There is, however, an important aspect of the story of the savings glut that often gets overlooked: if there is an excess supply of savings, what is the excess relative to?

The answer is: a demand for capital from companies. The flip side of the excess supply of savings is a deficiency of capital investment. As Federal Reserve governor Ben Bernanke said in a famous speech which popularised the notion of a savings glut, there has been a "dearth of domestic investment opportunities" in developed economies.

One sign of this dearth is that the share of capital spending in UK GDP has been trending downwards for years. Even before the recession began, business investment accounted for just 10 per cent of GDP, its lowest proportion since records began in the 1960s; it's now 8.5 per cent.

Granted, this has been in part a reflection of the fact that prices of capital goods - such as software - have fallen relative to prices generally. But it's unclear how relevant this is. It merely raises the question: why didn't companies respond to falling capital goods prices by spending even more?

Now, you might object that it makes no sense to speak of a lack of investment opportunities because profit rates have been high for years; in the second quarter of last year, non-oil, non-financial companies' net return on capital was 10.8 per cent - so, even in the worst recession since the 1930s, profit rates were higher than at any time in the 1970s or early 80s.

This objection, however, overlooks an important distinction - between existing investments and potential new ones. It's quite possible for capital in place to earn big returns while prospective new projects are expected to have low returns. There's no reason to suppose that what Keynes called the marginal efficiency of capital is close to the average profitability of existing capital; the idea that macroeconomic entities are stable, identifiable and smoothly differentiable is a pedagogic device, not (necessarily) an empirical reality.