SEXISM is among the prime suspects for the scarcity of female professors. Yet proving that bias against women is widespread in academia—or even exists at all—is tricky. But a forthcoming paper in the Journal of the European Economic Association rises to the task.

This paper’s authors, Friederike Mengel of the University of Essex, in Britain, Jan Sauermann of Stockholm University, in Sweden, and Ulf Zölitz of the Institute on Behaviour and Inequality, in Bonn, Germany, used data from nearly 20,000 student evaluations of instructors. These were made between 2009 and 2013 at the School of Business and Economics at Maastricht University, in the Netherlands. The students on each course had been assigned, randomly, either a male or a female instructor, and filled out end-of-course evaluations before they knew their grades. Half of the students involved were German, a third were Dutch and the rest mostly from other European and some Asian countries.

The results are both striking and disturbing. Ranked on a scale from zero to 100, the evaluations place female instructors an average of 37 slots below male ones. Students taught by women gave lower ratings even to teaching materials that were the same for all course instructors, such as the textbooks and the online learning platform.

Women did not appear to be worse teachers. On average, the students taught by women achieved the same course grades as those taught by men, and spent a similar amount of time studying for the course on their own. Neither did an instructor’s sex affect how much students’ overall grades improved—which suggests that female instructors added the same amount of value to students’ learning as did male instructors. And when the researchers stripped out the estimated gender bias from the evaluation scores, the difference in ranking between male and female instructors shrank from 37 rungs to five on the zero-to-100 ladder. That difference is statistically insignificant.

Both male and female students gave worse ratings to female instructors, though the men were much more prejudiced. Most worryingly, the bias was particularly pronounced in the case of junior instructors, for whom student evaluations are much more crucial for teaching awards, tenure decisions and even salary negotiations.

There is a broader lesson in all of this. The study was conducted at one of Europe’s top business schools. Its graduates, presumably, are destined for great things in the fields of commerce and industry. If the biases they hold against the competence of their female teachers stick with them at the office, equality of the sexes in the workplace has a steep hill to climb.