A key reason why boys lag behind in the classroom is revealed for the first time today – female teachers.

Ground-breaking 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.

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The findings, published by the Centre for Economic Performance today, could have immense repercussions for boys because of the dearth of male teachers in the profession. Only 15 per cent of primary school staff are men.

The findings were yesterday described as "fascinating" by one of the country's leading academic researchers, Professor Alan Smithers, of the Centre for Education and Employment at the University of Buckingham.

He said the research, carried out among 1,200 children in 29 schools across the country, had shown a possible reason for the glaring gap in performance between girls and boys right through schooling.

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 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 said: "It is fascinating research. It does stress the importance of independent marking for high-status examinations."