I know, I work for one of them. I certainly would not claim markets are 100% efficient.

It's joe_the_user's argument that I'm disputing, not his conclusion. In general, when you make an argument that the EMH is false and certain securities are priced incorrectly (I'm guessing he is referring to houses or MBS), you should have a track record of getting rich based on trading those securities.

Does that mean everyone who does not have such a track record should believe in EMH? Or should they just not make arguments that do anything to contradict it? Or something else?

In any specific case, you should believe in the EMH. It's the safest assumption.

And yes, once you are convinced that markets are inefficient, you should not make any public arguments contradicting the EMH. You should STFU until after you have earned so much money that you don't care about earning any more [1].

[1] Obviously you can talk to potential investors/employees/etc, keeping in mind that any one of them might steal your strategy.

> Some investors clearly have made returns over the last fifty years at substantially above the market rate.

That doesn't disprove the EMH as seemingly consistent winners and losers are consequences of any random system.

Note that the EMH just says that the market price reflects all public information. It doesn't say that the market price is the correct value wrt every individual.

For example, I may be spending down, perhaps because I have a terminal disease while you're saving for a new car. Those situations mean that the "value" that I'll get from certain purchases is very different from the value that you get for the exact same purchases.