Girls' growing brains 'more resilient', study suggests
Autism is less common in girls than in boys because their brains are more "resilient", research suggests.
DNA errors need to be more severe in girls to cause neurodevelopmental conditions, according to a large study.
The research in the American Journal of Human Genetics suggests this may help explain why males appear to be at greater risk of disorders like autism.
A UK autism charity said it highlighted the importance of developing gender-specific approaches to autism.
A gender difference is seen in disorders such as autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD), with males more at risk than females.
In high functioning ASD, for example, one woman is affected for every seven men.
This has led to the theory that females are protected to some extent biologically, although others have proposed that a social bias leads to more boys than girls being diagnosed with the likes of ASD.
This research studied genetic data from more than 15,000 individuals with neurodevelopmental disorders, and about 800 families affected by ASD.
They found females diagnosed with ASD had a greater number of harmful genetic mutations than males, but needed to reach a higher threshold of bad mutations for them to develop ASD.
This suggests the female brain requires more extreme genetic alterations than the male brain to produce symptoms, they say.
"The data suggests - and it would require additional experiments to really prove this - but it looks like there is a resilience in brain development that is much higher in females than in males," said lead author Sebastien Jacquemont of the University Hospital of Lausanne, Switzerland.
"You can 'break' neurodevelopment in males much easier than you can in females."
Co-author Evan Eichler of the University of Washington, Seattle, said there was a difference in the number of bad mutations between girls and boys.
"There are two ways of looking at it - boys are more susceptible or girls are protected," he said. "And so it takes more insult in the genome of a girl to push them over a threshold to develop autism or to develop developmental delay compared to a boy."
One possible explanation for this is that since females have two X chromosomes rather than one, one X may compensate for damage to the other, he said.
"Our idea is that if you are a male, you are essentially a one-hit wonder," he told the BBC World Service - Science in Action programme.
"Whatever you have on your X chromosome you're essentially stuck with in terms of your mutations."
The findings may lead to the development of more sensitive, gender-specific methods for diagnosis of neurodevelopmental disorders.
Carol Povey, director of the National Autistic Society's Centre for Autism, said historically, research on autism has given a "very male understanding of autism which can make it more difficult for women and girls to access a diagnosis and support".
"Research like this, which helps us to understand more about how autism affects both males and females, is therefore valuable," she said.
"The study highlights the importance of developing gender-specific approaches to autism. It is vitally important that gender does not remain a barrier to diagnosis and getting the appropriate, tailored support."