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Autism Risk in Younger Siblings May be Higher Than Previously Thought

Science Update

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Parents of a child with autism spectrum disorder (ASD) face about a 19 percent chance that subsequent children will also develop ASD, according to a study partially funded by NIMH. This estimate is much higher than previous reports but may also be more accurate due to the study's size and design, according to the researchers. Their study was published August 15, 2011, online ahead of print in the journal Pediatrics.


A few previous studies have explored the recurrence rate of ASD, or the likelihood of later-born siblings of children with ASD to also develop ASD. However, few studies addressed factors likely to influence risk estimates, such as:

  • Stoppage—the tendency for families to choose not to have more children after one child is diagnosed with ASD. Such families would not be included in research on ASD recurrence.
  • Overreporting—an error that can occur when researchers rely solely on parent reports or health records, which have been shown to inflate estimates.
  • Ascertainment bias—an example is overselection, which can occur when parents with one child who has ASD pay very close attention to a later child's development. They may be more likely to take part in ASD recurrence studies than other parents.

Taking a different approach, Sally Ozonoff, Ph.D., of the University of California-Davis, and colleagues evaluated data on 664 infants who were tested at 12 sites across the United States and Canada. All sites were members of the Baby Siblings Research Consortium (BSRC), an international network supported by the U.S. advocacy group Autism Speaks. All BSRC members contribute data to a centralized database that allows infant-sibling researchers to pool data across many sites and answer questions that require very large and geographically diverse samples to address.

The average age of the infant participants at the start of the study was 8 months, an age when signs of ASD are not usually present; two-thirds of the total study population were enrolled before age 6 months. All had at least one older sibling diagnosed with ASD, which was confirmed by a consortium doctor. The participants were themselves assessed for ASD multiple times in their first three years of life.

Results of the Study

Out of the total study sample, 18.7 percent of participants were diagnosed with ASD by age 3. Boys were nearly three times as likely as girls to be diagnosed with ASD. Participants who had more than one older sibling with ASD were about twice as likely to also be diagnosed with ASD, compared to participants who had only one older sibling with ASD.

Unlike some previous studies, the gender or IQ of the older sibling with ASD did not affect the later sibling's risk in the present study.


The findings indicate that ASD recurrence is 18 percent or higher, compared to 3-14 percent estimated in earlier studies. The researchers note that their study's size and design minimized the effects of stoppage, overreporting, and ascertainment bias.

Despite the strengths of their study, the researchers emphasize that recurrence estimates cannot provide information on an individual's risk. They highlight the need for careful and extensive counseling and thorough genetic work-ups for concerned parents, as well as close monitoring, especially of high-risk children, and prompt referrals for intervention by primary care providers.

What’s Next

According to the researchers, larger, population-based studies that include families of children with ASD who are not listed in the Baby Siblings Research Consortium may help to further refine recurrence estimates. Future studies will examine DNA collected from participants to examine genetic factors that may be associated with recurrence.


Ozonoff S, Young GS, Carter A, Messinger D, Yirmiya N, Zwaigenbaum L, Bryson S, Carver LJ, Constantino JN, Dobkins K, Hutman T, Iverson JM, Landa R, Rogers SJ, Sigman M, Stone WL. Recurrence Risk for Autism Spectrum Disorders: A Baby Siblings Research Consortium Study . Pediatrics. 2011 Aug 15. [Epub ahead of print] PubMed PMID: 21844053.