From a theoretical economic point of view, gratuities may solve the principal–agent problem (the situation in which an agent, such as a server, is working for a principal, such as a restaurant owner or manager) and many managers believe that tips provide incentive for greater worker effort. However, studies of the practice in America suggest that tipping is often discriminatory or arbitrary: workers receive different levels of gratuity based on factors such as age, sex, race, hair color and even breast size, and the size of the gratuity is found to be only very weakly related to the quality of service.