈列
室的汽车之间的价格存在巨大差别。对于这种情况最普遍的解释是人们有一种对“新”车的特殊偏好)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 （）. A new car may be a good car or a lemon, and of course the same is true of used cars。（假设(作出这种假设只是为了简化分析，而非从实际出发)用四种汽车：新车和旧车；高质量的车和低质量的车（低质量的车在美国被称为“柠檬”)。一辆新车可能是
高质量的，也可能是“柠檬固然一辆旧车也同样有两种情况）
The individuals in this market buy a new automobile without knowing whether the car they buy will be good or a lemon。But they do know that with probability q it is a good car and with probability （1-q) it is a lemon；by assumption，q is the proportion of good cars produced and （1 — q）is the proportion of lemons。（在这样的市场上，消费者买新车时并不知道车辆到底是高质量的还是“柠檬”，但是
他知道这辆车是高质量的概率是q，是“柠檬”的概率是“1—q”。这里假设q
是高质量车占所有汽车的比率，1-q是“柠檬的比率）
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; i.e。, the owner assigns a new probability to the event that his car is a lemon。This estimate is more accurate than the original estimate.(车主在拥有汽车后一段时间内就会了解到该车的质量，也就是说，此时车主可以赋予该车可能是“柠檬”的一个概率值，这个估计比初始的估
计更加准确）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。(于是，信息不对称发生了：卖主掌握了比买主更多的关于汽车质量的信息。对于买主来说，由于他不能在买车时区分汽车的质量，所以，高质量车和低质量车只能以同一个价格水平出售）It is apparent that a used car can—not have the same valuation as a new car — if it did
have the same valuation，it would clearly be advantageous to trade a lemon at the price of new car，and buy another new car, at a higher prob-ability q of being good and a lower probability of being bad.（显然，一辆新车和一辆旧车不应该有相同的评价.如果他们拥有相同的评价，车主通过以高质量车的价格出售“柠檬”后再
买一辆新车，该新车是高质量的概率q大于原先购买新车的概率）Thus the owner
of a good machine must be locked in。Not only is it true that he cannot receive the true value of his car，but he cannot even obtain the expected value of a new car。(这样，高质量车的拥有者将不会卖出汽车，因为如果卖出汽车他不仅无法得到汽车
的真实价值，也无法获得一辆新车的期望价值）
Gresham's law has made a modified reappearance。For most cars traded will be the ”lemons，
tend to drive out the good （in much the same way that bad money drives out the good）.（这里浮现了一个修正后的格莱欣定律.当“柠檬充斥汽车市场时，高质
量车根本不会成交，于是，如同劣币驱赶良币一样，低质量车将高质量车逐出市场）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；similarly, bad money drives out good because the exchange rate is even。(然而，该结果与格莱欣定律并不彻底一样：低质量车驱赶高质量车是因为低质量车和高质量车在相同的价格水平上出售.相似地，劣币驱赶良币是因为两者的交换利率相同）But the bad cars sell at the same price as good cars since it is impossible for a buyer to tell the difference between a good and a bad car; 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。
（但是低质量车之所以与高质量以同一个价格出售是因为买主无法区分一辆车到底是低质量的还是高质量的，这只有卖主才知道的信息。在格莱欣定律中，买主和卖主大概否可以区分劣币和良币。所以说，这个类比具有启示性，但非彻底相同）
(二）非对称信息
It has been seen that the good cars may be driven out of the market by the lemons。But in a more continuous case with different grades of goods, even worse pathologies can exist。（我们已经看到质量好的汽车会被“柠檬”逐出市场.但是，在不同质量等级的产品连续分布的情形下，更糟糕的情况也会发生）For it is quite possible to have the bad driving out the not-so—bad driving out the medium driving out the not—so-good driving out the good in such a sequence of events that no market exists at all.（质量最差的汽车在将质量最好的汽车逐出市
场后，会继续将质量较好、质量中等和质量稍差的汽车挨次逐出市场，从而导致二手车交易市场根本无法存在）
One can assume that the demand for used automobiles depends most strongly upon two variables — the price of the automobile p and the average quality of used cars traded，μ, or Q d = D(p, μ）。(假设对二手车的需求主要取决于两个变量-—二手车的价格p和进入交易的二手车的平均质量μ，即Q d
= D（p，μ)）Both the supply of used cars and also the average quality μ will depend upon the price, or μ=μ（p) and S=S（p)。（而二手车的供给和平均质量μ都取决于二手车的价格，即μ=μ（p) 和S=S(p)）And in equilibrium the supply must equal the demand for the given average quality，or S(p) = D （p, μ（p））。As the price falls, normally the quality will also fall。And it is quite possible that no goods will be traded at any price level.（在均衡状态下对于给定的平均质量，二手车的需求等于供给，即S（p) = D (p，μ（p)）.随着二手车交易价格下降，进入交易的二手车的平均质量固然也随之降低，最终导致在任何价格水平下都不存在二手车交易）
Such an example can be derived from utility theory。Assume that there are just two groups of traders: groups one and two. Give group one a utility function：(效用理论可以用开匡助我们解释二手车市场的例子。假定市场上有两类交易者，交易者1和交易者2，交易者1的效用函数为）
ᵄ1=M+∑ᵆ
ᵅ
ᵅ
ᵅ=1
Where M is the consumption of goods other than automobiles, ᵆ
ᵅis the quality
of the ᵅth automobile, and n is the number of automobiles.（其中，M代表交易者1对二手车之外其他商品的消费，ᵆ
ᵅ
表示第ᵅ辆汽车的质量，n为汽车的数量) Similarly，let（同理，交易者2的效用函数是）
ᵄ2=M+∑
3
2
ᵆ
ᵅ
ᵅ
ᵅ=1
Three comments should be made about these utility functions:（对于这些效用函数有三点需要说明)(1) without linear utility (say with logarithmic utility) one gets needlessly mired in algebraic complication（第一，这里采用线性效用函数（如对
数效用函数）来讨论问题，这样我们就无需处理复杂的代数运算）(2) The use of linear utility allows a focus on the effects of asymmetry of information；with a concave utility function we would have to deal jointly with the usual risk—variance effects of uncertainty and the special effects we wish to discuss here.（第二，现行效用函数还能让我们专注于对非对称信息的效果.如果在凹形效用函数下进行研究，我们将不得不在考察我们最希翼关注的事实的同时，考虑不确定性所带来的风险
变动的影响)(3) U
1 and U
2
have the odd characteristic that the addition of a second
汽车质量car, or indeed a kth car, adds the same amount of utility as the first.Again realism is
sacrificed to avoid a diversion from the proper focus。(第三,U
1和U
2
具有边际效用
不变的特征，即增加第二辆车的消费所带来的效用的增加都与增加第一辆车消费所带来的效用增加相等。在此，我们再一次为了研究的需要牺牲了边际效用变动的现实）
To continue, it is assumed (1）that both type one traders and type two traders are V on Neumann-Morgenstern maximizers of expected utility；（接下来假设：（1）两类消费者都是冯。诺依曼—摩根斯坦效用函数中的追求期望效用最大化者)(2) that group one has N cars with uniformly distributed quality ᵌ, 0≦ᵌ≦2，and groupt wo has no cars；((2)交易者1拥有N辆质量为ᵌ并且服从均匀分布的汽车，其中，交易者2没有汽车）(3）（（3）把其他商品M的价格看成为一个单位）
Denote the income （including that derived from the sale of automobiles）of all
type one traders as Y
1 and the income of all type two traders as Y
2
。(把各种类型的交
易者1的收入(包括销售二手汽车所带来的收入）记作Y
1，将交易者2的收入记
为Y
2）The demand for used cars will be the sum of the demands by both groups。
When one ignores indivisibilities, the demand for automobiles by type one traders will be(对二手车的需求即是两类交易者对二手车需求的总和。当我们忽稍不可整除的可能时，交易者1对二手车需求的数量可以表示为)
ᵃ1=ᵄ
1
ᵅ
⁄ ᵰᵅ
⁄>1
ᵃ1=0 ᵰᵅ
⁄<1
And the supply of cars offered by type one traders is（交易者1对二手车的供给是）
ᵄ2=ᵅᵄ2
⁄ ᵅ≦2 （1）
With average quality（二手车的平均质量为）
μ=ᵅ2
⁄（2）
(To derive (1) and (2），the uniform distribution of automobile quality is used.)(在(1)和（2）两个表达式中，我们使用了二手车质量服从均匀分布这一假设)
Similarly the demand of type two traders is （同样，交易者2的需求为)
ᵃ2=ᵄ
2
ᵅ
⁄ 3ᵰ2
⁄>ᵅ
ᵃ2=0 3ᵰ2
⁄<ᵅ
And（供给是）
ᵄ2=0
Thus total demand D （p, u) is （因此，总需求D (p，u）是）
D(ᵅ,ᵆ)=(ᵄ
2+ᵄ
1
)ᵅ 如果ᵅ< ᵆ
⁄
D(ᵅ,ᵆ)=ᵄ
2
ᵅ 如果ᵆ<ᵅ<3ᵆ2
⁄⁄
D(ᵅ,ᵆ)=0 如果ᵅ>3ᵆ2
⁄
However, with price p，average quality is p/2 and therefore at no price will any trade take place at all: in spite of the fact that at any given price between 0 and 3 there are traders of type one who are willing to sell their automobiles at a price which traders of type two are willing to pay.（尽管有这样的事实，即在0到3之间的某个价格下有的交易者1愿意卖掉他们的车，而有的交易者2愿意在这一价格下购买。但无论如何，在价格为p，平均质量为p/2这种质量均匀分布的情形中，在任何价格水平下都根本不会发生二手车交易）
（三)对称信息
The foregoing is contrasted with the case of symmetric information。Suppose that the quality of all cars is uniformly distributed， 0≦ᵌ≦2. Then the demand curves and supply curves can be written as follows:（上面的分析将与对称信息下的情形形成对照.假设所有汽车的质量是均匀分布的，0≦ᵌ≦2。那末，供给曲线和需求曲线就可以写成如下形式：)
Supply(供给曲线是)
S（p)=N p>1
S（p)=0 p<1
And the demand curves are(需求曲线是）
D（p）=（Y1+Y2)/p p〈1
D（p）=（Y2/p) 1<p<3/2
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