我们挨个来看：
第一篇关于讯息不对称的大名文章是G. Akerlof一九七零年发表的《柠檬市场》（The Market for Lemons）。
写的是什么呢？原文是这样说的：
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. 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. By nature, however, these institutions are nonatomistic, and therefore concentrations of power - with ill consequences of their own - can develop.
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.
二手车的例子讲的什么呢？
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......
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.
这里有个很强的假设：好的二手车和坏的二手车卖同样的价格 —— 先记住，后面会用到。
Akerlof推导的最终结果是好的二手车逐渐退出 —— 极端一点，如张五常老先生所说：
原则上可以推到旧车市场不存在。
那为啥现实世界中，二手车市场还存在呢？
观察所见，旧车市场举世皆是。
老先生认为：
没有遭淘汰是因为旧车的市值够高。
因为提供旧车的可靠讯息有利可图，专家会出现：旧车的代理商在检查车况后，稍作修理会提供担保，而懂车的修理专才会收费提供意见。
这是核心原因了 —— 因为价格够高，从而能让相对应的人工检查成本，这体现为讯息费用的一部分，能被价格吸收掉。而这背后的另外一层呢？是单位人工成本有基线，低价格的二手货，连基线成本也无法覆盖，拿也就没有成为市场的可能。
所以作者再次强调：
鉴证旧物的专家的出现要讲旧物值钱。
否则：
讯息费用在比重上高，没有专家鉴证，所以没有市场。
对应地，另一个极端是：
称得上是收藏品的旧物，例如旧书画、古瓷器之类，因为成为财富累积的仓库，很值钱，讯息费用奇高也有专家鉴证。
所以，作者再次强调自己的方法论：
我会选择以物为本，从而直接地带到专家与市场应对这些方面去。
而且：
有趣的是，以人为本和以物为本的分别，在逻辑上竟然推出相反的结论。
前面说过了，以人为本，是好的二手车逐渐退出；而后者：
因为劣旧车的讯息费用在市价的比重上较高，会先遭淘汰。
理论应该与现实符合 —— 从这个角度来说，以物为本更合理。
主线讲完了，再说一个分支线，老先生称：
奇怪没人指出，Akerlof的柠檬市场分析其实是故老相传的英国十六世纪货币观中的葛氏定律（Gresham's Law）的新版本。。。
然而并非如此，Akerlof自己写道：
Gresham's law has made a modified reappearance.
怎么说呢？
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 car; similarly, bad money drives out good because the exchange rate is even. 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.
是的，前面提过了，一个极强的假设是不管什么二手车，价格相同 —— 数学上就是兑换率相同，那么就回到了葛氏定律中的条件了：
劣币与良币之间有一个固定的兑换率，否则兑换率的变动会让优、劣二币共存。
然而，如果二手车市场不能实现劣币驱逐良币，那么货币市场应该也不能才对：
因为在市场购物可以讨价还价，提供劣币的要付固定兑换率之外较高之价，而提供良币的则可大手压价。
因而结果也是反过来的：
良币驱逐劣币人类历史屡见不鲜。
作者接着举例：
一九四八年在广州，市场的取向是收港币，不收当年贬值得快的金圆券。
中国改革开放初期，名牌宾馆指明收外汇券，不收人民币。
咳咳，后面这个，不多说了。另外，哈耶克在《货币的非国家化》中也和张五常先生得到了同样的结论，不赘述了。
然而，如果我们再想一想，为什么人们对于劣币驱逐良币这个比喻，这么执迷不悟呢？
还是现实中，有很多类似的现象了：
- 比如职场中的各种乱象
- 但这背后，一定是满足了“固定的兑换率”的假设
- 也就是说，所得不由绩效决定，而这
- 或者是由于管理者无从分辨，有员工擅长表演
- 或者是管理者喜欢吹捧、被迎合了而已
这里有些可以说是讯息费用，但更大的概率是管理者的不称职罢了。
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