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.
Professor Alan Smithers, a leading academic researcher, called the findings "fascinating".
The research was carried out among 1200 children in 29 British schools.
It also revealed that girls tried harder if they had a male teacher because they believed they would get better marks.
Their suspicions were not borne out, though, as the male teachers tended to give them exactly the same marks as the external examiners.
The study showed that boys believed their answers would be 3 per cent lower if marked by a woman. The girls thought their results would be 6 per cent higher if marked by a man.
Professor Smithers saidit stressed the importance of independent marking for high-status examinations.