Siblings of autistic children have a 20% chance of being autistic themselves, about seven times higher than the rate in babies with no autistic siblings.
That’s the main conclusion of a new article by researcher Sally Ozonoff of the MIND Institute at the University of California, Davis, and the Baby Sibling Research Consortium, a consortium made up of more than 20 research groups from universities around the world.
Ozonoff is a distinguished professor in the Department of Psychiatry and Behavioral Sciences. She has studied autism recurrence in families for decades.
The new study is based on a large and diverse group of families at research centers in the United States, Canada and the United Kingdom, and confirms the same research group’s 2011 findings on the likelihood of autism in siblings.
The new research was published this week in Pediatrics.
Rising autism rates prompt new study
“The rate of autism diagnosis in the general community has been steadily increasing since our previous paper was published,” Ozonoff said.
The latest estimates from the Centers for Disease Control and Prevention indicate that about 1 in 36 children has autism. In 2011, the estimate was 1 in 68.
Ozonoff noted that there have also been changes in the diagnostic criteria for autism over the past decade. In addition, there is growing awareness about autism in girls.
“Therefore, it was important to understand whether this had any impact on the likelihood of autism recurrence within a family,” she said.
The 2011 paper found a recurrence rate of 18.7%, while the new paper found a rate of 20.2%—a small but not significant increase.
“This should reassure providers who counsel families and monitor development. It should also help families plan and support future children,” Ozonoff said.
A larger and more diverse study
The new study included data from 1,605 infants at 18 research centers. All of the babies had an older sibling with autism.
“This study was much larger than the first one and included more racially diverse participants,” Ozonoff said. The original study included 664 children.
Researchers followed the children from 6 months of age for up to seven visits. Trained clinicians screened the children for autism at age 3 using the Autism Diagnostic Observation Schedule-2 (ADOS-2), a well-validated tool. Data were collected between 2010 and 2019.
Key factors: sex of the first autistic child and autistic siblings
Researchers found that the sex of the first autistic child influenced the likelihood of autism recurring within a family.
“If the first autistic child in a family was a girl, the likelihood of having another child with autism was 50 percent higher than if the first autistic child was a boy,” Ozonoff said. “This indicates genetic differences that increase the likelihood of recurrence in families that have an autistic daughter.”
Researchers also found that a child with multiple autistic siblings is more likely to have autism (37%) than a child with only one sibling on the spectrum (21%).
The sex of the baby was also associated with the likelihood of familial recurrence. If the baby born later was a boy, he was almost twice as likely to be diagnosed as a girl.
“The familial recurrence rate if the baby was a boy was 25%, while if it was a girl it was 13%,” Ozonoff explained. “This is consistent with the fact that boys are diagnosed with autism four times more often than girls in general.”
Race and maternal education level influence recurrence
Researchers found that the mother’s race and education level were also likely factors. In nonwhite families, the recurrence rate was 25%. In white families, the recurrence rate was about 18%. In families where the mother had a high school education or less, the recurrence rate was 32%. In families with some college education, the rate was 25.5%, and in families with a college degree, it was 19.7%. When the mother had a graduate degree, the rate dropped to 16.9%.
“These findings are new and it’s critical to replicate them,” Ozonoff said. “They mirror recent CDC findings that autism is more prevalent in children from historically underrepresented groups.” She noted that this reversed a long-standing trend of lower prevalence in those groups.
Most importantly, Ozonoff said, if these findings are replicated, they could indicate that there are social determinants of health that may lead to higher rates of autism in families. She emphasized that this study was not designed to answer those critical questions and that more research is needed.
Results tracking
Unlike the first study, the researchers also followed up with families who dropped out of the three-year study to see if their outcomes differed from those who did. “We wondered whether families who remained in the study might have had children who were more affected, making them more concerned about their development,” she explained.
This could have skewed the recurrence estimates and made them higher than they actually were. The current study showed that this was not the case.
“We now have two large independent studies reporting familial recurrence in the same range,” Ozonoff said. “This reinforces the importance of clinicians closely monitoring siblings of autistic children for delays in social development or communication. This is especially true in families who have reduced access to care, because early diagnosis and intervention are critical.”
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Co-authors of the study included Gregory Young and Rebecca Schmidt of UC Davis; Jessica Bradshaw of the University of South Carolina; Tony Charman of King’s College London; Katarzyna Chawarska of Yale University; Jana M. Iverson of Boston University; Cheryl Klaiman of Emory University; Rebecca Landa of Johns Hopkins University; Nicole McDonald of UCLA; Daniel Messinger of the University of Miami; Carol Wilkinson of Harvard University; and Lonnie Zwaigenbaum of the University of Alberta.