Norm returns to a theme of his - of wondering why the anti-war left couldn't and still cannot see that there was a case for overthrowing the fascist Saddam Hussein. I suspect that what's going on here is a common combination of two cognitive biases - groupthink and sampling bias.

The sampling bias is our tendency to forget that the people we associate with are more like us than the general population. Groupthink is the tendency to become overconfident about our beliefs simply because they are echoed by people we like.The combination of these two biases leads us both to over-estimate the prevalence of opinions like our own, and to be overconfident about their validity.

It's not just - or even mainly - the anti-war left that's prone to these biases. Republican pundits who expected Romney to win the presidential election made the same mistakes - though wishful thinking might also have been involved; being surrounded by Republicans led them to under-estimate Obama's support. It's a cliche that Hampstead liberals live in their own bubble. And one reason why chief executives often come across appallingly when interviewed on the radio is that, surrounded by like-minded people, they are unused to having their ideas challenged.

Luckily, there are ways we can counteract these twin biases.

First, for any strong belief you have, ask: what is the strongest case I can make against it?

It's important to do this because the most popular arguments for a particular policy are often mindless. But it doesn't follow that all arguments for them are. For example, the fact that the claim that Saddam had WMD was a lie should not discredit the claim that he was a tyrant we are better off without*. Similarly, just because talk of a structural deficit is drivel, it does not follow that there is no case to be made for fiscal austerity.

Secondly, we should open ourselves up more to opposing views. If you're a rightie, read the Guardian, and if you're a lefty, read the Torygraph. I subscribe to the Spectator but not the New Statesman.

Herein, however, lies a problem. There is a trade-off between inculcating rationality and building friendships. Rationality requires that we seek out people to challenge our worldview, but friendship and community requires that we move among like-minded people. This is another way of corroborating Jon Elster's observation that "the only persons who are capable of taking an unbiased view of the world are the depressed" (Alchemies of the Mind, p299).

Which only goes to show, yet again, the truth of Isaiah Berlin's view that the great goods are mutually incompatible.

* I don't say this to take sides in this debate; the case for or against the Iraq war is a matter of accounting, on which I have insufficient data.