A baby born into a family with an older sibling who has autism has a 19 percent chance of also developing the disorder, a new international study has found. Researchers previously estimated this sibling risk to be 5 to 10 percent.
For male infants, the new research showed, the risk is even higher, at 26 percent, and if a newborn has more than one sibling with autism, the risk is 32 percent. The study also analyzed the effects of socioeconomic status, birth order, ethnicity and parental education levels on autism risk.
"Some of what we found jibes with what has been found before," said study researcher Gregory Young, a developmental psychologist at the University of California, Davis. "But nobody else has found such a high recurrence risk for siblings before."
The findings suggest that younger siblings of autistic children should be screened early and often for signs of autism, the researchers said.
The new numbers were published online today (Aug. 15) in the journal Pediatrics.
Early signs of autism
Autism is a disorder that affects social and communication skills. The earliest known signs of autism appear around a baby's sixth month — if an infant's not smiling by this age, it raises a red flag. A lack of babbling and gesturing by the end of the first year, when most children have begun these forms of early communication, is the next sign, the researchers said.
The new study included more than 650 infants from around the world who had older siblings with autism. Most of the infants were enrolled in the study by the time they were six months old. At 3 years old, the infants were diagnosed as having autism or not, based on their score on a standard autism diagnosis test, as well as observations by expert clinicians.
By the study's end, 132 of the children were diagnosed with an autism spectrum disorder (the spectrum includes classic autistic disorder, as well as less severe forms such as Asperger's syndrome). Of these, 54 children — about 8 percent of the total enrolled — had the most severe form of autism, and 78 had a milder form of the disorder.
The risk of autism for the average child born in the United States today is about 1 in 110. The risk of autism for the children studied was much higher — 9 percent of girls and 26 percent of boys had some form of the disorder.
"This does suggest that there's this genetic component, because of this increased risk in families," Young said. "The counterargument if that these children also have a shared environment, so there's always that to untangle."
New numbers are a baseline
This first large, prospective study of autism risk in siblings offers a place to start for future studies, said Joachim Hallmayer, a psychiatrist at the Stanford School of Medicine.
"This is a baseline which you can build upon," Hallmayer said. "If you want to do a follow-up study now, you can see what causes that risk to go up or down."
Moreover, he said, the study suggests much could be learned by studying the genetics of this group of children.
"The big question is not the number," Hallmayer said. "Really, the question is what causes autism in these siblings? What differentiates the ones that become autistic from the ones that don't?"
Young said the results should also change how closely doctors screen younger siblings for early signs of autism.
"Ultimately, the important thing is how this can inform clinical practice, and how it can help parents and families," Young said. "Knowing that there is this significantly increased risk in these younger siblings tells us that maybe early monitoring needs to be taken more seriously with these kids."
Pass it on: The chance that a child develops autism if they have an older sibling with autism is 19 percent, higher than researchers previously estimated.