This post has been updated.

In the US, restaurant servers work under a very different pay system than most people. Beyond a small hourly rate, their employers don’t pay their wages—customers do. Essentially, these workers have dozens of different bosses each day that individually decide how they should be compensated. And there’s not even a requirement, beyond social mores, that these de-facto bosses pay their servers anything at all.

For years, Saru Jayaraman, co-founder and co-director of the Restaurant Opportunities Center United (ROC United) and director of the Food Labor Research Center at the University of California, Berkeley, has advocated and organized to end this system.

But it wasn’t until she started researching her new book, Forked: A New Standard for American Dining, that she discovered that the separate, lower minimum wage for tipped employees comes from a dark part of American history: slavery.

“The original workers that were not paid anything by their employers were newly freed slaves,” she tells Quartz. “This whole concept of not paying them anything and letting them live on tips carried over from slavery.”

Many Americans in post-slavery America initially resisted tipping, a custom that originated with European aristocrats. To tip was patronizing, Jayaraman writes in her book; it was seen as ”despicable, undemocratic and wholly un-American.”

But the idea that anyone who accepted tips was in a lower class held on into the early 20th century. Jayaraman quotes an American reporter, John Speed, who reflected on the tipping system in 1902 while traveling to the North for the first time. His words underscore the inherent racism to tipping: “I had never known any but negro servants. Negroes takes tips, of course; one expects that of them—it is a token of their inferiority. But to give money to a white man was embarrassing to me.”

Tipping eventually took hold in the hospitality industries, though—at restaurants, hotels, and rail companies with porters who served affluent travelers.

Today, the National Restaurant Association, a trade group representing more than half a million restaurants, encourages tipping. ”Tip-earning employees can be among a restaurant’s highest earners,” the association notes on its website. Its research “shows that on a national level, median hourly earnings for servers range from $16 to $22, depending on experience.”

According to the US Bureau of Labor Statistics, however, the median hourly wage for servers is actually $9.01, with an annual salary of $18,730. The NRA says the BLS data may skew lower because restaurateurs may not know they should include tips in their reports.

In Europe, tipping has gone mostly out of fashion, with service usually included in the rest of the bill. In the 1920s, US railroad car porters successfully fought for higher wages. But in the US restaurant industry, it remains. And while the standard minimum wage has risen from $4.75 in 1996 to $7.25 in 2009, representing a 53% increase, the national minimum wage for restaurant workers (which is lower to take tips into account) hasn’t budged since it was set 20 years ago at $2.13 an hour.

While the discrimination against tipped workers was originally rooted in racism, there is now a different demographic suffering because of it: according to Forked, “66 percent of the almost six million tipped workers in America are women.”

Update (Feb. 10): This post has been updated to include a response from the NRA to Quartz’s question regarding the discrepancy between its data and that from the BLS.