Freakonomics p.175

Economists might not be able to forecast business cycles, but we're always getting better at the making the obvious—and slightly risqué—seem like science, as this latest research demonstrates:

Michael Lynn, a Cornell University professor of marketing and tourism, surveyed 374 waitresses and asked them to assess their physical characteristics, including their breast size, and evaluate whether they perceived themselves as attractive.

Those with bigger breasts, slender waists and blond hair reported receiving the best tips. High-quality service, Lynn's analysis concluded, had less than a 2 percent effect on tip.
...
Lynn also suggested that restaurant managers might be wise to keep his research in mind during the hiring process, because servers who make better tips are more likely to stay at a given job.

"Ugly people are not a protected class, legally," he said. "It is not in fact illegal to hire only attractive waitresses."