THE “biggest bubble in human history comes down crashing,” tweeted Nouriel Roubini, an economist, gleefully. After an exhilarating ride skywards in 2017, investors in crypto-currencies have been rudely reminded that prices can plunge earthwards, too. In mid-December the price of bitcoin was just shy of $20,000; by February 6th, it had fallen to $6,000, before recovering a little (see chart).

And bitcoin is not the only digital currency to have fallen. Figures from CoinMarketCap, a website, show that the total market capitalisation of crypto-currencies has fallen by more than half this year, to under $400bn. This slide has taken place amid a flurry of hacks, fraud allegations and a growing regulatory backlash.

Perhaps the most damaging allegations surround Tether, a company that issues a virtual currency of the same name. Tether allows users to move money across exchanges and crypto-currencies without converting it back into “fiat” (central-bank-backed) money first. In theory, each...Continue reading