Think Nvidia looks dear? American shares could get pricier still

How can you tell it is time to get out of the market? In 1929 Joseph Kennedy, an American businessman and politician, supposedly realised the party was over upon hearing a shoeshine boy dispensing stock tips. In 2000 the exit doors beckoned after 17 “dotcom” firms paid millions of dollars each for brief advertising slots during the Super Bowl, an American-football extravaganza.

This article appeared in the Finance & economics section of the print edition under the headline “Buckle up, buttercup”