It has long been clear that autism strikes boys more often than girls. But when girls do get the
Now, a group of geneticists thinks they've figured out why.
Boys, it seems, can develop autism from a relatively small genetic hit, according to a study published today in the American Journal of Human Genetics. It takes more of a genetic wallop, though, to cause autism in girls – so when they do get it, they're worse off.
same explanation holds true, researchers think, for the gender
imbalance in ADHD (attention-deficit hyperactivity disorder),
intellectual disabilities and schizophrenia.
"In the male, maybe
more subtle things are enough to create a disorder," said geneticist
Mandel of the College de France in Paris and the Academic
Hospital in Strasbourg, France, who was not involved in the study.
finding, in a study of more than 16,000 people, confirms that autism is
not simply being missed in females – it is actually occurring less
often, said Kevin Mitchell, a geneticist at Trinity College in Dublin.
are about seven males with mild autism for every female, though the
gender gap is much smaller at the more severe end of the spectrum.
study also showed that the mutations behind the autism are either new
ones that develop in the child, or come from the parents – most likely
from the mother.
A man who is severely affected by an
autism-related genetic glitch is more likely to have trouble forming
relationships and therefore less likely to have children, Mitchell said,
so less likely to pass the mutation on. A woman, who can have the
glitch without noticing it, would be more likely to reproduce and
therefore pass on the mutation, he said.
The study's lead
researcher, geneticist Evan Eichler of the University of Washington,
said this is one more piece of the genetic puzzle of autism – which will
eventually lead to new diagnoses and treatments.
About 500 genes
have been connected to autism – "There's lots of different ways to
create an autistic child," he said. But those genes mostly fit into
about a dozen different pathways, suggesting different treatment
approaches may be most effective for each subtype.
"It's going to
be really important to know which pathway your child is in," he said. "I
would put money on it that not all drugs and not all behavioral
treatments will work the same depending on the basis for how that child
Genetics can also help families with autism who
are deciding whether to have another child. If the child has a rare
mutation that isn't present in the parents, their next child is probably
no more likely to have autism than any other family, Mitchell said. If
the mother has a risky mutation, however, the odds of having a second
child on the autism spectrum will be much higher.
The study was
quite large and well done, Mitchell and Mandel said. But not everyone is
convinced that genes tell the whole story. It's possible that the
hormones a fetus is bathed in during pregnancy also play a role in the
vulnerability of males, said Irva Hertz-Picciotto, an epidemiologist at
the University of California, Davis, and director of the school's MIND
Institute Program in Environmental Epidemiology of Autism and
"Boys are swimming in measurably more
testosterone than girls are," she said. "Some evidence suggests that
social behaviors are in part determined by such early life exposures to
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