Britain has an immigration problem – but not of the sort generally supposed.

Let’s be clear. The facts show that immigrants are a net fiscal benefit rather than a cost, and that immigration is, except for a small negative effect at the bottom end, a net positive for wages (pdf) and for economic growth (pdf).

Economically speaking, then, immigration is (net) a good thing.

So, what’s the problem? It’s that the public just don’t believe the economic evidence. I don’t think this is simply because they hold economists in low esteem: the man on the Clapham omnibus does not devote his waking hours to thinking “that Jonathan Portes talks some shite.”

Rather, I suspect the problem is a more general one – that irrationalism plays a big role in human affairs.

There’s an analogy with the MMR scare of a few years ago. The idea that immigration is an economic problem is like the belief that the MMR vaccine causes autism. Both views are founded upon a mix of anecdote and the post hoc ergo propter hoc fallacy: “my son got the jab and then was diagnosed with autism”; “a lot of Latvians arrived in town and my son now can’t find a job.”

In both cases, scientific evidence is weak against vivid anecdotes and well-organized campaigning.

You only have to listen to 6-0-6 to know that opinion and evidence coincide only accidentally. The belief that migrants are a drain on the economy is like the popularity of Boris Johnson, homeopathy or conspiracy theories.

It would be easy – and right – to complain that politicians and the media amplify such irrationalism rather than challenge it.

But there’s a caveat here. Irrationalism is not wholly unreasonable. Yes, the belief that immigration is doing serious economic damage is wrong, as it lacks a good evidence base. But an uneasy feeling that change can have unforeseeable effects isn’t so stupid, being rooted in a reasonable conservative disposition – and in the fact that, as Diane says, there’s much we don’t know about the effects of immigration.

One way to combat this instinct, as I’ve said, is to remind people that immigration is nothing new and is part of our “island story.”

But there’s something else. We know from financial markets that risk aversion rises in bad times, and falls in good. This is one reason why, as Ben Friedman has shown, people become more intolerant in recessions – it’s because downturns cause them to see the downside of change, openness and diversity more than the upside.

Perhaps, then, an economic recovery would soften attitudes to immigration – although it’ll take more than this to convert people to the case for free migration.