Of all the gin-joints in all the towns in all the world ...

Scientists say that our appreciation of the weirdness of 'coincidences' is flawed; our brains simply have not evolved to cope with big numbers. But Mark Rowe is still awestruck by the way he keeps meeting long-lost friends in unexpected places.

Two people, one big world. "Of all the gin-joints ..." moans Humphrey Bogart, as he spots Ingrid Bergman through a haze of smoke and whisky in that immortal scene from Casablanca. And it might well seem unlikely that the Hump and Ingrid characters bump into each other in an outpost of the Second World War.

A coincidence, you may well think. But most people reading this article have probably met someone they know while on holiday, whether in Newquay, New York or New Delhi. It may not always be in a gin-joint, but I am a serial people-meeter, and I am not the only one. Why does it happen?

During my childhood I invariably met schoolfriends, looking as sullen as myself, in tea shops in Cornwall or Skegness while spending rainswept family holidays. But during the last few years I have achieved several satisfying "meets".

After two months travelling around China in 1991, I decided on my last day to go for a cycle ride off the tourist trail on the outskirts of Peking. I wanted to see the "real China". Instead, along a dirt-track hutong, I met the real Western Europe, in the shape of a friend I had not seen for three years. She had left college with no real plans, and had gone to China to see what turned up. I assume she had hoped it would be something more exotic than me, but for the first time I pondered the odds of meeting a friend in a country of 1.2 billion people.

Another example: I once raised the cash for a trip to India by spending two months laying out motorway cones on the M25. One lad who shared biscuits and tea with me during the night shifts was also planning to go to India, and we joked about meeting in a country of 650 million people. Sure enough, as I slumped on to a bus travelling from Agra to Fatehpur Sikri, I was tapped on the shoulder and turned round to see Jim.

"It's all to do with social class," said Dr Susan Blackmore, a senior lecturer in psychology at the University of the West of England in Bristol. "The same sort of people are drawn to the same kind of holiday. Travelling through India, you will spend a lot of time in bus stations and on trains and at major sights - the places where there is a high concentration of foreigners.

"We think it is an amazing coincidence, but really it's just that our brains have not evolved to cope with big numbers. We are still used to the caveman mentality of small packs of people. To illustrate this, you can ask how many people you need in a room before there is a 50-50 chance of two sharing the same birthday. Most people might say 186 - or half of 365 - but in fact it is just 23, because that gives you the right number of possible pairs. The odds are lower than you think on such so-called coincidences."

Not always, though. Burma in 1992 remains my greatest achievement when it comes to meeting people in unexpected places. This was just before tourism to Burma expanded - and before I was fully aware of the atrocities committed by the military junta. In my first five days there I saw precisely two foreigners.

Then, one evening in Mandalay, I was sitting in my hotel's voluminous and silent reading room when a fellow tourist walked in. Enthused by finally getting the chance to talk about what we had seen, we started by establishing a few reference points. "What did I do?" asked Victor. I had just started working for the Grimsby Evening Telegraph, I told him. "Well, you've probably never heard of it but ..."

Victor had heard of it. He had been the Grimsby Telegraph's production editor, but had left the week before I started, in order to travel round the world. And here we were in Burma. No other tourists, just two people from Grimsby, a town which prides itself on its insularity, and whose population views a day trip to Hull as reason to get the passport out. Why go abroad when you've got Cleethorpes, I was often asked, during my three years in the town.

"That is weird, and does go off the scale of odds," said Dr Blackmore. "But it is only one coincidence, and you should think, 'isn't that nice', and remind yourself of the people you know who you didn't meet there, to keep it in perspective."

On my return to Bangkok I passed the night in the airport before an early flight home, talking with a man from Guildford who worked, and shared a desk with, my best friend from school; his veracity was established when he complained about the way my friend clinked his cup against his teeth when drinking coffee.

Dr Blackmore again: "The more outlandish the coincidence, the worse we are at being able to rationalise it. Most of us know about 1,000 people to recognise,and by association we have things in common with many more."

So spare a thought for poor old Bogie. Had he thought it through - Casablanca was, after all, a vital bolt-hole for those fleeing the Nazi advance - he would have stayed in Europe, and saved himself the heartache of meeting up with Ingrid again.