‘Good morning,’ the doctor says as you enter the surgery, ‘let me just go and get your file – won't be a moment.’

The doctor returns with your papers. ‘Good morning.’

Then, ‘Oops, I forgot my stethoscope. I'll be right back.’

The doctor comes back from the next room and sits down behind the desk. ‘Good morning.’

For a moment you start to think there's something strange about your doctor. Something almost Monty Python esque. But what?

There are many unwritten rules when it comes to saying ‘hi’ and saying ‘bye’. Why do you sound awkward if you greet someone more than once, while it is perfectly fine to say goodbye many times over?

Greeting customs is one of the ‘human universals’, as anthropologists call them. Perhaps farewelling customs is one too. This means that, among such things as jealousy and gift-giving and fear of death, there is no known human culture on earth that does not have rules about how to say hi and, perhaps, how to say bye.

People bow, rub noses, shake hands and kiss. Most raise their eyebrows. Polynesians rub each other's backs or sniff each other's breath; East Africans might spit on the ground in a greeting; Tibetans stick out their tongues. Some New Guinean tribesmen pat each other on the rump. Not long ago, Chinese people kowtowed nine times and Westerners learnt elaborate ways of removing their hats with a flourish or even threw themselves face-down on the ground in front of their superiors.