Registered nurses who are male earn nearly $11,000 more per year than RNs who are female, new research shows. Only about half of that difference can be explained by factors such as education, work experience, and clinical specialty.

That leaves a $5,148 annual salary gap that effectively discriminates against women, who make up the vast majority of the nursing workforce, according to a study published Tuesday in the Journal of the American Medical Association.

With nurses earning an average of $66,973 per year, that $5,148 amounts to an 8 percent bump in pay for men.

About 2.5 million women - and the families they support - are being shortchanged by the gender-based pay difference, say the researchers who conducted the study.

"Given the large numbers of women employed in nursing, gender pay differences affect a sizable part of the population," said study leader Ulrike Muench, a nurse practitioner with a doctorate from Yale who studies nursing, health policy and health-care economics at UC San Francisco.

"We hope that our results will bring awareness to this important topic," she said in a statement.

Muench and her colleagues examined two decades' worth of salary information from the National Sample Survey of Registered Nurses. Before the survey ended in 2008, it collected data once every four years from more than 30,000 RNs across the country. Altogether, the study sample included responses from 87,903 full-time RNs, 93 percent of whom were women.

In the raw analysis, the average salaries for men were $10,775 higher than for women, the researchers found. That discrepancy can be seen in every survey year going back to 1988. Though the gap appeared to narrow in the middle and late 1990s, it widened again after 2000.

Even after the researchers accounted for things like location, hours worked per week, years of experience and type of nursing degree, men still earned $5,148 more than women, on average.

For some nursing specialties, the gap was even greater. In cardiology, for instance, male RNs earned $6,034 more than their female counterparts. Only one specialty - orthopedics - had a pay gap too small to be statistically significant, meaning that the difference might have been due to chance.

Workplace mattered too. Nurses who cared for hospital patients took home $3,873 more per year if they were men, according to the study. In outpatient settings, men earned $7,678 more than women.

To see whether the situation had improved since 2008, the researchers compared the salaries of more than 200,000 RNs who took part in the U.S. Census Bureau's American Community Survey between 2001 and 2013. In this sample, 10 percent of the nurses were men - and they averaged $9,562 more per year than the women, based on the unadjusted analysis.