John Maynard Keynes once famously responded to the charge that he had changed his views on monetary policy by saying “When the facts change, I change my mind. What do you do, sir?” Today’s central bankers respond to changing “facts” by changing their target variables.

Incoming Bank of England governor Mark Carney is the most important among big-name economists who have been lining up to show their support for yet another target-based approach to monetary policymaking: nominal gross domestic product, or NGDP, level targeting. The basic idea is that a central bank should aim to fix GDP growth, unadjusted for inflation, at around 4.5% as a means of stabilising aggregate demand and avoiding recessions. Since the financial crisis of 2008 its myriad new supporters have come overwhelmingly from the left of the political spectrum - most notably former Obama administration economist Christina Romer.