Many of us economists are bald. One reason for this is that for the last 40 years we've been tearing our hair out whenever politicians liken government finances to those of a household. Although Thatcher popularized this woeful analogy (as early as 1949) it lingers on in Labour. Rachel Reeves' claims that "there's not a huge amount of money there" and (inverting Keynes) that "if we cannot afford it, we cannot do it" both appeal to it.

You all know this analogy is wrong. For one thing, households can cut their spending without cutting their income but governments sometimes cannot do so because cuts in public spending depress economic activity and hence tax revenues*. And for another, governments (in the UK if not euro zone) can print money, and so there is always "a huge amount of money there". The constraint on public spending is real resources - doctors, builders, management skill - not money.

For these reasons economists often say that the household analogy leads to bad policy.

And often they are bang right.

But not always. If the government were serious about managing the public finances as if they were a household, policy would actually improve in some ways.

I say this because of reports that Reeves is thinking of using private finance to fund the building of the Lower Thames Crossing, in exchange for which they would receive incomes from the tolls.

The problem here is simple. The government can borrow in the gilt market at a real cost of around 1% (depending on the maturity of the loan). Private financiers, however, will demand a higher cost than this; they wouldn't invest otherwise. The government will therefore in effect be borrowing at a higher cost than necessary. That's like a household choosing to take on expensive credit card debt rather than extending its mortgage at a lower rate. As Daniela Gabor has said, Reeves is in effect handing out subsidies to private financiers.

Of course, that might be her intention. But it's not something a sensible household would choose to do**.

She is doing this because of another way in which she is not running the public finances as a sensible household would. I'm referring to the fiscal rules, one of which is that "debt must be falling as a share of the economy by the fifth year of the forecast."

No sane individual would adopt such a rule, and not just because they know that one should never rely upon forecasts. It's also because what matters is the sustainability of debt over one's lifetime, not over five years. (There are only two things that can go wrong in personal finance; either you'll outlive your money or your money will outlive you).

Any fool can reduce debt in the short-term by selling their house, paying off the mortgage and renting a place. Whether you should do that depends upon whether the cost of renting is cheaper than that of borrowing, all things considered. That's a tricky calculation. But it's one that households wouldn't make if they were tied by that "non-negotiable" fiscal rule; they would just cut their mortgage and not worry about the fact that they'll have to pay the rent after five years.

This, though, is exactly what Reeves is doing in thinking of handing over future toll revenues to private financiers; she's giving up long-term income in order to meet a short-term target. As the Institute for Government has said, the fiscal rule "does little to promote fiscal sustainability." No sensible household would behave like that.

Such an absurdity is the result of another absurdity in the fiscal rules; they ignore the fact that balance sheets have two sides - assets as well as liabilities.

A sensible household would want to increase debt if in doing so it could buy an asset which yielded more than the cost of debt (adjusting for risk!) Buying a house - thereby saving on rent - or a business can be perfectly reasonable things to do.

Not so for Labour. Reeves has said that nationalizing utilities "just doesn't stack up against our fiscal rules". Which is drivel. Many utility companies have dividend yields well above the 1% it would cost to buy them, so nationalizing them would generate a net revenue for the government.

No household would rule out investing in a business on the basis of the level of its debt without also considering the cost of that debt relative to the income stream it is buying. But that's what Reeves is doing.

Of course, there are good arguments against nationalization, one being whether the companies would continue to yield so much under state control***. But this is exactly the question a sensible household would ask: is this business really as good as it seems? Can we manage it well? It would not rule out buying merely by saying "debt, aaarghh!"

There's another way in which the public finances would be better managed if governments took the household analogy seriously. It lies in Sir Keir's promise to raise military spending to 2.5% of GDP.

To see how daft this is, imagine a family decided that it wanted to eat more healthily. It would not be so stupid as to set a target of raising the share of its income it spends on fruit and veg. It could do that by spending £20 on a single carrot. Instead, it would buy a greater volume of fruit and veg and less of something else. It would be explicit about what it cuts from its weekly shop, and it would pay attention to value for money. Neither of these obviously sensible actions are contained in Sir Keir's target, which provokes the thought that just as Reeves wants to give handouts to financiers so Sir Keir wants to give them to military contractors.

You might object that there's a limit to my descriptions of what a sensible household would do. Sometimes, households are forced into bad decisions, such as by having to borrow at high rates if their creditworthiness is bad. But this doesn't apply to the government now. The fact that the gilt market is willing to lend to it at a real rate of just 1% tells us that the government is free to act sensibly. That it is not doing so is a policy choice.

Nor is any of this to say that Labour should run a significantly looser fiscal policy. I don't think it should, simply because there isn't enough slack in the economy to do so: boosting growth right now should be the job of supply-side reform and regional policy, not fiscal policy.

Instead, my point is simple. It's that if the government is to treat the public finances as if they were household finances it should be consistent in doing so, as this would actually lead to better policy-making.

* I say sometimes because if interest rates are high enough, monetary policy can be loosened sufficiently to offset the impact of spending cuts. This, however, was not the case with Osborne's austerity.
** In practice, however, many households do exactly this by paying too much in fees to fund managers - but few of us think they should do so.
*** There are other arguments. One is that nationalization just gives a future government something to sell off cheaply to its cronies. Another is that - given the pisspoor quality of UK management - the companies will be badly managed whoever owns them and its better than capitalists get the blame than that the government does.