Girls with autism may not be identified because they do not show traditional signs of the disorder, an expert warns.
Boys and girls with autism may show different signs of the disorder
Children with autistic spectrum disorders have poor social and communication skills.
Hyperactivity, and interests in technical hobbies have been seen as characteristics of the disorder.
But Christopher Gillberg, of the National Centre of Autism Studies, said girls were often passive and collected information on people, not things.
Around 535,000 people in the UK are estimated to have autistic spectrum disorders.
The number of boys diagnosed is much greater than the number of girls, but Professor Gillberg said the difference in incidence may not be as great as currently thought.
His theory is partly influenced by studies which did not find what they were expected to.
Researchers had looked at the male X chromosome, to see if genetic faults there could influence a boy's risk of developing the condition.
But no conclusive link has been found.
Professor Gillberg said: "Scientists had been very surprised that, so far, so little has come out of research into the X chromosome.
"But it may be that girls present differently to boys.
"The number of females with autism spectrum disorders may be under-diagnosed."
He said studies, including one his team had carried out into women with anorexia who were also autistic, as well as his own clinical practice, had shown the gender difference.
He added: "Autism may be behind many cases of anorexia. A girl may be withdrawn and uncommunicative, without attracting attention, but when she develops a calorie fixation it becomes a serious problem.
"Counting calories may be a manifestation of autism.
"I've seen quite a number of cases where the anorexia has become completely entrenched because people haven't understood that underlying the eating disorder is autism."
Professor Gillberg said that, at an earlier age girls with autism were likely to be more passive and not as active or aggressive as boys with autism are - and may be seen as simply shy.
"With some girls, there's a perception they are outsiders, someone who can't really mix with other children.
"They may tend to either avoid other children, or be on the periphery of the group."
He said boys were likely to show interest in technical or maths-related hobbies, whereas girls were more interested in people.
"They may have hobbies such as compiling books about their 'so-called' friends, and may make lists of their names and the colour of their eyes and hair, but not actually interact with them."
Professor Gillberg said girls may be perceived as simply shy, and parents and teachers may not realise there's a problem.
He added that differences in the way girls and boys learn to speak could also mask signs of autistic disorders.
"Girls tend to use language immediately, and use new words as soon as they hear them. Boys have longer periods of repeating what they know and processing what they are learning."
He said both genders may have the same combination of autism genes, but girls' natural linguistic ability may hide the associated language difficulties.
"Autistic spectrum disorders may be more difficult to pick up in girls, because they have superior linguistic abilities."
Judith Gould, director of the National Autistic Society's Diagnostic Centre, said: "We still know so little about this complex lifelong disability that it is essential we continue to question current thinking and suggest alternative theories for its prevalence.
"We would certainly agree we are probably missing autism in girls due to the different way in which it often manifests itself in females.
"We would also agree that anorexia, which is predominantly diagnosed in girls, could be linked to autism in an unknown proportion of cases."