If you believe the official statistics, something very strange is happening in the US economy. Output growth is bouncing along at about 2 per cent – below almost every economist’s estimate of US potential growth – but unemployment is not going up. In fact unemployment ticked down in March to 4.4 per cent.

This is odd, since one of the oldest rules of economics, Okun’s law, implies that, when output grows by less than its potential growth rate, unemployment should go up (to be precise, it should rise relative to the natural unemployment rate).