What has been achieved by the Federal Reserve’s $80bn-a-month bond-buying spree, and similar programmes elsewhere? The usual answer is that a tide of cheap money has brought down yields on safe assets, forcing investors to take risks in search of higher returns. The reality is different. The circumstances that forced central bankers to resort to this policy are the very conditions under which it cannot work.

True, asset prices have risen. That is not surprising: by printing money, central banks have in effect been signalling their intention to keep interest rates low for a long time. But a bond-buying programme is an extremely costly way for central bankers to communicate with markets. No wonder Mark Carney, the Bank of England governor who last year introduced a policy of offering formal guidance on the future path of interest rates, is searching for signals less fraught with risk.

Since 2008, the Fed has been buying bonds from the public with newly created cash in excess of $3tn, tripling the value of the assets it holds. The balance sheets of the European Central Bank, Bank of Japan and Bank of England have increased by similar proportion – though the expansion occurred over a longer period in Japan and the ECB has preferred to issue long-term repurchase agreements to banks.