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While scientists are still unraveling the many causes of autism spectrum disorders (ASD) — from exposure to air pollution during pregnancy to gestational diabetes — there’s one thing they know for sure: Boys are diagnosed with ASD much more often than girls.
In fact, for every four boys with ASD, only one girl has the condition. With high-functioning autism, the ratio is even higher, with seven boys for every one girl.
STORY: What Really Causes Autism?
So what role does sex play in autism? “It’s turning out to be a very important question,” David G. Amaral, PhD, research director of The MIND Institute at UC Davis, tells Yahoo Parenting. In recent years, researchers have been investigating these sex differences and have found that being female is actually protective of neurodevelopment disorders, such as autism.
Certain gene mutations — both inherited and new, spontaneous ones — increase the risk of developing autism. Several studies, including a 2013 study on twins published in the Proceedings of the National Academy of Sciences and a 2014 study published in the American Journal of Human Genetics, have found that females are able to withstand higher numbers of these harmful mutations — such as large amounts of missing or duplicated DNA — than boys before developing autism.
Amaral gives as an example: “You’d have to have, say, 10 mutations to get a diagnosis of autism in a boy, but if you looked at a girl with autism, you never find 10 mutations — you find 20. For a girl who had 10 genetic mutations, that wouldn’t be enough for her to get a diagnosis of autism. For some reason, she can deal with that. This has now been found over and over again.”
In other words, girls are able to endure a much greater genetic hit before developing autism. “Females are better at dealing with these severe mutations and males are more at risk of having them result in disease,” study author and human geneticist Evan Eichler, PhD, tells FoxNews.com.
Researchers don’t yet know why girls are more resilient when it comes to developing the disorder. Eichler guesses that females may benefit because they have two X chromosomes so if one is damaged, there’s another one that can serve as backup. “When you look at the X chromosome, there are 1,500 genes and five percent are important to brain development,” he says. “Imagine you’re a male, any mutation that even makes the protein produced a little weaker or less efficient, now you’re stuck with that. The female, because she has two X chromosomes, chances are if she has a defective mutation on one of those genes, she can compensate because she has one from the other parent.”
Another factor may be hormones. Girls naturally have higher levels of oxytocin, which may curb the loss of social skills associated with autism. “Oxytocin is a social hormone, so that would be protective,” Marjorie Solomon, associate professor of clinical psychiatry at the MIND Institute at UC Davis tells Time.
Although girls have more mechanisms in place to prevent ASD, the ones who do go on to develop the disorder are often diagnosed later than boys, around four years old on average, compared to 3.8 years old in males, according to the American Academy of Pediatrics. That may be because girls don’t exhibit all of the classic symptoms associated with ASD, such as repetitive behaviors, so they may be overlooked for a longer period of time.
“Not only can girls adapt to a greater genetic hit, but also their symptoms seem to be different,” notes Amaral. “With autism, you have two impairments, in social communication and repetitive behavior. Girls have far less of repetitive behavior even when they have a diagnosis of autism. Girls may be, in part, flying under the radar because they’re not showing one of the two [classic] features of autism.”
That’s why some scientists argue that more studies on girls are needed to improve diagnosis and interventions. “More boys are diagnosed, more boys are studied for research, interventions are developed based on that research and interventions are going on with boys,” Alycia Halladay, PhD, chief science officer at the Autism Science Foundation, tells CBS News. “There’s really been an increased awareness about the needs of girls and the need to develop specific interventions for girls that are gender appropriate.”