http://www.sciencedirect.com/science/article/pii/S1053535700000627 http://www.sciencedirect.com/science/article/pii/S1053535700... http://www.sciencedirect.com

http://www.sciencedirect.com/science/article/pii/S1053535700000627 http://www.sciencedirect.com/science/article/pii/S1053535700... http://www.sciencedirect.com/science/article/pii/S0148296301002065 http://www.sciencedirect.com/science/article/pii/S0148296301... While tipping is not an enormous driver of service quality (primarily for the reason you mentioned—that tips are expected even when service is poor), it does appear to be correlated with better service to some degree.