经济学人:信息不对称之柠檬市场理论与效应（上）Economics brief : six big ideas . 经济概要：六大经济思想。
Information asymmetry: Secrets and agents 信息不对称：秘密和代理George Akerlof's 1970 paper,“ The Market forLemons ” ,is a foundation stone of informationeconomics, thefirst in our series on seminal economic ideas.六大经济思想之一，乔治·阿克洛夫 1970 年的论文——《柠檬市场》，它是信息经济学的一块基石。
In 2007 the state of Washington introduced a new rule aimed at making the labour marketfairer: firms were banned from checking job applicants' credit s cores.2007 年，华盛顿州引入了一项旨在让劳动力市场更公平的新法律：企业被禁止核查求职者的信用得分。
Campaigners celebrated the new law as a step towards equality — an applicant with a low credit score is much more likely to be poor, black or young.发起这项运动的人欢呼庆祝，认为这是通向平等的一步——信用得分低的求职者极有可能是穷人、黑人或者年轻人。
Since then, ten other states have followed suit. 自那以后，又有十个州如法炮制。
But when Robert Clifford and Daniel Shoag, two economists, recently studied the bans, theyfound that the laws left blacks and the young with fewer jobs, n ot more.但是，日前，当两位经济学家——罗伯特·克利福德和丹尼尔·绍格——研究了这些禁令时，他们发现，这些法律带给黑人和年轻人的工作更少了，而不是更多。
Before 1970, economists would not have found much in their discipline to help them mull this puzzle .放在 1970 年之前，经济学家是不会在这门学科中找到太多的东西来帮助他们仔细思考这个迷的。
Indeed, they did not think very hard about the role of information at all. 实际上，他们当时根本就没有很努力地去思考信息的作用。
In the labour market, for example, the textbooks mostly assumed that employers know the productivity of their workers — or potential workers — and, th anks to competition , pay themfor exactly the value of what they produce .例如，那时的教科书大都假设，在劳动力市场中，雇主知道他们的工人——的生或者潜在的工人产率，而且，由于竞争，是按照工人所生产的东西的价值给他们开工资的。
You might think that research upending that conclusion would immediately be celebrated asan important breakthrough .人们可能认为，颠覆这一论断的研究立即会被誉为一项重大突破。
Yet when, in the late 1960s, George Akerlof wrote “ The Market for Lemons ”which did just that,and later won its author a Nobel prize, the paper was rejec ted by three leading journals.然而，当乔治·阿克洛夫于上世纪 60 年代末期写出恰恰实现了这种突破并为作者赢得了诺贝尔奖的《柠檬市场》时，这篇论文曾遭到三家主要期刊拒绝。
At the time, Mr Akerlof was an assistant professor at the University of Californ ia, Berkeley; hehad only completed his PhD, at MIT, in 1966. 那时，阿克洛夫是加州大学伯克利分校的一名助教；仅于 1966 年在麻省理工完成了博士学位。
Perhaps as a result, the American Economic Review thought his paper's insigh ts trivial .也许是因为这个原因，《美国经济评论》认为他的论文的观点微不足道。
The Review of Economic Studies agreed. 《经济研究评论》同意了这个看法。
The Journal of Political Economy had almost the opposite concern: it could not stomach thepaper's implications.《政治经济学杂志》有着几乎完全相反的担忧：它可能消化不了这篇论文深远意义。
Mr Akerlof, now an emeritus professor at Berkeley and married to Janet Yellen, the chairman ofthe Federal Reserve, recalls the editor's complaint:“ I fthis is correct, economics would bedifferent. ”如今已是伯克利分校名誉教授并同美联储主席珍妮特·耶伦成婚的阿克洛夫还记着那位编辑的不满：“如果这是正确的，经济学会截然不同。
”In a way, the editors were all right. 从某种程度上来说，这些编辑当时都是对的。
Mr Akerlof's idea, eventually published in the Quarterly Journal of Economics in 1970, was atonce simple and revolutionary .最终于 1970 年在《经济学季刊》上发表了的阿克洛夫的思想即是简单的，也是革命性的。
Suppose buyers in the used-car market value good cars —“ peaches ”—at $1,000, and sellers at slightly less.假设，二手车市场中的买方给好车——“桃子” ——估价 1000 美元，卖方要稍微少一点。
A malfunctioning used car —a “ lemon ”—is worth only $500 to buyers (and, again, slightly lessto sellers) .一辆瑕疵二手车——“柠檬”，对买方而言，只值 500 美元 (而且对卖方来说，又要稍微少一点 )。
If buyers can tell lemons and peaches apart, trade in both will flourish .如果买方能够区分柠檬和桃子，两者的交易都会红火起来。
In reality, buyers might struggle to tell the difference: scratches can be touched up, engineproblems left undisclosed , even odometers tampered with. 在现实中，买方可能会设法找出不同：刮痕可能被补上了，发动机问题可能未得到披露，甚至里程表都可能被篡改了。
To account for the risk that a car is a lemon, buyers cut their offers. 为了对车是柠檬的风险负责，买方大肆杀价。
They might be willing to pay, say,$750 for a car they perceive as having an even chance ofbeing a lemon or apeach .比如说，他们可能愿意为一辆他们认为有着或是柠檬或是桃子的同等几率的车支付 750 美元。
But dealers who know for sure they have a peach will reject such an offer.但是，确切地知道自己的车是桃子的交易者将拒绝这样的出价。
As a result, the buyers face “adve rse selection ” th e: only sellers who will be prepared to accept$750 will be those who know they are offloading a lemon.结果，买方面临“逆向选择” : 唯一准备接受 750 美元的卖家将是知道自己正在脱手柠檬的人。
Smart buyers can foresee this problem. 聪明的买方能预见到这个问题。
Knowing they will only ever be sold a lemon, they offer only $500. 由于知道自己永远只会被卖给柠檬，他们仅出价 500 美元。
Sellers of lemons end up with the same price as they would have done were t here no ambiguity .最后，柠檬的卖方得到的是与没有歧义时会得到的价格同样的价格。