A study in reciprocity found that a waitress who gives a single sweet to each dinner guest at the end of a meal leads to an average 3.3.% increase in the tip that she will get from the diners. Giving 2 sweets per patron leads to a 14.1%+ increase in the size of the tip compared to the 'no-sweets' control group. But if the waitress gives 1 sweet, and then (feigning spontaneity) returns to give a 2nd sweet to the diners that will lead to a 23% increase in the average tip given.
