We often tip, but not always. And we don't tip as generously on some occasions as on others. The British Psychological Society's Research Digest recently highlighted the work of Professor Michael Lynn, of Cornell University, in the US, who has studied tipping behaviour for many years.Lynn found, as expected, a tendency to tip individuals who were considered to be low-wage earners.He also noticed that customers were more likely to tip when they felt that the service provider had less to look forward to than they did.Lynn also found that if a waiter offered a customer a chocolate with the bill, the customer was likely to reciprocate with a higher tip . Two pieces resulted in even higher tips.We're likely to tip more if the service provider compliments us. John Seiter and Harry Weger, of Utah State University, asked waiters to compliment half the parties they served on their "excellent choice of order". As long as customers felt the compliment was sincere, they tipped significantly more.We're more likely to give more if the waiter attempts to establish a rapport. This is apparently even more important than the quality of the service. - © The Daily Telegraph..