A common practice among waiters in restaurants is to give their customers an unexpected gift, a treat in the form of candy, sweets or chocolate when the bill is handed over. Two studies were carried out in 2002 to assess the impact of this gesture on the tip received by waiters.
Study 2 found that the amount of tip varied depending on the amount of candy given to customers as well as the way it was offered.
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This study, like many others that link gift giving to larger tips, is grounded in the principle of reciprocity ('you do something for me, I do something for you'), as well as all those theories that focus on the power of the latter experience (which in this case attempts to mitigate the 'punishment' that is the bill).
So that's why many businesses in the restaurant industry offer something in the end, when the bill arrives.