Debate has been frenzied this week about how fast the US Federal Reserve plans to raise interest rates. But as investors look forward, it is also a good time to glance back and ask why rates have been so low this decade.

Conventional wisdom usually blames two factors: first, central banks such as the Fed have deliberately pushed down policy rates with startling quantitative easing experiments; second, rates have been depressed by the curse of “secular stagnation”, the phrase coined by Harvard economist Lawrence Summers.

More specifically, Mr Summers and others argue that the global economy is suffering from a stark structural decline in aggregate demand. Thus low (or negative) market rates are a consequence and a signal of collective investor forecasts about future economic gloom; or so this narrative goes.