Girls believe they are cleverer, better behaved and try harder than boys from the age of four, research suggests.

By the age of eight, boys had also adopted these perceptions, the study from the University of Kent found.

Researchers questioned 238 children in two Kent primary schools. They warn the stereotypes could become a "self-fulfilling prophecy".

Boys lag behind girls in nearly all subjects at A-level and GCSE.

The research is being presented by Bonny Hartley at the annual conference of the British Educational Research Association on Thursday.

In the study, children, aged between four and 10, were presented with a series of statements such as "this child is really clever" and "this child always finishes their work".

The children were asked to point to a picture, in silhouette, of a boy or of a girl to say who they thought the statement applied to.

'Silly boys'

On average, girls of reception age right through to Year 5 said girls were cleverer, performed better, were more focused and were better behaved or more respectful, the study found.

Boys in reception, Year 1 and Year 2 gave answers which were equally split between favouring boys and girls, but by Year 3 their beliefs were in line with those of the girls, the researchers said.

Ms Hartley said that children of both genders thought, in general, that adults believed that girls did better than boys at school.

In a separate investigation, she tested two separate groups of children in maths, reading and writing.

The first group was told that boys do not perform as well as girls, but the other was not.

Boys in the first group performed "significantly worse" than in the second group, which Ms Hartley says suggests that boys' low performance may be explained in part by low expectations.

She also warns against the use of phrases such as "silly boys" and "school boy pranks" or teachers asking "why can't you sit nicely like the girls?"

In GCSE results published last week, 25.5% of entries by girls were awarded grade A or A*, compared with only 19.5% for boys.

The gap is narrower at A-level, with 27.6% of entries by girls awarded an A or A*, compared to 26.1% of entries by boys.