Modern-day slumps in countries with stable political systems are supposed to last for a couple of years, before  policy makers step in to ease financial conditions and return countries to strong and steady expansion. But despite its spurt in mid-2014, we believe the U.S. economy shows little sign that it can go back to the 3 percent-plus average growth it enjoyed before the 2008 crash – without, at least, extremely low interest rates. The slow-flowing river of US economic performance has sparked talk that secular stagnation – a situation of permanently lower growth – is lurking somewhere in the depths.

That talk is probably right. We can no longer blame our problems on a series of unfortunate events that started with the bursting of its housing bubble. Its difficulties are much more profound.

Investors should also understand that the sinister beast of secular stagnation has two heads. The supply head of this hydra – the slow increase in the capacity of the economy to produce – is scary. But we believe it’s the demand head that we should really be nervous about – the possibility that low investment is hitting demand, and hence output and employment.