George Doyle/Getty Images

A review of starting salaries for new physicians leaving residency programs in New York State in 2008 found that men made $16,819 more per year than women. In 1999, that salary difference was only $3,600. In other words, the gender gap in doctor pay has more than quadrupled in less than 10 years.

The findings, published in the February issue of Health Affairs, are a bit of a mystery, considering that women are increasingly entering higher-paying fields in medicine. “It is not surprising to say that women physicians make less than male physicians because women traditionally choose lower-paying jobs in primary care fields or they choose to work fewer hours,” said lead researcher Anthony Lo Sasso, a professor at the School of Public Health at the University of Illinois-Chicago, in a statement. “What is surprising is that even when we account for specialty and hours and other factors, we see this growing unexplained gap in starting salary. The same gap exists for women in primary care as it does in specialty fields.”

The percentage of female doctors taking less lucrative primary care positions fell from 49% in 1999 to 34% in 2008, the researchers found. Yet, when broken down by specialty, women were still making less than their male counterparts in every field except general surgery, in which women earned an average of $11,000 more per year than men.

Researchers theorized that women are choosing quality-of-life benefits like flexible schedules and not being on call during certain hours, rather than higher pay, when negotiating starting salary.