Here's an excerpt:
Dr. William Michael Lynne, a full professor of consumer behavioral marketing at Cornell University's School of Hotel Administration has been examining the relationship between gratuities and race since the late-80s but due to what he saw as a politically sensitive topic, Dr. Lynne only recently began to discuss some of the implications attached to the subject. "It's a dirty secret in the industry that there is a wide spread perception that blacks don't tip well," says Lynne. Because of this perception, Lynne believes that African-Americans might often receive inferior service.
Beyond the perception however, Lynne who has conducted surveys in targeted areas of he U.S. as a well as telephone interviews across the nation, believes that at least some of the evidence from a compiled research paper titled, "Race Difference in Tipping", first published in 2006, supports the negative perception of blacks and tipping.
According to Lynne, one study from the paper concluded that among whites, blacks, Hispanics and Asians, blacks gave or claimed to give the lowest average tip. In another survey from the same report blacks were more than twice as likely as whites to leave a flat dollar amount instead of a gratuity reflecting a percentage of the bill. Part of Lynne's research also came from opinions and data collected from restaurant servers. (It is critical to note that Lynne's studies, at this point, did not take into account blacks or whites from other countries who may have adhere to different gratuity practices from abroad.)