Girls are routinely given more generous grades in class than boys with the same academic abilities, according to a new study.
Drawing on findings from tens of thousands of pupils and their teachers, researchers in Italy determined it was a systemic issue and set about theorising as to why it might be.
Their work, published in the British Journal of Sociology of Education, suggests teachers may subconsciously reward students exhibiting traditionally female behaviour, such as quietness and neatness, which makes life easier for them.
Another theory is that inflated grades in maths are a way of trying to encourage girls, who are often seen as weaker in this subject.
This bias against boys could mean the difference between a pass and a fail, and could have wider consequences in areas such as college admission, job choice and earnings, the researchers warn.
"There is a strong correlation between having higher grades and desirable educational outcomes, such as gaining admission to good colleges or having a lower probability of dropping out of school," said Ilaria Lievore.
How was the study carried out?
The researchers, based at the University of Trento, began their work based on test results which revealed an inconsistency between standardised exam grades and those awarded by teachers.
Girls typically outperform boys in humanities, languages and reading tests, while boys do better in maths.
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But when grades are awarded by teachers, girls do better in all subjects.
The researchers compared the scores 38,957 Year 10 students received in standardised tests of language and maths with the grades they achieved in their classroom exams.
The standardised tests were set nationally and marked anonymously, while the classroom exams were set in the classroom and marked by their teachers.
In line with previous studies, the girls performed better than the boys in the standardised tests of language, while the boys were ahead at maths.
But the teachers put the girls ahead in both subjects. The girls' average grade in language was 6.6 out of 10, while the boys got 6.2. In maths, the average for the girls was 6.3, while the boys averaged 5.9 - below the pass mark of 6.
The analysis also showed that when a boy and a girl were similarly competent at a subject, the girl would typically receive a higher grade.
What could be behind the gap in grades?
Researchers looked at a number of factors, from the type of school to the size of classes.
They also considered the characteristics of the teachers, such as how experienced they were and their gender.
The only factors found to have an impact were both in maths. The gender gap in grades was found to be greater when classes were larger, and girls were found to be further ahead of boys in technical and academic schools.
None of the other factors had any significant effect in reducing the gender grading gap, leading the researchers to warn of systemic problems.
