The surprising theory why boys are diagnosed with autism more often than girls

Blame it on the X factor.

Research has long shown that boys are over three times more likely to be diagnosed with autism than girls.

In fact, about 1 in 20 US boys are diagnosed with autism by age 8. For girls, the rate ranges from about 1 in 70 to 1 in 100. The complex neurodevelopmental condition, which typically affects social communication, interaction and behavior, is becoming more prevalent overall.

Scientists have pointed the finger at genetic and biological differences, as well as diagnostic biases, as reasons Y boys are so unlucky.

Now, the Whitehead Institute for Biomedical Research believes it has identified a more specific X-planation. The answer may lie in the X chromosome.

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You might remember from biology class that boys have an X and a Y chromosome, while girls have two X chromosomes. The long-time assumption is that the second X chromosome — also called Xi — is largely silent.

Whitehead researchers suggest that the “inactive X” chromosome actually plays an active role in shaping gene expression across the genome, including many genes tied to autism.

The theory — called the “female protective effect” — proposes that higher expression of certain regulatory genes from Xi allows girls to better buffer the effects of autism-associated mutations.

In short, girls generally require a greater “genetic load” — more, larger or more severe genetic mutations — to develop autism.

The Whitehead researchers believe the “female protective effect” extends to 17 other congenital and developmental disorders that primarily affect boys.

They gave the example of pyloric stenosis, a treatable gastrointestinal condition that causes infants to projectile vomit. Boys are affected four times more often than girls.

“Many of the other congenital or developmental conditions we’re pointing to aren’t subject to diagnostic inequities in the way autism is,” Harvard-MIT MD-PhD student Maya Talukdar said.

“This strengthens the idea that the female protective effect is emerging from genetic differences in males and females.”

The findings were published Monday in the journal Nature Genetics.

The study authors plan to further explore Xi to understand sex differences in disease to improve diagnosis and care.

In the meantime, they warn that girls and women are frequently overlooked or diagnosed late with autism.

They say that one reason is that much of autism research — and the screening tools born from this research — has long focused on boys.

As such, there’s limited scientific understanding of autism in females.

Autism symptoms include trouble making eye contact, difficulty understanding others’ feelings, struggles making friends, repetitive movements, obsessive interest in specific objects and sensory issues.

If you suspect you might have autism, you should consider consulting a doctor or mental health professional for a formal diagnosis or referral.