It's a well-documented fact that black workers in America earn less than white workers. It's a gap that has been explained by systemic discrimination and differences in education and employment expectations.

But as far as professionals go, doctors inhabit a somewhat unique space. The path to becoming a doctor in the US is very tightly regulated. Doctors all have to be educated in a similar way, pass the same board exams, and earn the same licenses at the end. They also typically come from similar (well-heeled) backgrounds and are rewarded with high incomes for their years of schooling and service.

Harvard Medical School associate professor Anupam Jena wanted to find out whether the black-white pay gap would persist among this homogenous group.

In a new study, published today in the BMJ, he and other researchers from Harvard and the University of Southern California used race and employment data from two nationally representative surveys to find out.

The picture they paint is alarmingly consistent with overall labor trends: Despite the uniform education levels and credentials among doctors, black physicians still earn significantly less than white physicians. The disparity between female black and white doctors is smaller, but female physicians of both races earned significantly less compared to men, the study found.

More specifically, the adjusted average annual income between 2010 and 2013 for white male physicians was $253,042 — nearly $65,000 more than what black male physicians earned ($188,230). White female physicians got $163,234 and black female physicians about $10,000 less at $152,784.

The researchers used two data sets to come to these conclusions: a larger American census that didn't account for physician specialty (which is an important limitation since there are sizable differences in pay among medical specialties) and a smaller doctor survey that did have specialty data (along with information about hours worked, practice setup, and insurance mix). When the researchers compared the findings from the larger census to the smaller but more detailed data set — they found the gap persisted. In other words, the inequality in income couldn't be explained by black and white doctors choosing different specialties or having different practice styles.

After adjusting the data to account for these factors, they still found that white male doctors more often brought home bigger salaries: 36 percent earned more than $250,000 while just more than a quarter of black male doctors did. Conversely, less than 10 percent of white male doctors earned less than or equal to $100,000, while nearly 15 percent of the black male doctors did.

"This was interesting," Jena said, "because physicians comprise a relatively homogenous group in terms of pay scale. So a lot of the reasons used to explain [racial pay] differences in the overall labor market shouldn't apply in medicine"

When it comes to the black-white pay gap, Jena added, "it's clearly the case that medicine has not been spared."

African Americans make up only 5 percent of the physician labor market

Thomas LaVeist, chair of the Milken Institute School of Public Health at George Washington University, said he wasn't entirely surprised by the research findings: "We know racial and gender discrimination has been a feature of American culture forever."

Part of the problem is that health professions generally do a really bad job at recruiting minority groups, he said. "There's been no increase in the percentage of African-American physicians who graduate from medical school since the 1960s." It's hovered around 5 percent.

But given the abysmally low supply of black doctors and the pressure throughout the economy to diversify the labor market, he thought black doctors may garner higher incomes as a result. "[The findings] go counter to market forces since supply and demand would suggest black male physicians, who are relatively rare, would have higher salaries."

Yet that's not what has happened with female doctors either, despite the fact that over that same period, there's been an enormous uptick in the number of women in medical schools. (Women now account for about half of all medical students.) Another recent study found that women in scientific careers earn 31 percent less than their male counterparts in their first year on the job market.

In LaVeist's view: "People need to know their worth, their value. [We need to do] a better job of educating people across the board — and physicians in particular — to become better educated about what their leverage is, what their value in the marketplace is, and how to negotiate to ensure they get the incomes that they deserve."