Researchers used two large databases of thousands of fraternal twins that included information about autistic behaviors, including problems with social interactions, communication, and repetitive behaviors. Since the siblings share similar genetic risk factors and environmental exposures, studying how the autistic traits the children in each family had was one way of trying to isolate the role gender could play in the disorder.
What the researchers found was a clear signal that girls were protected; in other words, females needed to have a greater burden of familial risk factors in order to manifest classical autistic behaviors. The researchers figured that out by comparing the siblings of two groups: girls whose behaviors put them in the top 10th percentile of autistic behaviors and boys who were similarly ranked. If gender had a protective effect, the researchers would expect girls to be more likely to have a sibling with autistic traits than boys in the same group. That's because girls would need more familial risk factors to overcome the protective effect, and those same risk factors would also be experienced by their siblings.
John Gabrieli, a neuroscientist at the McGovern Institute for Brain Research at the Massachusettts Institute of Technology, said that the study was striking because it shows evidence that something biological—in the genes or environment—is "muting" autistic traits in girls.
"It's worth studying, practically, because it is so impressive. Because if you understood some of these mechanisms, maybe it would be a suggestion of a treatment for boys or prevention for boys, or a naturally-occurring preventive treatment," Gabrieli said.
Image: Happy girl, via Shutterstock