Despite the fact that women overwhelmingly outnumber men in the nursing profession, a study of nurses has found that men still get paid more.

The gender gap in nursing has caused women to earn salaries about $5,000 less than their male counterparts on average, which has stayed constant over the last two decades, according to an Associated Press report.

While that is smaller than other professions where the gender gap is more pronounced, it still would add up to $150,000 over a long career, states Ulrike Muench, a professor at the University of California San Francisco who authored the study, according to the report.

Muench said she was “surprised” to see the gap so persistent, even in a female-dominated profession like nursing. Her study was published in the Journal of the American Medical Association.

The study found that male nurses in 2013 earned an average of about $70,000 annually, versus $60,000 for women, although the real gap is about half that because of factors like years of experience and cost of living based on geographic area.

The gap was most pronounced within the profession in nurse anesthetists, where the difference is a whopping $17,300 between the genders, versus $4,000 in middle-management nursing, the smallest gap.

The gap has remained the same despite the fact that wages have been rising steadily for both genders since 1988.

Just 10 percent of nurses nationwide are men out of 2 million who are registered, although that figure is rising. It once stood at 3 percent back in 1970.

So why is there still such a gap in a field where women are well-established? Muench offered some possible explanations. One is that female nurses sometimes leave the workforce to have children, which can impact the pay scale. Also, male nurses, like in other professions, may do a bettr job than their female counterparts in negotiating pay raises. Finally, there is the obvious possibility: gender discrimination. However, more studies will be needed to determine exactly what is the cause, Muench noted.