The causes of this attrition are manifold, but sexual discrimination is an indisputable part of it. Women in STEM repeatedly report experiencing sexual harassment, being mistaken for administrative staff, being forced to prove themselves to a degree that their male colleagues are not, being told to behave in more aggressive, outspoken masculine ways while simultaneously facing backlash for doing so.

And several careful experiments have shown that faculty members—both men and women—are more likely to spend their time mentoring men, to respond to emails from men, to call on men in classes, to rate (fictional) male applicants as more competent and hirable than identical female ones, and to hire a man for a job that requires math.

These biases, sometimes manifesting outrightly and sometimes insidiously, collectively create an environment where women feel like they don’t belong, like they aren’t valued, like the odds are set against them. Confidence falls, perseverance wanes, and careers die by a thousand cuts.

It begins early. Eddy has been studying the University of Washington’s undergraduate biology course for a few years to try and understand how biases play out among the students themselves. She teamed up with Daniel Grunspan, an anthropologist who’s interested in how information travels within groups. They surveyed three large classes of 196, 759, and 760 students respectively, asking them to nominate particularly strong peers at various points through the academic year. They found that men consistently received more nominations than women, and this bias only got worse as the year went on. The question is: Why?

Performance? Men got better grades than women in the three classes but the difference was only statistically significant in one; even then, the scores differed by no more than 0.2 of a grade-point average. Participation? The class instructors deemed more men than women to be “outspoken,” and Eddy’s previous work certainly showed that women comprise 60 percent of the students, but just 40 percent of the voices heard in class.

But even after adjusting for both these factors, the team found that male students still disproportionately nominated other men, giving them a boost equivalent to a GPA increase of 0.77. By contrast, the female students showed no such biases, giving other women a paltry boost of just 0.04 GPA points. As the team wrote, “On this scale, the male nominators’ gender bias is 19 times the size of the female nominators’.”

The team also found that the ‘celebrities’—the three students in each class with the most nominations—were all men. Sure, they had good grades and spoke up frequently, but they all had female peers who were equally outspoken, with grades just as high. As Grunspan and Eddy wrote, “It appears that being male is a prerequisite for students to achieve celebrity status within these classrooms.”