Researchers have found that the risk of autism in younger siblings was significantly higher if an older sibling had autism, but that the risk was comparable across race, ethnicity, and gestational age at birth. Risk did, however, remain higher for boys. The findings were published in the Journal of Developmental & Behavioral Pediatrics.
While past studies have shown that younger siblings who have an older sibling with autism have a much higher risk of also having autism, and several studies have shown that the rate of autism varies by race and ethnicity, these findings have not been evaluated in a large, racially and ethnically diverse population.
In an attempt to fill this gap, Fagen Xie, PhD, from 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.
They examined records of children born in the large health maintenance organization Kaiser Permanent Southern California hospitals between January 1, 2001 to 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.
Out of a total of 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. Furthermore, the [autism] recurrence was slightly higher when the older sibling was a girl.”
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.2-5 These results are similar to those of our previous study6 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.
Summary & Clinical Significance
This study examined a large, racially and ethnically diverse population and found that the risk of autism in younger siblings if an older sibling has autism. The increased risk was comparable across different races, ethnicities, and gestational age of birth, but the risks were higher for younger siblings who were boys.
“This study contributes to a better understanding of the influence of race/ethnicity, sex, and gestational age at birth in identifying children at highest risk of ASD,” the researchers concluded.
1. Children born in later years of the study may not have been followed for enough time to be identified as having autism, which may have led to an underestimation of the rates of autism.
2. Autism in older siblings was only identified if they were born between 2001-2010. Younger siblings could have had a sibling with autism born before then, which could have led to an underestimation of recurrence in siblings.
3. Some subgroups had smaller sample sizes, so their results should be interpreted with caution.
4. Multiple births were excluded from this study, so future studies are needed to determine risk in twin probands.
5. Researchers were unable to account for families with one autistic child who decided not to have more.
6. The study was limited to the diagnostic criteria in place at the time, the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV), which had less strict diagnostic criteria than the DSM-5.
7. Researchers were unable to compare recurrence risk between full and half siblings