Why do we really tip?

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Tipping in a restaurant is meant to reward great service, so why do many of us still hand over an extra 10 or 20 per cent when the waiters are mediocre? Michael Lynn, from Cornell University, has been studying tipping for 30 years and tells Graihagh Jackson it could be to do with guilt. (Picture: A customer puts cash in a tip jar. Credit: Getty Images)