All around the world, women studying geoscience are half as likely to receive outstanding recommendation letters as men, new research shows. This is true no matter what region they come from, or whether the person writing the letter is a man or woman.

Previous research had already shown that recommendation letters for women are usually weaker than the ones men get. But today’s study, published in Nature Geoscience, is the first to look at letters from around the world and to focus on the field of geoscience, also called earth science.

“The study uncovers a very real problem in the entire field,” said lead author Kuheli Dutt, assistant director of academic affairs and diversity at Columbia University’s Lamont-Doherty Earth Observatory. She hopes that the growing body of research on sexism in STEM will push universities and administrators to take a closer look at how they can eliminate gender bias.

Just like all of STEM, the field of geoscience is dominated by men. Women receive 40 percent of the geoscience doctorates, but they are only 10 percent of the professors. Previous research has shown the biggest “leak” in the pipeline, so to speak, happens to women applying to post-doctoral positions after receiving their PhD, says Dutt. So her team decided to zero in on this crucial part of the career ladder to see whether bias in recommendation letters might be holding women back.

These results hopefully make it easier for academics to talk about sexism

The researchers analyzed the tone and length of 1,224 recommendation letters written for applicants to a competitive geoscience post-doc position at a university in the US. These letters came from 54 countries. The team read through the letters, which had identifying information redacted, and ranked their tone as “doubtful,” “good,” and “excellent.” The same proportion of men and women received “doubtful” letters, which was 2.5 percent of the total. “Good” letters indicated that the candidate was a solid applicant who could do what was needed. “Excellent” letters, instead, used phrases like “outstanding,” “superior,” and “a scientific leader in the making.” They made the candidate sound like he was truly head and shoulders above the rest, not merely a competent scientist.

Next, the researchers crunched the numbers. They found that female applicants were only half as likely as male applicants to receive an “excellent” letter rather than a “good” one. No one region rated women more negatively, though. And the results were the same regardless of whether the letter-writer was male or female. Women didn’t write more positive letters for other women.

The study has some limitations. The authors didn’t see the resumes of the applicants, so they couldn’t statistically rule out the possibility that male applicants were actually better qualified. (It’s very unlikely that female geoscientists worldwide are all worse than their male peers, says Dutt.) They also couldn’t track the outcome, so they didn’t know if, perhaps, female applicants were consistently chosen despite more lackluster letters, notes Juan Madera, a University of Houston professor who wasn’t involved with the study.

These results hopefully make it easier for academics to talk about sexism, Dutt says, and eliminate gender bias in the field. Dutt now wants to repeat the study with recommendation letters from 2012 to 2017. (The letters used in this study were from 2007 to 2012.) “Over the last decade there has been a greater awareness in higher education of these sorts of issues of diversity, so it would be interesting to see whether we see any real differences now,” says Dutt.