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...

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