MOTIVATOR OF BEHAVIOR 

ratitude may also serve to reinforce future prosocial behavior in benefactors. For example, one experiment found that customers of a jewelry store who were called and thanked showed a subsequent 70% increase in purchases. In comparison, customers who were called and told about a sale showed only a 30% increase in purchases, and customers who were not called at all did not show an increase. In another study, regular patrons of a restaurant gave bigger tips when servers wrote "Thank you" on their checks.