The perceived wisdom is that autism is a male disorder, four times more common in boys than girls, yet many girls with autistic spectrum problems may be struggling at school because their condition is going undiagnosed, according to recent research from Great Ormond Street Hospital for Children in central London.
The study of nearly 600 children found that girls with higher functioning autism or Asperger's syndrome tend to behave very differently from boys with the same condition, trying to mask their difficulties by copying the behaviour and speech of others. Girls also appear to focus their obsessions on individuals or animals, while boys are more likely to fixate on objects such as traffic lights, timetables and characters, such as Thomas the Tank Engine.
Experts also suggest that girls with autistic spectrum disorders may experience more difficulties than boys in socialising and adjusting to puberty. "Girls tend to internalise their problems more than boys; they are more likely to be compliant, and less likely to express their difficulties through aggressive behaviour," says Judith Gould, director of the National Autistic Society Lorna Wing Centre for Autism in Bromley, Kent.
Instead, girls with Asperger's syndrome may seem extremely shy or withdrawn, sometimes becoming depressed or anorexic teenagers. An earlier study by American academics (Lainhart and Folstein, 1994) found that, despite the high male-to-female ratio for autism, half of the autistic patients with depressive illnesses were female.
Adolescence is a particularly testing time for girls on the autistic spectrum. "Puberty is far worse for girls with autistic spectrum disorders than boys," says Dr Gould. "Those with autism find any form of change difficult, yet girls have to learn how to cope with menstruation, as well as with the high demands our society places on teenage girls."
From her experience as a mother of two daughters with autism, Nicky Clarke believes it is far harder to diagnose girls who have Asperger's syndrome or higher functioning autism. Her 12-year-old daughter Emily, who has classic autism, was diagnosed at the age of three over a period of six weeks, but it took 18 months to reach a opinion about her elder daughter, by which time she was almost 11. Lizzy, now 15, has Asperger's syndrome.
"While Emily had no spoken language and was fixated on particular objects, Lizzy was very verbal, forward and confident," Mrs Clarke recalls.
"I think it is also a lot more difficult to diagnose girls as they don't show the aggression boys can sometimes display."
Shana Nichols, a US psychologist who works specifically with girls on the autistic spectrum, believes that adolescent females with autism disorders display more violent mood swings than their peers.
Girls' mood swings are not as easily identifiable as boys' either, says Jenny Wright, who is head of Hill House School, near Lymington in Hampshire, which specialises in teaching pupils of secondary school age who have severe autism.
"With boys, we can quickly work out where their frustration is coming from; with girls, it's not so obvious," she says. "This is not surprising as girls have many more changes to adapt to during puberty than boys. We tend to back off and have lower expectations if it's a girl's time of the month."
Socialising is another challenge for teenage girls with Asperger's syndrome. Dr Nichols suggests that boys tend to have more impairments earlier on in life, whereas for girls, social difficulties tend to emerge more in early adolescence.
The American research indicates that teenage girls tend to develop intricate social networks in which feelings play a crucial part. As a result, girls with autistic spectrum disorders in mainstream schools find it harder to join in, while boys are often able to forge links with their peer group simply through playing computer games or football.
"Girls usually are far more demanding socially; they are more catty and they use more jokes and innuendo than boys, which makes it much more difficult for girls to fit in with their peer group," says Dr Gould.
Girls with autistic spectrum disorders want to make friends, but they don't have the skills. As a result, they can fall victim to subtle forms of bullying from other girls, such as gossiping, spreading rumours or exclusion from the group. Ultimately, social rejection can spiral into depression.
One survival mechanism used by girls may be to keep quiet so as not to draw attention to themselves. The problem is that girls also become reluctant to ask for help. Dr Gould advises schools not to make a girl's diagnosis obvious to her peers and to use mentoring discreetly.
For parents, these issues make it difficult to choose an appropriate school. At first, Lizzy was happy at her mainstream primary, but when she was badly bullied at secondary school, her parents decided to send her to a small independent school.
"Lizzy had become borderline depressed," says Mrs Clarke. "The impact of being in a large mainstream secondary environment was overwhelming; the requirements to move between different rooms and different teachers.
"The pressure to conform socially was also stressful. The other girls were more interested in boys, fashion and music than Lizzy. They would mimic her, call her names or run away from her in the playground, and she would leave school in tears every day."
Funding constraints can limit the success of inclusiveness in mainstream schools, making it hard for them to adapt to a wide range of special needs.
"In the main, there is a determination and willingness to make it work, but there needs to be more flexibility and a distinction between inclusion and mainstreaming," says Mrs Clarke. "Too often you hear: `We need her to do this . `. It seems that the child must be seen to be adapting, rather than the school being flexible."
She would like to see more lunchtime clubs, as pupils with autistic spectrum disorders find it difficult to cope with unstructured time. Lunch breaks can be peak times for bullying.
A tricky issue for teachers can be that very bright teenagers with Asperger's syndrome sometimes seem arrogant and domineering as they have no idea of social hierarchy. Dr Gould suggests that teachers try not to take this personally and instead nurture the talent these pupils frequently display.
Because of their volatile behaviour, pupils with Asperger's syndrome, especially girls, can be susceptible to being bullied. However, difficulties with eye contact and other communication skills mean they sometimes find it impossible to tell staff when they are being bullied, so allowing them to describe what is happening to them in drawing or writing could help.
Whatever the difficulties, teachers need to see all pupils with autistic spectrum disorders as individuals, says Gillian Roberts, head of the National Autistic Society Robert Ogden School in Rotherham, South Yorkshire.
"It's important to know pupils and their families as individuals and respond to them all with empathy," she says. "Try to see where they are coming from."
For her part, Lizzy says she prefers those teachers who ask her privately if she needs extra help, otherwise it can be embarrassing. Her favourite teachers are those who encourage her. "When staff reward me in any shape or form, it makes me feel more confident," she says.
After taking part in Dustbin Baby, a recent BBC adaptation of Jacqueline Wilson's teen novel starring Juliet Stevenson, Lizzy now thinks she would like to become an actress, clearly demonstrating that, with the right help, it is possible to overcome the problems of being a teenage girl with autism.