It is a condition on the autistic spectrum that has long been known to affect boys, who may have obsessive interests or struggle to make friends. Now an expert says many more girls have it than was thought, and failure to diagnose them can lead to misery and self-harm. Amelia Hill reports

Doctors are failing to diagnose thousands of girls who have Asperger's syndrome, according to one of the world's leading experts. Dr Judith Gould has accused the medical world of missing and overlooking girls with the condition, condemning them to lives of such misery that many resort to extreme self-harm and anorexia.

Gould and her colleague, Lorna Wing, carried out ground-breaking research into the link between Asperger's syndrome, autism and other pervasive developmental disorders in 1979. Exploiting that insight, they pioneered the concept of the autism spectrum. Now Gould, a chartered consultant clinical psychologist with more than 35 years' experience in autism spectrum disorders, has called on the government for a packet of measures to help girls with Asperger's.

Gould, who is director of the National Autistic Society's Lorna Wing centre for autism and co-founder of the Centre for Social and Communication Disorders, said: "We're failing girls at the moment. We are doing many thousands of them a great disservice. They are either not being picked up in the first place, but if they ask for help they are being turned away. Even if they are referred for diagnosis, they are commonly rejected."

The government is about to launch a consultation on a new national strategy on autism. Gould and the National Autistic Society want the final strategy - due at the end of the year - explicitly to address the misconceptions about gender that can make accessing help, support and services particularly difficult for girls and women.

"Women tell us that these misconceptions can make their particular battles and struggles even more difficult," said Jane Asher, the society's president. "They say that getting a diagnosis in the first place can often feel like an insurmountable hurdle, with many doctors unaware that the condition can affect females."

More children are being diagnosed with Asperger's today than ever before. A decade ago one in 1,000 children in the UK was thought to have an autism spectrum disorder (ASD). Five years ago that had increased to one in 500. Today the figure stands at one in 100.

It remains unclear as to whether the increase in diagnoses is caused by a true increase in the disorder, or is the result of increased awareness of autism and its broad characteristics.

Even less well understood, said Gould, is the difference in prevalence rates between boys and girls. The statistic most commonly reported is that ASDs are four times more common in males than in females. Many clinicians, however, believe that the ratio is as high as 16 boys to every girl. But Gould believes that significantly more girls have the condition than is recognised; she estimates the ratio to be 2.5 boys to every girl.

"Girls are not being picked up because there is still a stereotyped view of what Asperger's is, which is based entirely on how boys present with the condition," she said. "Professionals are not up to speed in knowing how girls present. We are working with the government to ensure they highlight this concern in their upcoming consultation. We are hoping to convince them to target this much under-investigated but vitally important issue."

Tony Attwood, founder of the first diagnostic and treatment clinic for children and adults with Asperger's, and author of The Complete Guide to Asperger's Syndrome, agreed with Gould's estimation of a 2.5:1 ratio of boys to girls. "The bottom line is that we understand far too little about girls with ASDs because we diagnose autism based on a male conceptualisation of the condition. We need a complete paradigm shift," he said.

"We need to draw up a female version of Asperger's that identifies girls on the basis of the way they present, and we need to do this as a matter of urgency: undiagnosed Asperger's can create devastatingly low self-esteem in girls. In my experience, up to 20% of female anorexics have undiagnosed Asperger's."

Girls slip through the diagnostic net, said Attwood, because they are so good at camouflaging or masking their symptoms. "Boys tend to externalise their problems, while girls learn that, if they're good, their differences will not be noticed," he said. "Boys go into attack mode when frustrated, while girls suffer in silence and become passive-aggressive. Girls learn to appease and apologise. They learn to observe people from a distance and imitate them. It is only if you look closely and ask the right questions, you see the terror in their eyes and see that their reactions are a learnt script."

Girls also escape diagnosis, said Attwood, because they are more social than boys with the condition. Their symptoms can also be missed because it is the intensity of their interests that is unusual, and not the oddity of what they do.

"The impairments to their social life or interests tend not to stand out in the same way as boys' do," he said. "They might have one friend, while boys with the condition won't have any. Also, boys hyperfocus on facts and certain interests, such as trains or weather. Girls escape into fiction. They have imaginary friends, live in another world with fairies and witches, obsessively watch soap operas or become intensely interested in celebrities."

Professor David Skuse, head of the behavioural and brain sciences unit at the Institute of Child Health, teaches clinicians to diagnose the condition. "Increasingly fewer girls are diagnosed as their IQ reaches 100, the population average," he said. "Some people maintain this is because girls simply don't have Asperger's, but I would argue that brighter girls, especially those who are more verbal, are able to mask and compensate for their condition. I make sure I emphasise the difference in the ways boys and girls present when I train clinicians, because I am certain that girls are being failed by the system, especially those with higher IQs," he added. "My belief is that, if we can prove the ratio of boys to girls is as high as many of us suspect, it would be as significant a milestone in this field as the discovery that the condition is on a spectrum."