Girls in French secondary schools are benefiting from a marking bias by maths teachers, finds research.
The girls were given 6% higher marks than boys for similar work, says the study by the London School of Economics and Paris School of Economics.
The boost encouraged girls to take science subjects later in their school careers, say the researchers.
The study analysed the records of almost 4,500 11-year-olds at 35 secondary schools.
All French 11-year-olds take standardised tests in French and mathematics at the start of the school year.
This test is externally graded and the examiner has no information on pupils' names, gender, social background or schools so the results are free from any bias or stereotyping.
In these anonymised tests, the boys outperformed girls in mathematics.
The researchers compared these results with those from teachers' assessments of pupils on end-of-term report cards.
In the teacher assessments, the girls did better than boys in maths.
French schools perform another set of anonymised tests three years later, when pupils are 14.
Analysis of these results found the girls had caught up and even overtaken the boys by the age of 14 in maths.
The researchers suggest the improvement may result from "encouragement" generated by grade bias in teacher assessments during the intervening years.
The researchers note that not all teachers discriminate and some discriminate more than others.
The analysis suggests that girls who benefited most from biased marking in maths early on were more likely both to continue with academic education and to take science subjects at 16.
"Altogether, these results show that positively rewarding pupils has the potential to affect their progress and course choice," said author Camille Terrier from the London School of Economics.
"Since we note that marks in maths influence the progress of students, they could be a way to reduce the inequalities in achievement between boys and girls."
She said the strategic use of enhanced marks could also be a way of encouraging boys "to eliminate their lag" in humanities subjects.