Well, economists are arrogant people. And because they can’t explain something, it becomes irrational. The way I look at it, there were two crashes in the last century. One turned out to be too small. The ’29 crash was too small; the market went down subsequently. The ’87 crash turned out to be too big; the market went up afterwards. So you have two cases: One was an underreaction; the other was an overreaction. That’s exactly what you’d expect if the market’s efficient.
The word “bubble” drives me nuts. For example, people say “the Internet bubble.” Well, if you go back to that time, most people were saying the Internet was going to revolutionize business, so companies that had a leg up on the Internet were going to become very successful.
I did a calculation. Microsoft was an example of a corporation that came from the previous revolution, the computer revolution. It was hugely profitable and successful. How many Microsofts would it have taken to justify the whole set of Internet valuations? I think I estimated it to be something like 1.4.
Here it is, which is interesting throughout, hat tip to Mark Thoma. I do think Fama is skippiing a bit too quickly from "the efficient markets hypothesis is hard to test," to "there is a presumption in favor of efficient markets."