When Bethany Hughes was diagnosed with Asperger's syndrome, her parents were both relieved and devastated.
She showed many of the signs of autism even at preschool.
"The teachers told us that she was not mixing, she was sitting in a corner playing with little objects," said her mother, MaryAnn Hughes.
"Noise was a very big one. She didn't like the lawnmower, she didn't like trucks going past.
"Balloons were another thing. We never went to birthday parties because if there were balloons she'd melt down. She hated balloons popping.
"When we finally got the diagnosis, for me it was a relief because I knew that now we would get support. It was also devastating because we realised that her life would not be the same from now on."
But she was a girl. Autism or Asperger's syndrome, which was seen as a milder form of autism, is usually identified in boys. A recent American study found that 2.7 per cent of boys, but only 0.7 per cent of girls, have been diagnosed with the condition.
Bethany, who was diagnosed at 5 by an Invercargill paediatrician and turned 18 this week, doesn't know any other girl with autism.
But a new book, Girls and Autism , suggests that we are simply failing to identify most girls with the condition.
"Girls on the autism spectrum are typically diagnosed later, and with more extreme behaviours than autistic boys, and those girls without additional intellectual or behavioural problems are likely to 'fly under the radar'," the authors write.
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One of the authors Barry Carpenter , a professor of mental health at Britain's Oxford Brookes University, will speak on the issue at a seminar in Auckland next Saturday organised by the Special Education Principals' Association .
Organiser Judith Nel said about 90 boys, but only 30 girls, out of her roll of 150 at Parkside Special School in Pukekohe had been diagnosed with autism.
"Boys are more overt. Their behaviours are more readily diagnosed," she said.
"Often girls are able to hide their anxieties by just sitting there and being quiet."
The Ministry of Health says autistic people typically have difficulties understanding spoken and non-verbal language and social interactions, and "may engage in restricted, obsessive or repetitive behaviours".
Bethany Hughes finds it "difficult to pick up on social cues".
"I wouldn't be able to pick up on whether someone was angry with me or if they were pleased with me for something I had done," she said.
She likes things to be "very organised".
"If things don't happen the way I predicted, then that puts me out a lot," she said.
And she has an interest in conservation, especially of birds, which she doesn't see in her "neurotypical" classmates at Invercargill's Verdon College.
"It's an obsession," she said. "I just think looking after nature is so important. It's something I have always felt passionate about since about the age of 5."
Canadian psychiatrist Meng-Chuan Lai writes in the book that brain imaging studies have found typical differences in the brain between autistic and neurotypical people, and also between males and females.
Scientists had found some evidence of a "female protective effect" in typical female brain structures protecting against autism.
"An example is the so-called 'default network', a brain circuit linking midline and lateral structures, which critically underlies cognitive processes of understanding one's own and others' thoughts and emotions," he writes.
But the research is new, and Lai writes: "A definitive answer awaits further studies with autistic females and replicated findings."
In a speech to the support group Altogether Autism last month, Bethany Hughes said she was "deeply grateful for my Asperger's syndrome".
"It has given me a powerful imagination, unique insight for detail, honesty to say what needs to be said, and unparalleled curiosity and passion for what is important to me," she said.
"Without autism, I might never have experienced the awe of nature and the immense value of caring for it, or experienced the joy of my creativity coming to fruition.
"Every one of the challenges I have faced with autism has, in the long run, made me a much stronger, kinder and wiser person than I was before. I wouldn't trade my autism for anything."
People with autism have a delay or difficulty in three areas of development:
1. Language skills : they have trouble understanding and using spoken language and non-verbal communication such as facial expressions and body language.
2. Social behaviour : they have trouble understanding social interactions, which affects their ability to play or interact with others.
3. Cognitive and thinking skills : they have trouble thinking and behaving flexibly, and may engage in restricted, obsessive or repetitive behaviours.
Source : Ministry of Health .