When it comes to developmental disorders of the brain, men and women are not created equal.

Decades of research have shown that males are at far greater risk for neurodevelopmental disorders such as autism spectrum disorder (ASD) than females. Boys, on average, are five times more likely to have autismthan girls.What causes this disparity has largely remained unknown.

Now scientists haveuncovered compelling genetic evidence to explain why the biological scales arent balanced.

According to a team of geneticists in the U.S. and Switzerland,it all boils down to whats called the female protective model. This suggests that girls have a higher tolerance for harmful genetic mutations and therefore require a larger number ofthem than boys to reach the diagnostic threshold of a developmental disorder. With identical genetic mutations, then, a boy could show symptoms of ASD while a girl could show none.

But because the female mutation threshold is higher, when girls are diagnosed with ASD, they tend to fall on the more severe end of the spectrum.

Researchers believe the same dynamic could explain why more boys are diagnosed withADHD, intellectual disabilities and schizophrenia. The findings were published Thursday in the American Journal of Human Genetics.

Geneticists analyzed DNA samples from 16,000 boys and girls with neurodevelopmental disorders.They found that, on average, females diagnosed with ASD had 1.3 to 3 times more harmful genetic alterations than males diagnosed with the disorder.

The findings suggest that as the male brain develops, smaller and more subtle genetic changes can trigger autism spectrum disorders. Female brains require a greater number or severity of mutations before showing symptoms, so their symptoms tend to be worse.

Theres no application in terms of treatment, said study author Sbastien Jacquemont of University Hospital of Lausanne in Switzerland, but it does help understand the inheritance dynamics in families.

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Genetics May Explain Why Autism Is More Common in Boys