I know how blue David Cameron feels when he goes red

You could make a rather cruel party game out of this. Ask a table full of people whether they blush, and when some sucker pipes up to say that she did when young but has grown out of it, subject her to beady scrutiny until she goes quite pink. That's what happened to me yesterday when the subject of blushing came up — in the context of politics, of all things.

It was Gordon Brown who first raised the matter, at Prime Minister's Questions this week. He was jeering at David Cameron's posters and then pointed at the Tory leader. "You're getting even redder than you are in your photograph!" he shouted.

Cue, obviously, for Dave to blush even pinker. Which made me realise that although the Conservative leader may be privileged in several ways, he belongs, in fact, to a disadvantaged minority for which we should feel only pity: those who go red in embarrassing situations and at the least propitious times.

A friend of mine calls blushing "nature's own lie-detector". It's not true, you know. I can blush when I'm perfectly innocent of telling fibs, when I'm not doing anything wrong, when I'm trying hard to be cool and aloof. But just subject me to unwanted attention or put me on the spot in some way and I'll turn anything from a pretty rose to some sort of crimson. And though you can't see it, the blushing has a concomitant physical sensation: a kind of constriction in the solar plexus. There's no accounting for it, unless the roots lie very deep back in the playground.

The good thing is that you really do blush less as you get older. When I was younger and I knew I'd be passing a building site, I'd start to blush at a distance of about 300 yards and get pinker and pinker as I got closer, in the expectation that the men would make some personal observations. By the time I got to them, I'd be so crimson, the unkinder ones would simply hold their hands up, as if beside a stove, as I passed.

Other people can go from red to white when embarrassment is replaced by rage. I saw it when I was at school and the nun was remonstrating with us. "Your manners is appalling," she said. A hundred adolescent girls murmured simultaneously "Are appalling", and honestly, she did the red-to-white routine before our eyes.

Traditionally, blushing is associated with modesty, with bashfulness. Dickens's Mr Podsnap observed of his daughter that she was "liable to burst into blushes when there was no need for it at all". But both sexes do it. A friend of mine who is a happily married man would blush furiously whenever the office beauty approached him.

Of course there's a scientific explanation for all this. It's down to an overactive sympathetic, or autonomic, nervous system, apparently.

But that doesn't help. It's like saying that when you're in love (and lovers in the first flush of passion do, in fact, blush when they hear the name of the person they've got a crush on) you show particular physical sensations. It doesn't explain why some do, some don't.

All I can say is that David Cameron is now a twin soul, a blusher. Politically, he's blue; on top, he's a right pinko.