Suppose that there are just four kinds of cars. There are new cars and used cars. There are good cars and bad cars (“lemons”). A new car may be a good car or a lemon, and of course the same is true of used cars.
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.
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. 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. It is apparent that a used car cannot 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 probability q of being good and a lower probability of being bad. 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.
The Market for Lemons: Quality Uncertainty and the Market Mechanism
by George A. Akerlof
The Quarterly Journal of Economics, Vol. 84, No. 3. (Aug., 1970)
http://www.iei.liu.se/nek/730g83/artiklar/1.328833/AkerlofMarketforLemons.pdf
II. The Model with Automobiles as an Example
A. The Automobiles Market
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 for this phenomenon is the pure joy of owning a “new” car. 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.
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.
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. 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. It is apparent that a used car cannot 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 probability q of being good and a lower probability of being bad. 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.
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; 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.