Some trades unions want the UK to keep Trident to save jobs. For example, the GMB's Paul Kenny says:

Everybody keeps talking about the wonderful principles of Trident. But there are tens of thousands of British jobs involved here...There are about 50 sites around the UK whose livelihoods depend on defence contracts and we are going to ask those people what they think about the Labour party effectively shutting down their jobs

I don't know which is more depressing - the possibility that the unions are wrong, or that they are right.

Econ 101 says they're wrong. If we don't spend billions on Trident, we could spend the money on other things that will create maybe more or better jobs. Sam Brittan used to argue for a clampdown on arms dealing in part because the jobs lost in that industry would be replaced by other, less immoral, ones.

This, however, overlooks two things. One is that workers aren't fully fungible. Trident employees can't swiftly retrain or move to where there are jobs. And there are already skill shortages in the construction trade, which suggests that switching money from Trident to housebuilding might not be as easy as it seems.

Secondly, people are loss averse: shipyard workers who lose their jobs will feel genuinely unhappy whilst those who get jobs - even if they are just as good - won't get an equivalent increase in happiness. Wolfgang Maennig and Markus Wilhelm have estimated that losing one's job reduces well-being by five to ten times as much as does finding a job.

On balance, though, I'm inclined against the union position:

- If the only way to preserve or create good jobs is to spend billions on arguably-pointless weapons of mass destruction, then the UK economy is even sicker than any of us thought. Are things really that bad?

- Workers must shift jobs sometimes. If they didn't, we still be making stovepipe hats and using quill pens to keep ledgers. The task for the left is to make such shifts as easy as possible, not to prevent them.

- Jobs aren't the only thing that matter. Morality does too. I suspect ISIS has an equivalent of Paul Kenny who is defending burning people to death and pushing them off high buildings because these create work for his members.

Of course, Sir Paul is only doing his job and defending his members' interests. This, though, draws our attention to an embarrassment for the left - that, despite their many virtues, unions can be sometimes be very conservative.