Katy Guest: In London we already walk by on the other side

There comes a point for every newcomer when suddenly, irrevocably, London has got into your soul. For some, it's growling at people who stand on the left on escalators. For others, it's squeezing on to a Tube train when another will arrive in one minute.

I knew it had happened to me when I was back for a weekend in Derby and caught myself fighting my way through a perfectly placid bus queue. And for Boris Johnson, that moment arrived three days ago.

Only two months into his new job, he has changed forever from a carefree provincial who wouldn't say boo to a bendy bus into an (Oyster) card-carrying, prototypical Londoner. The Mayor has got his spurs. And in this city, he's going to have to use them.

It's a sad day when a part of you dies and is replaced by sharp elbows, black snot and an ability to recite the stops of the Northern Line in your sleep (no lift service at Bank, exit at Borough only during peak hours...). But obviously the capital has got to Boris. Until 3 May, he was a chilled-out optimist with faith in human nature, who encouraged people to stand up to bullies and "take a risk". But that was when he lived in Henley. One summer in the city and it's sod the bullies and look after number one. And to be fair to him (though why am I breaking the habit of a lifetime?) it happens to us all.

Boris's London moment came at a press conference at City Hall this week, when he ditched the Good Samaritan views of his more innocent days and started talking tough. "I'm afraid that [this] may sound like a lack of public spirit if someone is being badly attacked," he said, "but I say to kids who are going out this evening and they see a fight, don't get involved, move away." It's all to do with personal risk, he explained.

Whereas last year Boris said that one's chances of being set upon for stepping in were "microscopic", he now thinks that's a chance not worth taking. "There will be cases where it is the public duty of all of us to help those that are the victims of crime. But if I was giving my kids advice about what to do in a bar in Islington, it would be to look after themselves." If I were giving Boris's kids advice about what to do in a bar in Islington it would be mine's a Hoegaarden and get out of the pub, you're 12. But really, Boris doesn't have to tell Londoners to look the other way.

Here's an experiment anyone can try: board a train at St Pancras, weeping openly and travelling north. As you pull out of the station, nobody will notice as they unpack their laptops and grind their teeth. At Luton Airport Parkway, everyone will still be looking away. But by about Leicester, as more new passengers get on, something strange starts to happen. Concerned glances will be flicked in your direction. A tissue may be offered. By Long Eaton it becomes ok to talk; at Chesterfield somebody cracks open the little wine bottles left in the buffet carriage and by the time you reach Sheffield passengers will be joined in a rousing chorus of "I Will Survive". Now try the experiment in reverse. By the time you reach St Pancras, people "don't want to get involved".

It is not Londoners' fault that we are so mean. There are too many people here, and not enough time, and so everyone who lives in the capital is very, very busy. Add to that the self-fulfilling prophecy that only crazy people talk to each other in London and you can understand why we walk on by. I once came near to screaming when I lost a contact lens on Tooting Common and nobody stopped to help, but really I can't have looked like a very good bet. My knees were muddy, one eye was closed, my bicycle wheel span in the road where I had dropped it and I was picking with my bare hands among nettles. And maybe the two Balham ladies who crossed the road to avoid me were actually registered blind and couldn't see a person who needed their help.

The trouble with self-preservation is that it's catching. Like the Prisoner's Dilemma, the Samaritan's Dilemma is not knowing what the other guy will do. Stop to help and you may give others the courage to join you; but stop on your own and you're a goner. The more people turn away, the more people have to turn away.

It's hell out there, and for some London kids there are much worse fears than missing a Tube. But it's a London moment too far to give up altogether. Bullying and violence happen because everyone is afraid to be the only one to step forward, and politicians ought not to accept that. I understand why Boris tells his children not to intervene to help a stranger – anyone would tell their children the same thing – but I hope it doesn't catch on. And I hope that the next time I need help I happen to need it in the North.

k.guest@independent.co.uk