A key reason boys lag behind in the classroom is revealed for the first time - female teachers.
Research shows that boys lower their sights if they think their work is going to be marked by a woman because they believe their results will be worse.
It also shows their suspicions are correct - female teachers did, on average, award lower marks to boys than unidentified external examiners.
Male teachers, by contrast, awarded them higher marks than external examiners.
The findings, published by the Centre for Economic Performance, an arm of the London School of Economics, could have immense repercussions for boys because of a disparity in male-female teachers.