The risk of autism in younger siblings is significantly higher if an older sibling has autism, according to findings published in the Journal of Developmental & Behavioral Pediatrics. The risk is comparable across race, ethnicity, and gestational age at birth; however it remains higher for boys. 

While past studies have shown that younger siblings who have an older sibling with autism have a much greater risk of also having autism, these findings have not been evaluated in a large racially and ethnically diverse population.

In an attempt to better understand this association, Fagen Xie, PhD, of the Department of Research and Evaluation at the Kaiser Permanente Southern California Medical Group in Pasadena, California and colleagues aimed to examine the risks of autism in younger siblings of older affected siblings, and to examine whether their risk was affected by sex, race or ethnicity, and gestational age at birth.

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They examined records of children born in Kaiser Permanente Southern California hospitals between January 1, 2001 and December 31, 2010, and who remained in the system until 2 to 11 years of age, to examine the risk of autism surfacing in younger siblings.

Among the 53,336 children born within the specified time frame with at least 1 older sibling also born during this time period, 592 (1.11%) were diagnosed with autism. The rate of autism diagnosis was 11.3% for those with an older sibling with autism, and .92% for those without. Younger boys with autism with older brothers (44.59%) were more likely to be affected than younger girls with autism who had older sisters (8.11%).

“In this study, we demonstrate that the risk of receiving a diagnosis of autism spectrum disorders is approximately 14-fold higher in younger siblings with affected older siblings,” the authors wrote. “The magnitude of association also held within each subanalysis stratified by the child’s race/ethnicity and gestational age at birth, after further adjustment for potential confounding factors.”

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The researchers also found that the highest absolute risk of autism among siblings was in mixed racial/ethnic groups. The risk was still present after limiting the analysis to member patient only and controlling for maternal socioeconomic factors, which makes other factors more likely to be responsible.

“A potential explanation to the observed disparity may be related to the lack of education about ASD. These results are similar to those of our previous study on attention-deficit hyperactivity disorder (ADHD) that showed a higher rate of ADHD diagnosis among children of white families,” the researchers wrote. “Children of highly educated parents tend to have their children examined for various learning disabilities when they fail to meet parental expectations regarding school work.”

They also noted that differences in environmental exposures may play a role in the different rates of diagnosis in race.


Xie F, Peltier M, Getahum D. Is the Risk of Autism in Younger Siblings of Affected Children Moderated by Sex, Race/Ethnicity, or Gestational Age? J Dev Behav Pediatr. 2016; doi: 10.1097/DBP.0000000000000341. [Epub ahead of print]

This article originally appeared on Psychiatry Advisor