For years, central bankers in the developed world have been trying to battle poor economic growth and weak demand with ultra-low, and even negative, interest rates. It’s not working. More than 17 years after Japan cut interest rates to zero, and eight years after the financial crisis prompted central banks in the US and Europe to slash their policy rates to record lows, growth has stubbornly refused to recover to pre-crisis levels and deflation remains an ever-present threat. The response among central bankers has been to cut further, defying all historical precedent to push interest rates into negative territory.