John McDonnell tells us that we'll all make massive profits from government spending

There are two - useful anyway - possible responses to this latest assertion of John McDonnell. One is hollow laughter, the second is a more robust stream of Anglo Saxon. For what he's saying is that it doesn't really matter what government spends borrowed money upon, it's all still bound to make us richer.

Well, yes.

By focusing on the cost of government borrowing (currently close to an all-time low, thanks to tiny interest rates) rather than the enormous social and financial returns on investing that money, the right creates a narrative that investment costs society rather than benefits it.

Lurking behind this is an assumption that a government cannot invest productively – the belief that government borrowing is akin to burning money. This is nonsense used to support the economic approach that has led our country to this pass. Very clearly, government investment can and should be used to support economic growth, as the OECD and others recommend – as indeed this Tory government, in some small way, is coming to recognise, committing a small amount of borrowing for investment. The meaningful question is whether that investment is wise, given the costs – rather than presuming that only costs exist.

Or as he's expanded upon:

Asked by BBC Radio 4’s Today presenter Mishal Husain how much extra it would cost to service public debt under Labour, McDonnell would not give a figure, saying extra borrowing would “pay for itself”.

The problem with this argument is that it is nonsense.

We can, dependent upon how Keynesian we want to be, make the argument that in a depression and the like then government spending upon near anything can make us all richer. It's not a view we subscribe to particularly but we'll acknowledge that it's out there. That isn't where we are though, is it? We're at or at least about full employment, there's little to no spare capacity in the economy. Thus a simple and pure increase in spending isn't the answer to whatever remaining problems we have - that would just cause inflation.

What is necessary for McDonnell's argument is that whatever the borrowing is spent upon is worth more than the cost of the borrowing. For purists we should also be insisting that it creates more value than alternative uses of the same money - opportunity costs or crowding out. This is possible, certainly. The amount spent upon enforcing property rights is the very basis of our having a functioning economy at all thus it's obviously true that some at least government spending passes this test.

But this isn't the hurdle which must be leapt. Rather, we need to know that the marginal spending will pass it. At which point we might look at the two major infrastructure projects currently under discussion, HS2 and the Swansea Lagoon. Both of which fail their cost benefit analyses. That is, they make us poorer, not richer. As they do so they clearly and obviously do not manage to pay back their construction costs, let alone the interest on the borrowings to finance them.

In the past this was also not true of the Edinburgh tram system, the Humber Bridge, the Tanzanian ground nut scheme and that multitude of other bright ideas that the political process has spent our money upon.

It is indeed theoretically possible that government can add value. It even does so, which is why we continue to have it. We've little to no evidence that this is true of the various building sets that politicians love so much. Therefore, how about not spending our money upon them? 

After all the private sector will already be doing those things that are clearly and obviously identifiable as profitable....