Angela Ahrendts is about to leave Burberry; Liv Garfield is soon to run Severn Trent; the privatised Royal Mail may soon join the FTSE 100 club with Moya Greene at the top. But one thing never seems to change: there are only a handful of female chief executives of FTSE 100 companies. Why?

There is, of course, more than one force at play. Discrimination surely remains important, as anyone can see if they pay attention to how women are often treated in business environments. For those wanting a higher standard of evidence, consider a famous study by economists Claudia Goldin and Cecilia Rouse. They demonstrated that when the top US orchestras, which were dominated by men, introduced blind auditions for new members, women became several times more likely to be offered jobs.

Another obstacle to women’s achievement is motherhood. Goldin, with Lawrence Katz and Marianne Bertrand, studied alumni from Chicago’s Booth School of Business. Men and childless women have almost indistinguishable earnings; once women start having children, the gap between them and men begins to widen. (Why this might be is itself a good question.)