At U.S. medical schools affiliated with public universities, female physicians get paid tens of thousands of dollars less each year than their male colleagues, according to a new study.

The average male doctor at these institutions earns at least $50,000 a year more than the average female, researchers found.

A large pay gap remained even after accounting for factors that influence salary, such as age and medical specialty.

"When you account for those, you can explain about 60 percent of the gap, but about 40 percent of the gap remains," said lead author Dr. Anupam Jena, of Harvard Medical School in Boston.

Previous research has found differences in pay between male and female doctors, but those studies were often based on survey results.

For the new study, the researcher used data from 24 public medical schools in 12 U.S. states that require salary information be made available to the public through freedom of information requirements.

"The states that we looked at had this information online," Jena told Reuters Health.

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Male doctors at those schools earned on average $257,957, compared to $206,641 among female doctors.

Female doctors who were the top professors at their institution were making as much as male doctors who held lower positions.

Women were less likely than men to be full professors, to have funding from the National Institutes of Health and to have conducted a clinical trial.

They were also more likely to be younger, and to specialize in internal medicine, obstetrics and gynecology and pediatrics.

After accounting for those factors, women still made nearly $20,000 less, on average, than men, the researchers report in JAMA Internal Medicine.

"They were still left with that pay gap," said Dr. Vineet Arora, who wrote an editorial accompanying the new study.

"This study does point to some bright spots that merit further discussion and understanding," added Arora, who is in the Department of Medicine at the University of Chicago.

For example, not every institution had a pay gap. Also, women working in radiology were paid as much as their colleagues.

"When something is going well some places, we need to figure out why and that may help poor performers," Arora told Reuters Health.

Jena suggested several possible reasons for the gap. For example, he said, women negotiate salary differently than men. They may also be less likely to seek out outside job offers to bolster their current salary. They may also be the victims of conscious or unconscious biases.

Arora said the next step is to look at ways to reduce the pay gap between male and female doctors.

"I think what we need to realize is that women physicians aren't alone," she said. "It's important to understand it. It can help women in other fields."