Good judges are sceptical of Theresa May's repetition of her failed promise to reduce migration. The Migration Observatory says the target is "not feasible in the short term and difficult to achieve in the long-term." And Stephen Bush says that even if it could be achieved, the effects would be "pretty terrible".

This is true. And it's irrelevant. It misses the point that the Tories need an immigration target, and they need it to fail. This is because it's a deflection strategy.

The Tories want to blame immigration for poor public services and low wages. For the most part, however, this is just plain false. Immigration is only a minor contributor to low wages and to pressure on the NHS. What's far more important for the latter is austerity. And wages have been depressed by many things other than immigration, such as declining trades unions, austerity, financialization (pdf), power-biased technical change and the countless factors (among them bad management, the effects of the financial crisis and low investment) that have caused productivity to stagnate.

The Tories, however, cannot say this. They can't blame their own policies, and nor can they comfortably point out that capitalism is failing. They need a scapegoat. Immigration is it. Hence the need to talk up the need to reduce migration.

By the same token, though, they need to fail to do so. What if migration were to fall whilst public services remain stretched and real wages continue to stagnate? It would then be clear that immigration was indeed a red herring and that our problems owed more to Tory policy and capitalist failure. In failing to meet the target, however, the Tories can maintain the pretence that, if only they could reduce migration, then pressure on public services and wages would be relieved.

From this perspective, failure works better than success.

Now, you might object here that this looks like a conspiracy theory, or at least like the sort of functional explanation of which Jon Elster, one of my great intellectual heroes, was sceptical.

I think I can escape this charge. I'm not arguing that the Tories originally adopted the target because of this motive. They're not that clever. Cameron chose it simply because he was pandering to the right. By sheer luck, however, the target serves a useful function - all the more so because it won't be hit.