The writer is editor-in-chief of MoneyWeek

For those confused by the market this year, I have a suggestion. Invest in the Practical History of Financial Markets course run by the Edinburgh Business School (you can do it online — no need to come to chilly Scotland). One of the modules focuses on the history of extreme market valuations — what causes them and what crashes them.

The first thing to note is that, while we love to talk about bubbles, periods of extreme valuation in the stock market don’t really happen very often. Of the 29 business cycles in the US since 1881 only a few have ended in one, according to Professor Russell Napier. But, while each has had its own peculiarities, the basic driver has been much the same: the ability of investors to believe absolutely in something that always turns out to be impossible. Namely that, thanks to some “marvels” of technology, corporate profits will stay high (and probably rise) indefinitely and that interest rates will also stay low indefinitely.