Legend has it that Joe Kennedy, father of former president John F Kennedy, avoided the stock market crash of 1929 by selling his entire portfolio just days before prices collapsed. He did this, the story goes, after receiving a share tip from a shoeshine boy. When taxi drivers, lift attendants and young lads on the sidewalk are speculating in stocks, he concluded, “the market is too popular for its own good”.

The stock market is riding high today but I’d be amazed if your next cab ride results in a share recommendation. This unloved bull market has failed to generate much excitement. It’s quite unlike the explosion of interest in shares in the late Nineties when everyone knew someone who had made a killing in some obscure internet stock and fear of missing out was intense.

So, if Joe Kennedy were having his shoes polished today he would not be talking equities. What he might well be steered towards, however, is an investment in bitcoin, the cryptocurrency that is tracing the classic parabola of every bubble in history, from tulips to internet incubators. Two years ago, you could have bought a bitcoin for $300 (£230); today, it would set you back $7,000. Its price has doubled in six weeks. It’s not just the price of bitcoin that is increasing but the speed of the gains is accelerating. The chart is turning left and heading straight up the page as asset prices always do in the final stages of an irrational mania.