I was at college with a bloke called Ben. The first time I met him, before he even opened his mouth, I knew he was very posh. This wasn't a value judgement; just a statement of fact. And, though he wore a blazer over a pinstripe shirt and ripped jeans above suede brogues, it wasn't his clothes that told me, but his face.

Ben wasn't particularly handsome or ugly. He had small, bright blue eyes that were wider set than most and a slightly piggy nose. Prominent, tombstone teeth gave him a prominent upper lip that held its place a few millimetres forward of the lower, and a good half inch in front of his chin. Most of all, though, he had a certain clean-scrubbed innocence of expression; as if, I thought, his face had never done a hard day's work in its life. - posh. This last piece of description is, of course, projection on my part ... and somewhere therein lies my point.

Ben was a nice guy and I came to like him a lot. But I was peculiarly fascinated by his face and often I found myself staring at him in a way that probably wasn't polite.

I mean, Ben had to be posh. There was no way those teeth and that lip had not been forced forward by a childhood sucking a silver spoon. But how could that be? Isn't a face just a face - an accident of genes, no more definitive of class than a talent for music or a sweet tooth?

Of course, there are only two possible explanations. Either the features we associate with the upper classes (or any other social group) are, indeed, the product of breeding, or our lifestyles don't just affect our bodies, bearing and demeanour, but our very features too.

I thought about this yesterday on a rush hour tube at Holborn. The carriage was full of white men between the ages of 25 and 50 and we all avoided each others' eyes and read the free papers and fiddled with our phones and iPods. And didn't we all look alike, with our complexions like old ham and expressions fixed in brittle confidence? I'd swear that these attributes were not changeable; at least, not any more. They were now hard coded into our skin, tissue and skulls.

Mrs McGarry, my primary school teacher, told me that if I pulled a face and the wind changed I'd be stuck like that forever.  Mum always reassured me that this was an old wives' tale. But I'm beginning to think Mrs M was on to something.

I thought about it again this morning after I brushed my teeth. I looked at my reflection and examined my brittle confidence. I now have unusual and permanent lines on my forehead - not horizontal wrinkles, but vertical gashes that could be scars, but aren't. I don't know if it would be better if they were scars. I suspect maybe it would. I have prominent veins at my temples that make me think of sci-fi movies and invasive alien procedures. My jaw is heavier than it should be, jutting and stubborn. My ears are bigger than they used to be, flapping assemblages of cartilage and flesh. My mouth is smaller, but it doesn't yet look mean. My nose is enormous. I can't decide whether my eyes look haunting or haunted. Fortunately, I quite like my face. After all, I'm beginning to think it's the one I chose.

Book Slam this evening. Good fun. I couldn't have more admiration for William Boyd as writer and person and Netsayi's new stuff sounded fantastic. But the thing that's stuck in my head is one of the estimable Don Paterson's aphorisms ... something like (and I'm going to misquote), "it's bad when a man looks at his face in the mirror and sees his father, worse when he looks at his body in the mirror and sees his mother". Very good, Don. All things considered, I've no right to complain.