In recent decades, women have been making significant headway in becoming dentists, doctors, and lawyers—professions which require a significant amount of education and postgraduate training. According to some theories, this growing number of women with advanced degrees should help bridge the gender pay gap. And yet, for all three professions, not only does the gender pay gap persist, the differences that can’t be explained by simple factors, such as hours worked or age, might actually be more pronounced than they are for women overall.

In the 1970s, women’s educational progress improved their earnings. However, more recently, researchers have found that education only helps so much. In other words, women can’t educate themselves out of the gender gap.

A new study by the American Dental Association (ADA) takes a look at dentists and physicians to determine whether these unexplained differences have gone up as the proportion of female workers grew. In 1982, only 3 percent of dentists were women; that number is now at 30 percent. The study used Census data to look at earnings in these three professions in 1990, 2000, and 2010.

“Dentistry is about to hit a tipping point where half of dental students are women. There are a lot of myths out there about gender differences among dentists and we wanted to see what the data showed in terms of labor supply and wages,” said Marko Vujicic, an economist at ADA and one of the authors of the study along with Thanh An Nguyen Le and Anthony T. Lo Sasso at the University of Illinois at Chicago.

The authors found that after taking into account the observable differences between male and female dentists—such as age, hours worked, number of children, and self-employment—the pay gap was still there. And the difference in earnings for female dentists was much larger than that of female doctors and lawyers.

For dentists and physicians, while the overall pay gap narrowed, the discrepancies that they aren’t able to attribute to concrete factors has grown. In 1990, 62 percent of salary differences between male and female dentists were due to unobservable factors. (This generally points to data limitations that prevent researchers from attributing the discrepancy to something that’s easy to deduce, such as different majors or hours worked. ) That’s compared with 66 percent in 2000 and 64 percent in 2010. For physicians, that number has risen from 54 percent in 1990 to 57 percent. A different pattern emerged for lawyers: Unexplained salary differences dropped from 66 percent to 55 percent. Overall, the researchers found that differences in age and hours worked were significant factors, which is in line with the body of research on the pay gap. They also found that being self-employed was a significant factor for female dentists.

There are caveats to this study’s findings. Plenty of variables are hard to measure, and Vujicic says that two significant factors might be a dentist’s area of specialty and the way dentists get paid: “We know women tend to go into lower paying specialties in dentistry. We also do not include whether you accept Medicaid patients. We know women are much more likely to accept Medicaid, and Medicaid pays low fees. So we definitely are missing some important variables and that is part of why the unexplained part is so high.”

These stubborn, unexplained gender differences in salary across a range of medical and other occupations call for further study. It’s also worrisome that past research has found a persistent trend of pay drops as women take over a profession. While statisticians usually refrain from speaking about what these unexplained differences are, economists are sometimes more candid: One of the complicated reasons that the gender gap persists might well be that female careers are shaped differently by cultural norms and societal forces. And those may take even more effort and time to remedy.