Yet again, the BBC gave airtime this morning to the scaremongering Andrew Green. This raises the point that there is adverse selection in political debate: fanatics are given attention whilst sober, rational voices are overlooked.There are four channels through which this happens:

- Fanatics think their beliefs are so important and true that they set up lobbying groups and "thinktanks" to promote them, whilst rational people devote less time and organization to pushing their opinions. Sir Andrew set up MigrationWatch (and Richard Murphy the Tax Justice Network if you want a leftist example - I'm not making a partisan point here) but people with more reasonable, liberal, views confine themselves to occasional articles (though Philippe Legrain wrote a good book in praise of immmigration).

- Producers want "good" TV/radio, and this means having a violent debate between people with well-defined positions who can talk in soundbites. Why else does the silly Peter Hitchens get on air? This tends to squeeze out those who take evidence-based positions, as evidence is often messy and nuanced.

- People mistake confidence for knowledge, and so give too much credence to the irrationally overconfident.

- A tendency has emerged for people to respect strongly-held opinions; this is what gave us the law against religious hatred. This, of course, in the opposite of what should be the case. The fact that someone believes strongly in something is a reason for us to disrespect their belief and to discount it as the product of a fevered, fanatical and irrational mind.

What I'm suggesting here is an adjunct to something Mancur Olson said in the 1960s.He pointed out that small numbers of people with large interests would organize themselves better than large numbers with smaller interests. The upshot, he said, was that politics would give too much weight to small vested interests to the detriment of aggregate well-being. I'm saying that what Olson thought true of material interests is also true for beliefs. Small groups with strongly-held beliefs are given more credence and deference than they should have.

And this, in turn, implies that the mass media can sometimes undermine rational political discourse rather than promote it.