By PsyPost

Researchers from the UC Davis MIND Institute and the Baby Siblings Research Consortium have discovered that siblings of children with autism are at a much higher risk of being diagnosed with the condition than children in the general population.

The study, published in Pediatrics, determined that about 20% of later-born siblings in families with an autistic child are diagnosed with autism themselves, a rate roughly seven times higher than that of children without autistic siblings.

Autism spectrum disorder is a complex developmental condition that affects communication, behavior, and social interaction. Families with an autistic child often want to know the chances that a future sibling will also have the condition, especially given the significant life changes that come with managing the diagnosis.

The researchers were motivated by the increasing prevalence of autism diagnoses in the general population—rising from about 1 in 110 children in 2011 to 1 in 36 today. This spike raised the question of whether similar trends might influence the recurrence of autism within families, where genetic and environmental factors could play a larger role.

In 2011, a study led by the same research group found that 18.7% of younger siblings of autistic children were later diagnosed with autism. Over the past decade, advances in diagnostic criteria and greater awareness of autism, especially among girls and cognitively able children, have led to increased identification. The researchers wanted to update their previous estimates to reflect these changes.

“Our team previously found that autism is more likely to recur in a family that already has a child with the condition than a family that doesn’t have a child with autism (about 2.5% risk). Since our initial report in 2011, the ASD general population prevalence rate has almost doubled and we were curious whether the same factors operating to increase prevalence (e.g., improved awareness, broader diagnostic criteria, etc.) might have also affected the likelihood of recurrence within a family,” explained study author Sally Ozonoff, a distinguished professor at UC Davis and principal investigator of the Early Detection Lab.

In the new study, the research team analyzed data collected from 18 different research sites worldwide, all part of the Baby Siblings Research Consortium, an international network of autism researchers. This study followed 1,605 infants born into families that already had one or more autistic children. These infants were recruited between 2010 and 2019 and were followed until they were 3 to 5 years old, which is the typical age range when autism symptoms begin to emerge and can be formally diagnosed.

To ensure consistency in diagnosis, each research site used the same assessment tools: the Autism Diagnostic Observation Schedule (ADOS-2) and the Diagnostic and Statistical Manual of Mental Disorders (DSM) criteria. These tools are considered reliable for identifying autism. In addition to assessing whether these children developed autism, the researchers collected data on their developmental progress using the Mullen Scales of Early Learning (MSEL), which measures cognitive, motor, and language skills.

Key demographic data were also gathered from the families, including information on race, parental education levels, and family structure (e.g., whether the family had one or more autistic children). These factors were included to better understand how they might influence the likelihood of an autism diagnosis in younger siblings.

The study confirmed that the recurrence rate of autism in families remained high, with 20.2% of younger siblings of autistic children receiving an autism diagnosis. This rate is significantly higher than the general population’s autism diagnosis rate, which is about 2.5%. Although the overall recurrence rate was slightly higher than the 18.7% reported in 2011, the increase was not statistically significant, suggesting that despite the rising population prevalence of autism, the likelihood of autism recurring within families has remained relatively stable.

“We replicated the previous finding that the familial recurrence rate is about 20%,” Ozonoff told PsyPost. “In 2011, we found a rate of 18.7%, which is not statistically different. The current sample was much larger and more diverse, giving greater reliability to the finding. The stability of the findings over more than a decade should reassure families and clinicians that the 1-in-5 likelihood of having another child with ASD is a reliable rate.”

“This reinforces the notion that autism runs in families, likely due to genetic causes. Given the elevated likelihood that a younger sibling will develop ASD, close developmental surveillance and prompt referrals are warranted. This was the conclusion of our 2011 paper as well but not necessarily followed in practice yet. Providers need to carefully monitor the development of all younger siblings of autistic children.”

One key finding was that the recurrence rate varied based on certain factors. For instance, male infants in these families were more likely to be diagnosed with autism than female infants—25.3% of boys were diagnosed compared to only 13.1% of girls. This aligns with broader autism research, which consistently shows that boys are more likely than girls to be diagnosed with autism.

However, the study found that girls from families with more than one autistic sibling (known as multiplex families) had a much higher risk of autism compared to girls from families with only one autistic child. In multiplex families, the autism recurrence rate for girls was 39.5%, compared to 12.6% in simplex families (those with only one autistic child).

Interestingly, the sex of the older autistic child (known as the proband) also influenced recurrence rates. Families with an autistic daughter had a recurrence rate of 34.7%, whereas families with an autistic son had a recurrence rate of 22.5%. This supports the theory that girls may require a greater number of genetic or environmental risk factors to develop autism, a phenomenon known as the female protective effect.

Another important finding was the impact of race and maternal education on recurrence rates. The study showed that autism recurrence was higher in families of non-white infants (24.3%) compared to white infants (17.4%). Additionally, maternal education was found to be a predictor of recurrence: children whose mothers had less than a college education were more likely to be diagnosed with autism than those whose mothers had higher educational attainment.

We found that the race of the family affected the likelihood that they would have another child with the autism,” Ozonoff said. “The recurrence rate in White families was 18% while the recurrence rate in families of color was significantly higher, 25%. We also found that recurrence likelihood was affected by the educational attainment in the family.”

“These are new and surprising findings, so it is critical that they are replicated by an independent research team. If replicated, these findings may indicate that there are social factors and social determinants of health that could lead to higher rates of autism in families who experience adversities (systemic racism, economic disadvantage, etc.).”

The study also highlighted the need for more research into the interaction between genetic and environmental factors in autism. For example, why do girls in multiplex families face such a high risk of developing autism? More research into the genetic underpinnings of autism could help answer this question and provide clearer guidance for families.

The Autism Science Foundation was one of several organizations that funded this study. Alycia Halladay, the foundation’s chief science officer and program officer for the Baby Siblings Research Consortium, emphasized the importance of early monitoring, saying: “Given the increased risk to younger siblings, it is critical that subsequent children in families with a child diagnosed with ASD be closely monitored and referred promptly for diagnostic evaluation if early warning signs are present.

“Younger siblings who are male, have an affected female sibling, or multiple affected siblings should be very closely watched during early development. More must be done to ensure that infants with a higher likelihood of diagnosis receive continuous monitoring, especially those from economically disadvantaged communities.”

The study, “Familial Recurrence of Autism: Updates From the Baby Siblings Research Consortium,” was authored by Sally Ozonoff, Gregory S. Young, Jessica Bradshaw, Tony Charman, Katarzyna Chawarska, Jana M. Iverson, Cheryl Klaiman, Rebecca J. Landa, Nicole McDonald, Daniel Messinger, Rebecca J. Schmidt, Carol L. Wilkinson, and Lonnie Zwaigenbaum.