In the row over the "mad, swivel-eyed loons" remarks, something is being missed - that, from an economists' perspective, we'd expect political activists to be disproportionately mad.

By "mad" I do not mean the content of activists' beliefs, but rather the intensity with which they are held; I'm thinking here of fanaticism rather than extremism. I don't think opposition to EU membership or to gay marriage are mad. What is mad is attaching great importance to such views. As Adam Smith said, "there is a great deal of ruin in a nation." We can live well with a lot of suboptimal policies.

But here's the thing. Political activists are selected for passionate intensity. Stuffing envelopes and canvassing often-hostile voters is a dull and unpleasant way of passing the time. So what sort of people would volunteer to do it? The sort who over-rate the importance of politics - who believe their favoured policies will have huge pay-offs and their opponents' will lead to disaster - that's who. As Eugene McCarthy said:

Being in politics is like being a football coach. You have to be smart enough to understand the game, and dumb enough to think it's important.

I suspect there are two other selection effects:

- When the main parties have so much in common - similar managerialist ideologies - a narcissism of small differences emerges, with both sides exaggerating the benefits of their side and costs of their opponents'. In the 70s and 80s, when there really were big differences between Labour and Tory, it was possible for rational people to think that party politics was very important. It is less possible to do so now.

- Party political activism is now a minority activity. This, I fear, creates positive feedback effects. When party membership was a mass activity, peer pressure attracted sensible people to become activism. Now it is regarded as eccentric, only the eccentric are attracted.

All of this applies to both Labour and Tory parties. But there's another mechanism which makes me suspect that Tories (and Ukippers) are more likely to be loons. If you're a middle-aged, moderately wealthy person working in the private sector, then party politics doesn't much affect you personally: a few pounds here or there on tax rates doesn't much affect the middle classes, but changes to benefit rules can make a huge difference to recipients. The typical Tory activist, therefore, is likely to be someone who over-estimates the importance of party politics.

Now, I don't say all this to claim that all party activists are loons. That phrase "from an economists' perspective" is doing some work; I'm speaking here of cost-benefit considerations and abstracting from the crooked timber of humanity which causes some sane people to become activists.However, these thoughts are consistent with a recent empirical finding - that political activism, unusually amongst voluntary activities, does not make people happier.