Autism runs in families to a much greater degree than previously thought, according to new research that has tracked the younger siblings of children with the developmental disorder.
Researchers at the University of California, Davis (UCD) found that among 664 children in the U.S. and Canada with at least one older sibling with autism, the risk of the disorder was 18.7%, considerably higher than previous sibling recurrence estimates of 3% to 10%.
Younger male siblings were three times more likely to have autism than girls: 26.2% versus 9.1%. The 37 children with more than one older sibling with autism were most likely to be diagnosed, with a 32.2% risk — twice that of kids with only one older autistic sibling.
The results of the study, published online Monday by the journal Pediatrics, are intended to inform, not scare parents, says lead author Sally Ozonoff. She expects that parents will welcome data from what is the largest group of siblings of autism ever tracked. “If a family has a child with autism, it’s the first question they ask,” says Ozonoff, a professor of psychiatry and behavioral sciences at UCD’s MIND Institute, a research center that studies causes and treatments for autism. “But even now with better estimates, it’s important to convey this was the average risk.”
Autism, inasmuch as scientists understand it, has many causes. An estimated one in 110 U.S. children is diagnosed with autism, and 1.5 million Americans live with the complex brain condition. Although genetics clearly plays a role, it’s not the only factor, as shown in previous research that has found that identical twins — who share the same genome — don’t always both have autism.
“Every child’s autism has a different combination of risk factors,” says Ozonoff. “Some families’ risks are much higher than 18% and some are much lower.”
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Laurisa Stuart, a Folsom, Calif., mom of four boys, was taken aback by the odds. Her second child, 4-year-old Bryson, has autism; her youngest, 15-month-old Cooper, is enrolled in the UCD sibling study. “I was very worried with Cooper,” says Stuart, who knew that boys run a greater risk of having autism. Of Cooper’s 1-in-4 chance, however, she says: “I had no idea.”
Every few months, Cooper’s head is measured and his weight is recorded; Stuart answers questions about his development. So far, the toddler has shown no signs of autism.
Like Cooper, all 664 children in the study were enrolled before 18 months; two-thirds were younger than six months old, which is a critical element of the study’s design. Researchers wanted children to join the study at an age before parents typically become concerned about possible signs of autism. The first signs are often noticed around age 2 — in some infants as young as 1 — and the disorder is typically diagnosed around age 4 or 5.
The children were pulled from the 12 sites of the Baby Siblings Research Consortium, a mutinational autism research project that is tackling issues including the earliest age at which autism can be reliably diagnosed and whether it may be possible to predict which children will develop the disorder before signs become evident.
Most sites saw the babies between four to seven times until their third birthday. If, at any point, a child demonstrated signs of autism, he could potentially be diagnosed. At 36 months, however, all participants were thoroughly evaluated for symptoms even if their behavior had raised no red flags.
Much lower estimates of sibling recurrence date to the 1980s, but that was before diagnostic criteria had improved and awareness had increased. Earlier studies, including some of those that looked at the likelihood of parents having multiple siblings with autism, didn’t take into account that parents who have a child with autism may choose not to have more kids — a phenomenon known in genetics as “stoppage.” The UCD study followed only families with subsequent children.
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Those parents who decide to have more kids should connect with a pediatrician who will step up developmental surveillance of future babies. “These babies should potentially be treated differently than other infants,” says Ozonoff. At a typical “well baby” check-up for a healthy infant, doctors often ask general questions such as, How is she doing? For babies who have older siblings with autism, doctors should be more specific, inquiring about eye contact, babbling, smiling at others and playing peek-a-boo — all developmental milestones whose absence could be worrisome. Early intervention services should be recommended if parents or pediatricans are concerned.
“The study really defines younger siblings with older brothers or sisters with autism as a risk group and allows their health-care providers to explore their early development and possibly provide earlier diagnosis,” says Alycia Halladay, director of research for environmental sciences for Autism Speaks, the world’s largest autism science and advocacy organization. “There is definitely evidence that early intensive behavioral intervention is an effective treatment for autism-spectrum disorders.”
That’s exactly why Stuart enrolled her son, Cooper, in the study. “We could find out earlier if he had it, and if he didn’t, we could help other children out by being a part of this,” says Stuart, who credits behavioral services with helping her son with autism progress to the point that he will be “mainstreamed” this year in a preschool for typically developing children.
Stuart’s family also appears to illustrate the key point of Ozonoff’s study, with three of her four boys showing no signs of autism. The research, as a whole, should give families hope. “These findings are much higher than any of us anticipated, but the flip side is that over 80% of these children did not develop autism,” says Ozonoff. “That is really important for these families to hang on to. The glass is still more than half full.”
Bonnie Rochman is a reporter at TIME. Find her on Twitter at @brochman. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.