Boys perform worse at school because they are constantly being told that girls are more intelligent, according to a new study.
Stereotypes about boys being intellectually inferior to girls affect children during the first years of primary school and prevent them fulfilling their potential, researchers found.
Teachers should assure boys that they are just as academic as girls and avoid doing anything which could make them feel inferior such as splitting classes by gender, they said.
In the first stage of the study, researchers presented 238 boys and girls aged four to 10 with a range of scenarios related to behaviour or performance, such as "this child really wants to learn and do well at school".
The children were asked to guess who the situation applied to by pointing to a silhouette of either a boy or a girl.
The results, published in the 'Child Development' journal, showed that by the time girls are aged four and boys are seven, they equate girls with better behaviour and higher achievement at school.
Researchers also found that the children believed adults shared the same opinion as them, meaning that boys felt they were not expected by their parents and teachers to do as well as girls and lost their motivation or confidence as a result.
Further tests revealed that belief in their own academic inferiority could translate into lower school grades among boys.
A group of 162 seven and eight year olds was asked to sit a series of reading, writing and mathematics tests but half were told beforehand that boys were expected to score worse than girls.
Boys who were told they should expect lower marks performed worse in the test than those who were not given any information but girls' marks were unaffected, suggesting stereotypes about male inferiority harm boys but do not help girls.
In contrast, when boys were told that they were expected to perform equally well as girls, their marks improved compared with those who were not given any expectations. Girls' performance was again unaffected.
Official figures have shown that boys begin to lag behind girls by the age of five, with past research blaming the "gender gap" on biological differences, different learning styles, teachers' attitudes, a lack of male role models and even the "feminisation of the classroom".
Researchers said the new findings could advance our understanding of why boys fall behind at school by highlighting the significant role of stereotypes.
Bonny Hartley, a PhD student at the University of Kent who led the study, said: "A lot of the theories so far have been much more biologically based – the idea that boys and girls have different brains or that you need to teach them differently.
"Even if there are biological differences, as soon as you believe there are differences they can become self-fulfilling, so if anything we want to challenge those differences rather than telling boys and girls they are different."
Parents, teachers and even television programmes should be careful to avoid encouraging stereotypes which could harm boys' development, she added.
"These studies suggest that negative academic stereotypes about boys are acquired in children's earliest years of primary education and have self-fulfilling consequences.
"They also suggest that it is possible to improve boys' performance, and so close the gender gap, by conveying egalitarian messages and refraining from such practices as dividing classes by gender."