Behaviours relevant to autism are more frequently observed in boys than in girls, because of naturally occurring sexual difference in characteristic of all children, research has found.

Autism spectrum disorder (ASD) is characterised by social impairments, communication difficulties and restricted, repetitive and stereotyped patterns of behaviour.

"The results imply that there may be an over representation of boys with autism, based on sex differences that affect all children," said principal investigator of the study Daniel Messinger, professor of psychology at University of Miami in the US.

The study followed a large sample of boys and girls at high-risk for the disorder and other children at low risk beginning before 18 months of age.

One in four high-risk boys were identified with the disorder at three years, compared to one in 10 high-risk girls.

The researchers examined how this difference in male-to-female risk of autism spectrum disorder emerged.

They found that boys with ASD had higher levels of a particular autism symptom (stereotyped behaviours) than girls.

The boys with ASD generally had less advanced cognitive and language functioning than the girls.

However, the sex differences in stereotyped behaviours and cognitive functioning were also present in children without ASD.

"We found that girls look a little better than boys in almost every area," Messinger said.

"Children with ASD show typical differences between boys and girls, even though - by virtue of having ASD - they clearly have higher symptoms and cognitive difficulties."

"Our results are important because they show that naturally occurring sex differences characteristic of all children are behind the sex differences we see in autism," he added.

The study was published in the journal Molecular Autism.