"More than six million workers are worried their jobs could be replaced by machines over the next decade" says the Guardian. This raises a longstanding paradox - that, especially in the UK, the robot economy is much more discussed than observed.

What I mean is that the last few years have seen pretty much the exact opposite of this. Employment has grown nicely whilst capital spending has been sluggish. The ONS says that "annual growth in gross fixed capital formation has been slowing consistently since 2014." And the OECD reports that the UK has one of the lowest usages of industrial robots in the western world.

My chart, taken from the Bank of England and ONS, puts this into historic context. It shows that the gap between the growth of the non-dwellings capital stock and employment growth has been lower in recent years than at any time since 1945. The time to worry about machines taking people's jobs was the 60s and 70s, not today.

Of course, we shouldn't set much store by the precise numbers here: measuring the capital stock is a fool's errand. But these data are consistent with other facts. Households are saving less than they used to, which is not what you'd expect if they feared losing their jobs. Companies are still building up cash quickly and borrowing little, and of course real interest rates are low. All this is consistent with low capital growth.

If we looked only at the macro data, we'd fear that people are taking robots' jobs - not vice versa.

So why is investment so weak (a fact which long pre-dated Brexit uncertainty? There are thousands of firms that aren't investing in new tech, and therefore thousands of potential explanations. Among them are:

- There is, as Bloom and Van Reenen say, "a long tail of extremely badly managed firms (pdf)" which lack the confidence or competence to invest in new technology.

- The 2008 recession had a scarring effect upon animal spirits; it raised the fear of future recessions and so deterred investment.

- Fiscal austerity depressed aggregate demand and hence the motive to invest. And in squeezing real wages, it reduced companies' incentive to invest in labour-saving technology: this was one reason why investment boomed in the 60s.

- Talk of a robot age might be self-defeating, as it increases fears of future competition; why spend £10m on robots if a rival will undercut you by spending £5m on better ones in a few months' time? Maybe firms have wised up to Nordhaus' finding - that innovation doesn't pay very well except for a tiny handful of firms. (Hendrik Bessembinder has estimated that 4% of firms account for all of the rise in the US stock market since 1926).

Whatever the reason for low investment, we have a genuine paradox here - that whilst there's much talk of a robot economy there is little evidence of it in the data or on the ground. There might, therefore, be a mismatch between the vast productive potential that technology might offer us on the one hand, and the poor performance of today's capitalism on the other.

Marx famously wrote:

At a certain stage of development, the material productive forces of society come into conflict with the existing relations of production...From forms of development of the productive forces these relations turn into their fetters.

One of the neglected questions of our time is: could it be that we have now reached this stage?