Large Study Confirms: Siblings of Autistic Children Have 20% Chance of Autism

Introduction

A comprehensive study led by UC Davis MIND Institute researcher Sally Ozonoff, in collaboration with the Baby Siblings Research Consortium, has provided significant insights into autism recurrence in families. The research confirmed that siblings of autistic children have a 20% chance of being autistic themselves, a rate approximately seven times higher than in infants with no autistic siblings. This finding underscores the heightened risk within families where autism is already present, prompting a deeper exploration into the genetic and environmental factors that contribute to the condition. The study’s robust methodology and extensive sample size lend credibility to these findings, providing a critical resource for parents, healthcare providers, and researchers.

Building on previous findings, this research highlights several important factors that influence the recurrence of autism within families. The team’s comprehensive approach included genetic analysis, developmental assessments, and longitudinal tracking to identify patterns and predictors of autism spectrum disorder (ASD). These insights are not only crucial for early diagnosis and intervention strategies but also for understanding the complex interplay of hereditary and external elements that contribute to autism. The study’s outcomes advocate for increased awareness and support for families with a history of autism, emphasizing the need for tailored resources and continuous monitoring to better manage and potentially mitigate the impacts of ASD.

Increasing Autism Rates Prompt New Study

Background and Motivation

“The rate of autism diagnosis in the general community has been steadily increasing since our previous paper was published,” Ozonoff explained. The latest estimates from the Centers for Disease Control and Prevention (CDC) show that about 1 in 36 children has autism, compared to 1 in 68 in 2011. Changes in diagnostic criteria and increased awareness of autism in girls also necessitated a fresh look at the recurrence of autism within families.

Reassurance for Families

The 2011 paper reported a recurrence rate of 18.7%, while the new study found a slightly higher rate of 20.2%. “This should reassure providers who are counseling families and monitoring development. It should also help families plan for and support future children,” Ozonoff said.

A Larger, More Diverse Study

Study Design and Participants

The new study included data from 1,605 infants at 18 research sites across the United States, Canada, and the United Kingdom. All infants had an older autistic sibling. “This study was much larger than the first and included more racially diverse participants,” Ozonoff noted. The original study included 664 children.

Methodology

Researchers followed the children from as early as 6 months of age through up to seven visits. Trained clinicians assessed the children for autism at age 3 using the Autism Diagnostic Observation Schedule (ADOS-2), a well-validated tool. The data were gathered from 2010 to 2019.

Key Findings: Factors Influencing Autism Recurrence

Sex of First Autistic Child

The study found that the sex of the first autistic child significantly influenced the likelihood of autism recurrence within a family. “If a family’s first autistic child was a girl, they were 50% more likely to have another child with autism than if their first autistic child was a boy,” Ozonoff said. This points to genetic differences that increase recurrence likelihood in families with an autistic daughter.

Multiple Autistic Siblings

A child with multiple autistic siblings has a higher chance of autism (37%) than a child with only one sibling on the spectrum (21%).

Sex of the Infant

The sex of the later-born infant was also a factor. If the new baby was a boy, they were almost twice as likely as a girl to be diagnosed with autism. “The familial recurrence rate if the new baby was a boy was 25%, whereas it was 13% if the new baby was a girl,” Ozonoff explained. This aligns with the general trend of boys being diagnosed with autism about four times as often as girls.

Race and Maternal Education Level

Race and the mother’s education level were also influential. In non-white families, the recurrence rate was 25%, compared to 18% in white families. The recurrence rate was 32% for mothers with a high school education or less, 25.5% for some college, 19.7% for a college degree, and 16.9% for a graduate degree.

“These findings are new — and critical to replicate,” Ozonoff explained. “They do mirror the recent CDC findings that autism is more prevalent in children of historically underrepresented groups.”

Tracking Outcomes and Bias Concerns

Addressing Potential Bias

Unlike the first study, researchers also tracked families who dropped out of the three-year study to check if their outcomes differed from those who remained. “We wondered whether families who stayed in the study may have had children who were more affected — making them more worried about their development,” Ozonoff explained. This could have biased the estimates of recurrence to be higher than they really were. The current study showed that this was not the case.

Confirmation of Previous Findings

“Now we have two large, independent studies that report familial recurrence in the same range,” Ozonoff said. “This reinforces how important it is that providers closely monitor the siblings of autistic children for delays in social development or communication.”

Implications for Healthcare Providers

Early Diagnosis and Intervention

Early diagnosis and intervention are critical, especially for families with reduced access to care. This study highlights the importance of monitoring siblings of autistic children closely to ensure timely support and intervention.

Societal and Health Determinants

Ozonoff emphasized the need for further research to understand the social determinants of health that may lead to higher rates of autism in certain families. “More research is needed to answer these critical questions,” she said.

Study Collaborators

The study’s co-authors included Gregory Young and Rebecca Schmidt of UC Davis; Jessica Bradshaw of the University of South Carolina; Tony Charman of Kings 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.

Conclusion

This large-scale study confirms that siblings of autistic children have a significantly higher chance of being diagnosed with autism themselves. The findings underscore the importance of long-term monitoring and support for these families, considering various factors such as sex, race, and maternal education level. As research continues to evolve, healthcare providers must stay informed to offer the best possible guidance and care for families affected by autism.