When it comes to U.S. doctors’ paychecks, race and gender may be factors, according to a new study.

“Black male physicians earn substantially less than white male physicians, while white and black females have comparable salaries,” said senior author Dr. Anupam Jena, of Harvard Medical School in Boston. “Black and white female physicians both have lower incomes than either black or white male physicians.”

In a paper published online in the BMJ, Jena and his colleagues say there is limited evidence about race-based difference in physician incomes. But national estimates from 2011 show people in non-Hispanic white households earning an average of $76,063 per year, compared to $47,255 per year among people in black households.

There is a larger body of evidence suggesting that female physicians earn less than their male counterparts, however.

For the new study, the researchers used data from two surveys of physicians. One was conducted between 2000 and 2013 and involved 43,213 white males, 1,698 black males, 15,164 white females and 1,252 black females. The second, conducted between 2000 and 2008, involved 12,843 white males, 518 black males, 3,880 white females and 342 black females.

Overall, race played a factor in how much male physicians got paid, with white males paid more than their black colleagues. Race was not a factor among female physicians, but they were paid less than their male colleagues.

For example, half of the white males were paid at least $253,042, while the median income for black males was only $188,230. White females had a median yearly income of $163,234, compared to $152,784 for black women.

Jena told Reuters Health that possible explanations include workplace discrimination, differences in negotiating pay and preferences in where people choose to work.

One researcher who wasn’t involved in Jena’s research but is familiar with the data said the findings are more likely due to what type of medicine participants practice, rather than race.

“The real issue that’s driving their results is that there are many more white specialists than black specialists, and specialists make a lot more money than primary care physicians,” said Dr. William Weeks, of The Dartmouth Institute in Lebanon, New Hampshire.

The question is whether black doctors choose not to go into specialties or if there are barriers keeping them from those fields, said Weeks, who is also affiliated with Aix-Marseille University in France.

Jena, however, said doctors’ specialties only explained part of the income gap.

He said to guarantee equality in doctors’ incomes, there needs to be accountability at an organizational level, such as when healthcare facilities are going through their accreditation processes.

“What needs to drive that accountability is unclear,” said Jena. The next step is to collect more comprehensive and timely data on black and white physician income, he said.