Philosopher of Science On The Efficient Market Hypothesis
The efficient markets thesis is that the market makes complete use of all relevant information, and the “proof” is roughly that in a perfectly competitive market among perfectly rational agents prices invariably and instantaneously reflects all agents’ real beliefs and real desires. Any one who knows anything that can make him or her money acts on it—buys or sells—and that signal is picked up by every one else, who also acts on it, thus preventing any one from making excess profits—rents–long-term.
The first thing a philosopher notes about this notion is that since most people have false beliefs, especially about the future, an efficient market doesn’t internalize knowledge, but only beliefs. If they are mostly false, then the market isn’t efficient at internalizing (correct) information, it’s efficient at internalizing mostly false beliefs. If false beliefs are normally distributed around the truth, then they’ll cancel out and the proof of a probabilistic version of the efficient markets theorem will go through—market prices reflect the truth most of the time. Too bad false beliefs don’t always take on this tractable distribution. Even worse, when enough people notice the skewed distribution of false beliefs, they can make rents, as the markets crash. This is what Cochrane seems to think can’t happen. How many times will it have to happen for the Chicago School to give up the efficient markets hypothesis?
There are so many way the assumptions of the efficient markets theorem can go wrong—different ones at different times, often even cancelling one another out, that it’s easy for a complacent economist to see in the long term trend a vindication of the efficient markets theorem. And all Chicago economists have been taught to be complacent with their mother’s milk—Milton Friedman’s famous insistence that the falsity of assumptions doesn’t matter.