Warren Buffett said most of what you need to know about efficient markets. “Observing correctly that the market was frequently efficient, they [academics, investment professionals and corporate managers] went on to conclude incorrectly that it was always efficient. The difference between the propositions is night and day.”

The difference between these propositions is also the difference between a $50bn fortune and the returns of the average investor. Mr Buffett has made his money not from the part that is frequently efficient, but from the part that is infrequently inefficient.