Boys are falling further behind girls at GCSE because they "lack the maturity" to aim for top grades, it has been suggested.
National GCSE results have revealed a widening gender gap, especially at A* and A grade as boys struggle to keep up with their female classmates.
More than one in four (26.5%) of girls' entries were awarded an A or A* this summer, compared with 19.8% of boys' exams, according to data published by the Joint Council for Qualifications (JCQ).
This is a gap of 6.7 percentage points - the widest it has been since the A* grade was introduced in 1994. It is the opposite of A-levels, where last week's figures showed that boys are closing the gap in top grades.
The gender gap has also widened at A*-C for GCSE. This year 66% of boys' exams gained at least a C compared with 73.5% of girls', a difference of 7.5 percentage points. Last year the gap was 7.2%.
Brian Lightman, general secretary of the Association of School and College Leaders, said: "At A-level, boys are clearly very focused on the grades they need for university entry. Boys will focus on a means to an end and they will therefore aim for the end, and we've seen an increase in that.
"At GCSE perhaps that sort of maturity that girls have at that stage is not quite there with the boys, and they can't see that obvious reason to aim for the A*."
Andrew Hall, chief executive of the AQA exam board, admitted he did not know why there was such a gap between boys and girls at GCSE level, saying: "We've scratched our heads over this. I think there's a good question for some social psychologists."
Around 650,000 teenagers in England, Wales and Northern Ireland received their GCSE results on Thursday in what was another record year for results.
The pass rate rose for the 23rd year in a row, with almost seven in 10 entries (69.8%) gaining at least a C grade, and nearly one in four exams (23.2%) were awarded a coveted A or A*, up from 22.6% in 2010.